uni_store/runtime/

writer.rs

// SPDX-License-Identifier: Apache-2.0
// Copyright 2024-2026 Dragonscale Team

#[cfg(feature = "lance-backend")]
use crate::backend::table_names;
use crate::runtime::context::QueryContext;
use crate::runtime::flush_coordinator::{
    FinalizeFn, FlushCoordinator, FlushOutcome as AsyncFlushOutcome, RotatedFlush, SharedFlushCtx,
};
use crate::runtime::id_allocator::IdAllocator;
use crate::runtime::l0::{L0Buffer, serialize_constraint_key};
use crate::runtime::l0_manager::L0Manager;
use crate::runtime::property_manager::PropertyManager;
use crate::runtime::wal::WriteAheadLog;
use crate::storage::adjacency_manager::AdjacencyManager;
use crate::storage::delta::{L1Entry, Op};
use crate::storage::main_edge::MainEdgeDataset;
use crate::storage::main_vertex::MainVertexDataset;
use crate::storage::manager::StorageManager;
use anyhow::{Result, anyhow};
use chrono::Utc;
use metrics;
use parking_lot::{Mutex as PlMutex, RwLock};
use std::collections::{BTreeMap, HashMap, HashSet};
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::{Arc, OnceLock};
use tracing::{debug, info, instrument};
use uni_common::Properties;
use uni_common::Value;
use uni_common::config::UniConfig;
use uni_common::core::fork::ForkId;
use uni_common::core::id::{Eid, Vid};
use uni_common::core::schema::{ConstraintTarget, ConstraintType, IndexDefinition};
use uni_common::core::snapshot::{EdgeSnapshot, LabelSnapshot, SnapshotManifest};
use uni_xervo::runtime::ModelRuntime;
use uuid::Uuid;

/// Whether `property` on `label` is a multi-vector (`List<Vector>`) column — the
/// late-interaction (ColBERT) shape that auto-embeds per-token via the multi-vector model
/// (issue #104). A plain `Vector` column uses the dense single-vector model.
fn is_multivector_property(
    schema: &uni_common::core::schema::Schema,
    label: &str,
    property: &str,
) -> bool {
    schema
        .properties
        .get(label)
        .and_then(|p| p.get(property))
        .is_some_and(|m| {
            matches!(&m.r#type, uni_common::DataType::List(inner)
                if matches!(**inner, uni_common::DataType::Vector { .. }))
        })
}

/// Convert per-token embedding vectors into the stored `List<Vector>` representation:
/// a `Value::List` whose elements are each a `Value::List<Float>` (one token vector).
fn multivec_to_value(tokens: &[Vec<f32>]) -> Value {
    Value::List(
        tokens
            .iter()
            .map(|tok| Value::List(tok.iter().map(|f| Value::Float(*f as f64)).collect()))
            .collect(),
    )
}

/// One coalesced auto-embed group: all targets that share an `alias` + `source_properties`,
/// split into dense (`Vector`) vs multi-vector (`List<Vector>`) target columns. When a group
/// has BOTH kinds, a single hybrid inference fills both (single forward pass).
struct EmbedGroupSpec {
    source_properties: Vec<String>,
    document_prefix: Option<String>,
    dense: Vec<String>,
    multi: Vec<String>,
    sparse: Vec<String>,
}

/// Convert one xervo sparse embedding (`(term_id, weight)` pairs, possibly
/// unsorted / with duplicate terms) into a `Value::SparseVector`. Sorting (via
/// `BTreeMap`) and summing duplicates yields the sorted-unique invariant the
/// index and codec require; non-finite weights are dropped (never poison a row).
fn sparse_pairs_to_value(pairs: &[(u32, f32)]) -> Value {
    let mut by_term: std::collections::BTreeMap<u32, f32> = std::collections::BTreeMap::new();
    for &(term, weight) in pairs {
        if weight.is_finite() {
            *by_term.entry(term).or_insert(0.0) += weight;
        }
    }
    Value::SparseVector {
        indices: by_term.keys().copied().collect(),
        values: by_term.values().copied().collect(),
    }
}

/// Run ONE embedding inference for a coalesced group of auto-embed targets sharing an alias +
/// source text, returning `(dense_per_text, multi_vector_per_text)` (each `Some` iff that head
/// was requested). When both heads are needed this uses the hybrid model
/// (`hybrid_embedder`), so a multi-functional model (e.g. BGE-M3) produces the dense + ColBERT
/// heads from a SINGLE forward pass; otherwise it uses the single-head dense / multi-vector
/// embedder (today's behavior for non-mixed groups).
#[allow(clippy::type_complexity)]
async fn embed_group(
    runtime: &ModelRuntime,
    alias: &str,
    texts: &[&str],
    want_dense: bool,
    want_multi: bool,
    want_sparse: bool,
) -> Result<(
    Option<Vec<Vec<f32>>>,
    Option<Vec<Vec<Vec<f32>>>>,
    Option<Vec<Vec<(u32, f32)>>>,
)> {
    use uni_xervo::traits::hybrid::HeadSet;
    let heads_wanted = u8::from(want_dense) + u8::from(want_multi) + u8::from(want_sparse);
    if heads_wanted > 1 {
        // Single-pass hybrid: one model, one inference, all requested heads
        // (e.g. BGE-M3 fills dense + sparse from one forward pass).
        let mut heads = HeadSet::empty();
        if want_dense {
            heads |= HeadSet::DENSE;
        }
        if want_multi {
            heads |= HeadSet::MULTI_VECTOR;
        }
        if want_sparse {
            heads |= HeadSet::SPARSE;
        }
        let embedder = runtime.hybrid_embedder(alias).await?;
        let res = embedder.embed(texts, heads).await?;
        let dense = if want_dense {
            Some(res.dense.ok_or_else(|| {
                anyhow!("hybrid model '{alias}' returned no dense head for a Vector target")
            })?)
        } else {
            None
        };
        let multi = if want_multi {
            Some(res.multi_vector.ok_or_else(|| {
                anyhow!(
                    "hybrid model '{alias}' returned no multi-vector head for a List<Vector> target"
                )
            })?)
        } else {
            None
        };
        let sparse = if want_sparse {
            Some(res.sparse.ok_or_else(|| {
                anyhow!("hybrid model '{alias}' returned no sparse head for a SparseVector target")
            })?)
        } else {
            None
        };
        Ok((dense, multi, sparse))
    } else if want_multi {
        let embedder = runtime.multi_vector_embedder(alias).await?;
        Ok((None, Some(embedder.embed(texts).await?.vectors), None))
    } else if want_sparse {
        let embedder = runtime.sparse_embedder(alias).await?;
        Ok((None, None, Some(embedder.embed(texts).await?.vectors)))
    } else {
        let embedder = runtime.embedding(alias).await?;
        Ok((Some(embedder.embed(texts).await?.vectors), None, None))
    }
}

/// Group a label's auto-embed configs by `(alias, source_properties)`, classifying each target
/// column as dense vs multi-vector. A group with both kinds is a single-pass hybrid source.
fn collect_embed_groups(
    schema: &uni_common::core::schema::Schema,
    label: &str,
) -> std::collections::BTreeMap<(String, Vec<String>), EmbedGroupSpec> {
    use std::collections::BTreeMap;
    let mut groups: BTreeMap<(String, Vec<String>), EmbedGroupSpec> = BTreeMap::new();
    fn spec_for<'a>(
        groups: &'a mut std::collections::BTreeMap<(String, Vec<String>), EmbedGroupSpec>,
        emb: &uni_common::core::schema::EmbeddingConfig,
    ) -> &'a mut EmbedGroupSpec {
        groups
            .entry((emb.alias.clone(), emb.source_properties.clone()))
            .or_insert_with(|| EmbedGroupSpec {
                source_properties: emb.source_properties.clone(),
                document_prefix: emb.document_prefix.clone(),
                dense: Vec::new(),
                multi: Vec::new(),
                sparse: Vec::new(),
            })
    }
    for idx in &schema.indexes {
        if let IndexDefinition::Vector(v_config) = idx
            && v_config.label == label
            && let Some(emb) = &v_config.embedding_config
        {
            let g = spec_for(&mut groups, emb);
            if is_multivector_property(schema, label, &v_config.property) {
                g.multi.push(v_config.property.clone());
            } else {
                g.dense.push(v_config.property.clone());
            }
        } else if let IndexDefinition::Sparse(s_config) = idx
            && s_config.label == label
            && let Some(emb) = &s_config.embedding_config
        {
            spec_for(&mut groups, emb)
                .sparse
                .push(s_config.property.clone());
        }
    }
    groups
}

/// On a partial / `SET` write, when the write touches a *source* property of an
/// auto-embed group, drop that group's target columns from `props` so the
/// `!contains_key` guard in `process_embeddings_*` re-embeds them (otherwise the
/// re-read old embedding is preserved → stale), and add the targets to
/// `touched_keys` so the partial Lance write persists the refresh. Mirrors the
/// MUVERA derived-column touched-keys handling. A target the caller set
/// explicitly in the SAME write (already in `touched_keys`) is left intact.
fn refresh_touched_embed_targets(
    schema: &uni_common::core::schema::Schema,
    props: &mut Properties,
    touched_keys: &mut HashSet<String>,
    labels: &[String],
) {
    let Some(label) = labels.first() else {
        return;
    };
    let user_touched = touched_keys.clone();
    for (_key, group) in collect_embed_groups(schema, label) {
        if !group
            .source_properties
            .iter()
            .any(|s| touched_keys.contains(s))
        {
            continue;
        }
        for target in group
            .dense
            .iter()
            .chain(group.multi.iter())
            .chain(group.sparse.iter())
        {
            if user_touched.contains(target) {
                continue;
            }
            props.remove(target);
            touched_keys.insert(target.clone());
        }
    }
}

#[derive(Clone, Debug)]
pub struct WriterConfig {
    pub max_mutations: usize,
    /// Enable the partial-column MergeInsert path for SET-only flushes.
    ///
    /// When `true`, `Writer::insert_vertex_partial` records the touched
    /// property keys into `L0Buffer::vertex_partial_keys` and the flush
    /// routes those VIDs through Lance `MergeInsertBuilder` with a
    /// subset-of-schema source, skipping the read of (and write of)
    /// the unchanged columns — including wide ones like embeddings.
    ///
    /// When `false`, `insert_vertex_partial` falls back to the
    /// read-modify-write `insert_vertex_with_labels` path (preserving
    /// bit-for-bit equivalence with prior releases). Default `false`
    /// for the first release; flip to `true` after telemetry on the
    /// issue #72 ingest workload confirms the win.
    ///
    /// See the soundness probe at
    /// `crates/uni-store/tests/common/storage/lance_merge_insert_probe.rs`.
    pub partial_lance_writes: bool,
}

impl Default for WriterConfig {
    fn default() -> Self {
        Self {
            max_mutations: 10_000,
            partial_lance_writes: false,
        }
    }
}

/// Parent state captured atomically at a fork point under `flush_lock`.
///
/// Holds the allocator high-water marks and every existing dataset's
/// Lance main-branch version at the instant the parent's L0 was
/// flushed. Because [`Writer::flush_and_capture_fork_point`] reads these
/// while still holding `flush_lock`, no concurrent commit or flush can
/// advance the allocator or any dataset tip between the flush and the
/// reads. A fork built from these values therefore cannot collide VIDs
/// with the parent nor inherit rows committed after the fork point.
#[derive(Clone, Debug, Default)]
pub struct ForkPoint {
    /// Next vertex id the parent would allocate at the fork point.
    pub vid_hwm: u64,
    /// Next edge id the parent would allocate at the fork point.
    pub eid_hwm: u64,
    /// `dataset_name` → Lance main-branch version at the fork point.
    ///
    /// Keys use the same dataset naming as the fork branch loop
    /// (`vertices`, `edges`, `vertices_{label}`, `deltas_{type}_{dir}`,
    /// `adjacency_{type}_{dir}`). A dataset with no `.lance` directory
    /// on disk at the fork point has no entry.
    pub dataset_versions: BTreeMap<String, u64>,
    /// Parent's MVCC version high-water-mark at the fork point: the
    /// largest `_version` any inherited row can carry. A fork bootstraps
    /// its own version counter to this floor so a fork transaction's
    /// `_version <= pin` read still sees inherited (base_paths) rows,
    /// while the fork's own writes get versions above it.
    pub version_hwm: u64,
}

/// RAII latch on [`StorageManager::flush_in_progress`].
///
/// Sets the flag to `true` on construction (via CAS) and back to `false` on
/// drop, so any `?` early-exit inside `flush_to_l1` cannot leave the flag
/// stuck. Returns `None` if a flush is already in progress, providing
/// forward-compatible exclusion once the outer writer-RwLock is removed in
/// Phase 4 of the concurrent-writer refactor.
// FlushInProgressGuard moved to storage/manager.rs so flush_coordinator.rs
// can hold it on RotatedFlush without a writer.rs back-import cycle.
pub use crate::storage::manager::FlushInProgressGuard;

/// Output of [`Writer::flush_l0_rotate`]: the to-be-flushed L0 buffer,
/// captured WAL LSN, current_version, and the in-progress guard whose
/// lifetime spans the full flush (including the future async stream
/// phase that runs on a spawned task).
struct RotateOutput {
    old_l0_arc: Arc<RwLock<L0Buffer>>,
    wal_lsn: u64,
    current_version: u64,
    flush_in_progress_guard: FlushInProgressGuard,
}

/// Project a property map to a subset selected by `keys`. Used to
/// run `touched_needs_full_read` against just the SET-touched keys
/// when the caller passes a fully-merged `props` map.
fn props_subset(props: &Properties, keys: &HashSet<String>) -> Properties {
    let mut out = Properties::new();
    for k in keys {
        if let Some(v) = props.get(k) {
            out.insert(k.clone(), v.clone());
        }
    }
    out
}

/// Join a storage base URI and a dataset name into a `.lance` URI.
///
/// Mirrors the fork branch loop's `join_uri` so the versions captured
/// by [`Writer::flush_and_capture_fork_point`] key the exact same
/// datasets the fork later branches.
fn join_lance_uri(base: &str, dataset: &str) -> String {
    if base.ends_with('/') {
        format!("{base}{dataset}.lance")
    } else {
        format!("{base}/{dataset}.lance")
    }
}

/// Cheap on-disk existence check for a dataset `.lance` directory.
///
/// Local-fs heuristic: a URI with a `://` scheme is assumed remote and
/// reported present, deferring the real check to `current_version`.
/// Mirrors the fork branch loop's `path_exists`.
fn lance_path_exists(uri: &str) -> bool {
    if uri.contains("://") {
        return true;
    }
    std::path::Path::new(uri).exists()
}

/// Output of [`Writer::flush_stream_l1`]: the built (but not yet
/// published) snapshot manifest and its id. Finalize is responsible
/// for `save_snapshot` + `set_latest_snapshot` + `cached_manifest`
/// update.
struct FlushOutcome {
    manifest: SnapshotManifest,
    snapshot_id: String,
}

pub struct Writer {
    pub l0_manager: Arc<L0Manager>,
    pub storage: Arc<StorageManager>,
    pub schema_manager: Arc<uni_common::core::schema::SchemaManager>,
    pub allocator: Arc<IdAllocator>,
    pub config: UniConfig,
    /// Optional embedding runtime. `OnceLock` so the initializer can run
    /// on `&self` after the `Writer` has been wrapped in `Arc<Writer>`
    /// (Phase 4 of concurrent_writer.md). Read through
    /// [`Writer::xervo_runtime`] — the field itself is private to keep
    /// callers oblivious to the OnceLock representation.
    xervo_runtime: OnceLock<Arc<ModelRuntime>>,
    /// Property manager for cache invalidation after flush
    pub property_manager: Option<Arc<PropertyManager>>,
    /// Adjacency manager for dual-write (edges survive flush).
    adjacency_manager: Arc<AdjacencyManager>,
    /// Timestamp of last flush or creation. Interior-mutable so that
    /// `&self` callers can update it; uncontended in practice because all
    /// writes happen inside the single-flusher critical section.
    /// Arc-wrapped so it can travel into the SharedFlushCtx that the
    /// async-flush coordinator passes to spawned stream/finalize tasks.
    last_flush_time: Arc<PlMutex<std::time::Instant>>,
    /// Background compaction task handle (prevents concurrent compaction races)
    compaction_handle: Arc<RwLock<Option<tokio::task::JoinHandle<()>>>>,
    /// Optional index rebuild manager for post-flush automatic rebuild scheduling.
    /// `OnceLock` for the same reason as `xervo_runtime`.
    /// Wrapped in `Arc` so the async-flush finalize path can read it
    /// from a spawned task via `SharedFlushCtx`.
    index_rebuild_manager: Arc<OnceLock<Arc<crate::storage::index_rebuild::IndexRebuildManager>>>,
    /// Cached snapshot manifest from the last flush. Avoids re-reading from
    /// object store on every flush_to_l1 call. Wrapped in a `Mutex` for
    /// `&self` access; uncontended because all access is inside the
    /// single-flusher critical section.
    cached_manifest: Arc<PlMutex<Option<SnapshotManifest>>>,
    /// Identifier of the fork this writer serves, if any. `None` for
    /// primary's writer. Set by [`crate::fork::writer_factory::new_for_fork`]
    /// and read in `flush_to_l1` to emit fork-tagged metrics and to fire
    /// the fragment-count guard rail (Phase 2 Day 12).
    pub fork_id: Option<ForkId>,
    /// Number of `flush_to_l1` calls since this writer was constructed.
    /// Used as a proxy for L1 fragment growth on the fork's branches:
    /// each flush typically appends ~1 fragment per touched dataset, so
    /// the count tracks the order of magnitude of fragment accumulation.
    /// Reading the actual `Dataset::manifest().fragments.len()` per
    /// flush would add a per-dataset object-store roundtrip on the hot
    /// commit path; the proxy keeps the guard rail purely observational
    /// (Phase 5 introduces fork compaction proper). Only meaningful when
    /// `fork_id.is_some()`. `Relaxed` is sufficient — observational only.
    fork_flush_count: Arc<AtomicU64>,
    /// Whether the fork-fragment warning has already fired at the
    /// configured threshold. One-shot per writer lifetime. `Relaxed` is
    /// sufficient — observational only.
    fork_fragment_warn_fired: Arc<AtomicBool>,
    /// Dedicated lock for the genuinely-exclusive flush path. Acquired by
    /// the [`Writer::flush_to_l1`] entry and by `commit_transaction_l0`
    /// across its WAL-append + L0-merge window. Replaces the outer
    /// `Arc<RwLock<Writer>>` for flush exclusion once Phase 4 drops it.
    /// Arc-wrapped so async-flush coordinator's finalize path can
    /// re-acquire it from a spawned task via SharedFlushCtx.
    flush_lock: Arc<tokio::sync::Mutex<()>>,
    /// Coordinator for async-flush pipeline. Owns the back-pressure
    /// semaphore, rotate-order sequence, single-finalizer task, and
    /// pending-flush counter. Always present even when async flush is
    /// disabled — the sync `flush_to_l1` path uses it for the future
    /// `FlushInProgressGuard`/permit ownership model.
    /// Coordinator is `None` when `async_flush_enabled = false`. The
    /// coordinator's finalizer task captures `SharedFlushCtx` which
    /// includes `Arc<StorageManager>`; on a fork-scoped Writer that
    /// also pins the fork's `ForkScope` via `storage.fork_scope`, so
    /// the holder count never drops. Constructing it only when the
    /// feature is actually on avoids that side-effect for all
    /// existing sync-flush paths. When async-flush graduates from
    /// opt-in to default (Commit 12), `drop_fork` (Commit 8) handles
    /// the drain explicitly.
    #[allow(dead_code)] // first production use lands in Commit 6/7
    pub(crate) flush_coordinator: Option<Arc<crate::runtime::flush_coordinator::FlushCoordinator>>,
    /// Optimistic-concurrency commit-sequence counter (SSI). Incremented once
    /// per successful commit under `flush_lock`; a transaction captures the
    /// current value at begin as its read sequence (`L0Buffer::occ_read_seq`).
    ///
    /// Always allocated; consulted only when `config.ssi_enabled` is `true`.
    ///
    /// Typed through the [`crate::runtime::sync`] shim so the OCC commit core can
    /// be model-checked under loom/shuttle; aliases to `std::AtomicU64` normally.
    commit_sequence: Arc<crate::runtime::sync::AtomicU64>,
    /// Bounded log of recently-committed write-sets for OCC conflict detection.
    /// Read and updated only under `flush_lock`.
    ///
    /// Always allocated; consulted only when `config.ssi_enabled` is `true`.
    committed_writes: Arc<PlMutex<crate::runtime::occ::CommitRegistry>>,
    /// Per-row pessimistic locks for `FOR UPDATE` (SSI escape hatch), keyed by
    /// canonical (label, key-props) bytes. A transaction holds the lock from
    /// MATCH until commit/rollback, serializing concurrent `FOR UPDATE` writers
    /// on the same key (avoiding optimistic abort-retry on hot keys).
    ///
    /// Always allocated; populated only when `config.ssi_enabled` is `true`.
    for_update_locks: Arc<dashmap::DashMap<Vec<u8>, Arc<tokio::sync::Mutex<()>>>>,
}

/// Number of recent commits retained for OCC conflict detection. Large enough
/// that under-run — and the resulting conservative abort — is rare in practice;
/// each entry is a small set of touched ids.
const OCC_REGISTRY_CAPACITY: usize = 4096;

impl Writer {
    pub async fn new(
        storage: Arc<StorageManager>,
        schema_manager: Arc<uni_common::core::schema::SchemaManager>,
        start_version: u64,
    ) -> Result<Self> {
        Self::new_with_config(
            storage,
            schema_manager,
            start_version,
            UniConfig::default(),
            None,
            None,
        )
        .await
    }

    pub async fn new_with_config(
        storage: Arc<StorageManager>,
        schema_manager: Arc<uni_common::core::schema::SchemaManager>,
        start_version: u64,
        config: UniConfig,
        wal: Option<Arc<WriteAheadLog>>,
        allocator: Option<Arc<IdAllocator>>,
    ) -> Result<Self> {
        let allocator = if let Some(a) = allocator {
            a
        } else {
            let store = storage.store();
            let path = object_store::path::Path::from("id_allocator.json");
            Arc::new(IdAllocator::new(store, path, 1000).await?)
        };

        let l0_manager = Arc::new(L0Manager::new(start_version, wal));

        let property_manager = Some(Arc::new(PropertyManager::new(
            storage.clone(),
            schema_manager.clone(),
            1000,
        )));

        let adjacency_manager = storage.adjacency_manager();

        // Hoist the Arc'd fields so we can both stash them on Writer and
        // hand the same Arcs to the SharedFlushCtx that FlushCoordinator
        // captures. Single-source-of-truth for each piece of mutable
        // shared state.
        let last_flush_time = Arc::new(PlMutex::new(std::time::Instant::now()));
        let cached_manifest = Arc::new(PlMutex::new(None));
        let fork_flush_count = Arc::new(AtomicU64::new(0));
        let fork_fragment_warn_fired = Arc::new(AtomicBool::new(false));
        let flush_lock = Arc::new(tokio::sync::Mutex::new(()));
        let compaction_handle = Arc::new(RwLock::new(None));
        let index_rebuild_manager: Arc<
            OnceLock<Arc<crate::storage::index_rebuild::IndexRebuildManager>>,
        > = Arc::new(OnceLock::new());

        let flush_coordinator = if config.async_flush_enabled {
            let shared = SharedFlushCtx {
                storage: storage.clone(),
                l0_manager: l0_manager.clone(),
                adjacency_manager: adjacency_manager.clone(),
                property_manager: property_manager.clone(),
                schema_manager: schema_manager.clone(),
                cached_manifest: cached_manifest.clone(),
                last_flush_time: last_flush_time.clone(),
                fork_id: None,
                fork_flush_count: fork_flush_count.clone(),
                fork_fragment_warn_fired: fork_fragment_warn_fired.clone(),
                fork_fragment_warn_threshold: config.fork_fragment_warn_threshold,
                flush_lock: flush_lock.clone(),
                index_rebuild_manager: index_rebuild_manager.clone(),
                compaction_handle: compaction_handle.clone(),
                compaction_config: config.compaction.clone(),
                index_rebuild_config: config.index_rebuild.clone(),
                auto_rebuild_enabled: config.index_rebuild.auto_rebuild_enabled,
            };
            let finalize_fn: Arc<dyn FinalizeFn> = Arc::new(WriterFinalizer);
            Some(Arc::new(FlushCoordinator::new(
                config.max_pending_flushes,
                shared,
                finalize_fn,
            )))
        } else {
            None
        };

        let commit_sequence = Arc::new(crate::runtime::sync::AtomicU64::new(0));
        let committed_writes = Arc::new(PlMutex::new(crate::runtime::occ::CommitRegistry::new(
            OCC_REGISTRY_CAPACITY,
        )));
        let for_update_locks = Arc::new(dashmap::DashMap::new());

        Ok(Self {
            l0_manager,
            storage,
            schema_manager,
            allocator,
            config,
            xervo_runtime: OnceLock::new(),
            property_manager,
            adjacency_manager,
            last_flush_time,
            compaction_handle,
            index_rebuild_manager,
            cached_manifest,
            fork_id: None,
            fork_flush_count,
            fork_fragment_warn_fired,
            flush_lock,
            flush_coordinator,
            commit_sequence,
            committed_writes,
            for_update_locks,
        })
    }

    /// Returns the shared pessimistic lock handle for a `FOR UPDATE` row key,
    /// creating it on first use. The caller `.lock_owned().await`s the returned
    /// mutex and holds the guard for the transaction's lifetime.
    pub fn row_lock_handle(&self, key: &[u8]) -> Arc<tokio::sync::Mutex<()>> {
        self.for_update_locks
            .entry(key.to_vec())
            .or_insert_with(|| Arc::new(tokio::sync::Mutex::new(())))
            .clone()
    }

    /// Prunes `FOR UPDATE` lock-map entries for `keys` that no live transaction
    /// holds anymore, so the map does not grow without bound across the keyspace.
    ///
    /// Called when a transaction ends, **after** its guards have been dropped.
    /// `remove_if` evaluates its predicate under the DashMap shard lock, which is
    /// the same lock `row_lock_handle` takes to clone an entry — so the check
    /// `strong_count == 1` (only the map holds the `Arc`) is race-free: a
    /// concurrent acquirer either already cloned the `Arc` (count ≥ 2 → we skip
    /// removal) or has not yet taken the shard lock (it will mint a fresh entry
    /// after we remove). Either way no two transactions ever lock different
    /// `Mutex` instances for the same key.
    pub fn release_for_update_locks(&self, keys: &[Vec<u8>]) {
        for key in keys {
            self.for_update_locks
                .remove_if(key, |_, handle| Arc::strong_count(handle) == 1);
        }
    }

    /// Number of live entries in the `FOR UPDATE` lock map. Introspection for
    /// tests that the map does not leak entries across transactions (G5).
    pub fn for_update_lock_count(&self) -> usize {
        self.for_update_locks.len()
    }

    /// The current OCC commit sequence. A `FOR UPDATE` acquisition re-stamps a
    /// fresh transaction's `occ_read_seq` to this so its conflict-detection
    /// baseline advances to lock-acquisition time (read-latest under the lock).
    pub fn current_commit_sequence(&self) -> u64 {
        self.commit_sequence
            .load(crate::runtime::sync::Ordering::Relaxed)
    }

    /// Build a fresh `SharedFlushCtx` from this Writer's current state.
    /// Used by the async-flush stream/finalize paths to pass into spawned
    /// tasks without smuggling `Arc<Writer>` (which would create a cycle
    /// with `flush_coordinator -> FinalizeFn -> Writer`).
    pub(crate) fn shared_ctx(&self) -> SharedFlushCtx {
        SharedFlushCtx {
            storage: self.storage.clone(),
            l0_manager: self.l0_manager.clone(),
            adjacency_manager: self.adjacency_manager.clone(),
            property_manager: self.property_manager.clone(),
            schema_manager: self.schema_manager.clone(),
            cached_manifest: self.cached_manifest.clone(),
            last_flush_time: self.last_flush_time.clone(),
            fork_id: self.fork_id,
            fork_flush_count: self.fork_flush_count.clone(),
            fork_fragment_warn_fired: self.fork_fragment_warn_fired.clone(),
            fork_fragment_warn_threshold: self.config.fork_fragment_warn_threshold,
            flush_lock: self.flush_lock.clone(),
            index_rebuild_manager: self.index_rebuild_manager.clone(),
            compaction_handle: self.compaction_handle.clone(),
            compaction_config: self.config.compaction.clone(),
            index_rebuild_config: self.config.index_rebuild.clone(),
            auto_rebuild_enabled: self.config.index_rebuild.auto_rebuild_enabled,
        }
    }

    /// Borrow the flush coordinator if async flush is enabled.
    /// Returns `None` when `config.async_flush_enabled = false`.
    /// External callers (`drop_fork`) use this to drain pending streams.
    pub fn flush_coordinator(
        &self,
    ) -> Option<&Arc<crate::runtime::flush_coordinator::FlushCoordinator>> {
        self.flush_coordinator.as_ref()
    }

    /// Set the index rebuild manager for post-flush automatic rebuild scheduling.
    ///
    /// One-shot: returns `Err` if already set. The receiver is `&self` so this
    /// can be called after the `Writer` has been wrapped in `Arc<Writer>`.
    pub fn set_index_rebuild_manager(
        &self,
        manager: Arc<crate::storage::index_rebuild::IndexRebuildManager>,
    ) -> Result<()> {
        self.index_rebuild_manager
            .set(manager)
            .map_err(|_| anyhow!("index_rebuild_manager already set"))
    }

    /// Replay WAL mutations into the current L0 buffer.
    pub async fn replay_wal(&self, wal_high_water_mark: u64) -> Result<usize> {
        let l0 = self.l0_manager.get_current();
        let wal = l0.read().wal.clone();

        if let Some(wal) = wal {
            wal.initialize().await?;
            let mutations = wal.replay_since(wal_high_water_mark).await?;
            let count = mutations.len();

            if count > 0 {
                log::info!(
                    "Replaying {} mutations from WAL (LSN > {})",
                    count,
                    wal_high_water_mark
                );
                let mut l0_guard = l0.write();
                l0_guard.replay_mutations(mutations)?;
                // Rebuild the UNIQUE constraint index over the recovered rows
                // (Bug #9 Mechanism B). `replay_mutations` restores
                // vertices/properties/labels but never repopulates
                // `constraint_index` (its only other caller is the live insert
                // path). Without this, a unique key that lives only in the WAL
                // (committed but not yet flushed to Lance) is invisible to
                // `check_unique_constraint_multi` after recovery and a
                // duplicate of it could be created.
                self.rebuild_constraint_index(&mut l0_guard);
            }

            Ok(count)
        } else {
            Ok(0)
        }
    }

    /// Rebuild the UNIQUE constraint index on a recovered L0 buffer.
    ///
    /// Scans every recovered vertex's properties and, for each enabled UNIQUE
    /// constraint whose target label the vertex carries and whose member
    /// properties are all present, inserts the same constraint key the live
    /// insert path builds (`serialize_constraint_key`). Tombstoned vertices are
    /// skipped. Called after [`L0Buffer::replay_mutations`] under the buffer's
    /// write lock; the schema is already loaded on the `Writer`.
    fn rebuild_constraint_index(&self, l0_guard: &mut L0Buffer) {
        let schema = self.schema_manager.schema();
        // Collect entries first to avoid borrowing `vertex_properties`
        // immutably while mutating `constraint_index` through the same guard.
        let mut keys: Vec<(Vec<u8>, Vid)> = Vec::new();
        for (&vid, props) in &l0_guard.vertex_properties {
            if l0_guard.vertex_tombstones.contains(&vid) {
                continue;
            }
            let Some(labels) = l0_guard.vertex_labels.get(&vid) else {
                continue;
            };
            for label in labels {
                for constraint in &schema.constraints {
                    if !constraint.enabled {
                        continue;
                    }
                    let ConstraintTarget::Label(l) = &constraint.target else {
                        continue;
                    };
                    if l != label {
                        continue;
                    }
                    let ConstraintType::Unique {
                        properties: unique_props,
                    } = &constraint.constraint_type
                    else {
                        continue;
                    };
                    let mut key_values = Vec::new();
                    let mut all_present = true;
                    for prop in unique_props {
                        if let Some(val) = props.get(prop) {
                            key_values.push((prop.clone(), val.clone()));
                        } else {
                            all_present = false;
                            break;
                        }
                    }
                    if all_present {
                        keys.push((serialize_constraint_key(label, &key_values), vid));
                    }
                }
            }
        }
        for (key, vid) in keys {
            l0_guard.insert_constraint_key(key, vid);
        }
    }

    /// Allocates the next VID (pure auto-increment).
    pub async fn next_vid(&self) -> Result<Vid> {
        self.allocator.allocate_vid().await
    }

    /// Allocates multiple VIDs at once for bulk operations.
    /// This is more efficient than calling next_vid() in a loop.
    pub async fn allocate_vids(&self, count: usize) -> Result<Vec<Vid>> {
        self.allocator.allocate_vids(count).await
    }

    /// Allocates the next EID (pure auto-increment).
    pub async fn next_eid(&self, _type_id: u32) -> Result<Eid> {
        self.allocator.allocate_eid().await
    }

    /// Allocates multiple EIDs at once for bulk operations.
    /// This is more efficient than calling next_eid() in a loop.
    pub async fn allocate_eids(&self, count: usize) -> Result<Vec<Eid>> {
        self.allocator.allocate_eids(count).await
    }

    /// Install the embedding runtime exactly once. Receiver is `&self` so it
    /// can be called after the `Writer` has been wrapped in `Arc<Writer>`.
    pub fn set_xervo_runtime(&self, runtime: Arc<ModelRuntime>) -> Result<()> {
        self.xervo_runtime
            .set(runtime)
            .map_err(|_| anyhow!("xervo_runtime already set"))
    }

    pub fn xervo_runtime(&self) -> Option<Arc<ModelRuntime>> {
        self.xervo_runtime.get().cloned()
    }

    /// Create a new empty L0 buffer for transaction-scoped mutations.
    ///
    /// Only reads the current version — no exclusive lock required on Writer.
    /// The returned buffer has no WAL reference; mutations are logged at
    /// commit time via [`Self::commit_transaction_l0`].
    pub fn create_transaction_l0(&self) -> Arc<RwLock<L0Buffer>> {
        let current_version = self.l0_manager.get_current().read().current_version;
        // Transaction mutations are logged to WAL at COMMIT time, not during the transaction.
        let buf = L0Buffer::new(current_version, None);
        // SSI: stamp the OCC read sequence at begin so commit can detect any
        // transaction that committed since. Gated on the runtime `ssi_enabled`
        // toggle — when off, `occ_read_set` stays `None` and every downstream
        // read-set recording / commit validation self-gates to a no-op.
        let buf = if self.config.ssi_enabled {
            let mut buf = buf;
            buf.occ_read_seq = self
                .commit_sequence
                .load(crate::runtime::sync::Ordering::Relaxed);
            // The read path records observed ids here for SSI antidependency
            // detection; commit consults it.
            buf.occ_read_set = Some(Arc::new(parking_lot::Mutex::new(
                crate::runtime::l0::OccReadSet::default(),
            )));
            buf
        } else {
            buf
        };
        Arc::new(RwLock::new(buf))
    }

    /// Resolve the target L0 buffer for a mutation.
    ///
    /// When `tx_l0` is `Some`, the mutation targets a transaction-private buffer.
    /// When `None`, it targets the global L0 from the manager.
    fn resolve_l0(&self, tx_l0: Option<&Arc<RwLock<L0Buffer>>>) -> Arc<RwLock<L0Buffer>> {
        tx_l0
            .cloned()
            .unwrap_or_else(|| self.l0_manager.get_current())
    }

    fn update_metrics(&self) {
        let l0 = self.l0_manager.get_current();
        let size = l0.read().estimated_size;
        metrics::gauge!("l0_buffer_size_bytes").set(size as f64);
    }

    /// Overlay-aware issue-#77 edge-endpoint validation.
    ///
    /// The current buffer alone does not hold all committed-but-unflushed
    /// tombstones — a flush rotation moves them onto `pending_flush` until
    /// the Lance write completes. A vertex is effectively deleted iff,
    /// walking newest-first (tx → current → pending newest→oldest), the
    /// first buffer that knows the vid says "tombstoned" (an insert clears
    /// the tombstone within a buffer, so props/tombstone are mutually
    /// exclusive per buffer).
    ///
    /// Must run under `flush_lock` so the overlay cannot change before the
    /// merge, and BEFORE the durable WAL flush (see the call site).
    fn validate_edge_endpoints_overlay(&self, tx_l0: &L0Buffer) -> Result<()> {
        let chain = self.l0_manager.get_pending_flush();
        let current = self.l0_manager.get_current();
        let effectively_deleted = |vid: &Vid| -> bool {
            if tx_l0.vertex_properties.contains_key(vid) {
                return false;
            }
            if tx_l0.vertex_tombstones.contains(vid) {
                return true;
            }
            {
                let cur = current.read();
                if cur.vertex_properties.contains_key(vid) {
                    return false;
                }
                if cur.vertex_tombstones.contains(vid) {
                    return true;
                }
            }
            for frozen in chain.iter().rev() {
                let g = frozen.read();
                if g.vertex_properties.contains_key(vid) {
                    return false;
                }
                if g.vertex_tombstones.contains(vid) {
                    return true;
                }
            }
            false
        };
        for (eid, (src_vid, dst_vid, _etype)) in &tx_l0.edge_endpoints {
            if tx_l0.tombstones.contains_key(eid) {
                continue; // a deletion, not an insertion — never resurrects a vertex
            }
            if effectively_deleted(src_vid) {
                anyhow::bail!(
                    "Cannot insert edge {}: source vertex {} has been deleted (issue #77)",
                    eid,
                    src_vid
                );
            }
            if effectively_deleted(dst_vid) {
                anyhow::bail!(
                    "Cannot insert edge {}: destination vertex {} has been deleted (issue #77)",
                    eid,
                    dst_vid
                );
            }
        }
        Ok(())
    }

    /// Seed `main_l0` (the current buffer) with the newest pending-overlay
    /// value for each CRDT property the transaction writes, so the commit
    /// merge MERGES against the committed CRDT state instead of shadowing it
    /// (the carve-out lets concurrent CRDT writers commit on the assumption
    /// that the merge sees the committed value — true only while that value
    /// lives in current, not mid-flush on `pending_flush`). No-op when
    /// nothing is pending or the property already exists in current. Vertex
    /// properties only, mirroring the carve-out itself.
    fn seed_crdt_state_from_chain(&self, tx_l0: &L0Buffer, main_l0: &mut L0Buffer) {
        let chain = self.l0_manager.get_pending_flush();
        if chain.is_empty() {
            return;
        }
        for (vid, props) in &tx_l0.vertex_properties {
            Self::seed_crdt_props(&chain, *vid, props, main_l0);
        }
    }

    /// Per-vertex CRDT seeding (see [`Self::seed_crdt_state_from_chain`]).
    /// Also used by the non-transactional vertex write path, which CRDT-merges
    /// into the current buffer directly and has the same shadowing hazard
    /// during a flush window.
    fn seed_crdt_props(
        chain: &[Arc<RwLock<L0Buffer>>],
        vid: Vid,
        props: &Properties,
        target: &mut L0Buffer,
    ) {
        for (key, value) in props {
            if crate::runtime::l0::try_as_crdt(value).is_none() {
                continue;
            }
            if target
                .vertex_properties
                .get(&vid)
                .is_some_and(|p| p.contains_key(key))
            {
                continue;
            }
            // Newest generation first: the first hit is the live state.
            for frozen in chain.iter().rev() {
                let g = frozen.read();
                if let Some(v) = g.vertex_properties.get(&vid).and_then(|p| p.get(key)) {
                    if crate::runtime::l0::try_as_crdt(v).is_some() {
                        target
                            .vertex_properties
                            .entry(vid)
                            .or_default()
                            .insert(key.clone(), v.clone());
                    }
                    break;
                }
            }
        }
    }

    /// Commit an externally-owned transaction L0 buffer.
    ///
    /// Writes mutations to WAL, flushes, merges into main L0, and replays
    /// edges into the AdjacencyManager. Returns the WAL LSN of the commit
    /// (0 when no WAL is configured).
    /// Commit a transaction's private L0 buffer into main L0.
    ///
    /// Returns `(wal_lsn, flush_pending)`. When `flush_pending == true`, the
    /// post-commit `should_flush()` predicate fired but no flush ran — the
    /// caller is expected to spawn a background `flush_to_l1`. This is the
    /// shape used when `UniConfig::async_flush_enabled` is set, so commits
    /// don't block on L1-streaming I/O.
    pub async fn commit_transaction_l0(
        self: &Arc<Self>,
        tx_l0_arc: Arc<RwLock<L0Buffer>>,
    ) -> Result<(u64, bool)> {
        // Hold `flush_lock` across WAL append + flush + main-L0 merge.
        // Two concurrent commits serialize here; in Phase 3 the outer
        // `Arc<RwLock<Writer>>` already provides this exclusion, so the
        // acquisition is uncontended. Phase 4 drops the outer lock and
        // this becomes the load-bearing serialization point.
        let _flush_lock_guard = self.flush_lock.lock().await;

        // Crash-recovery seam: simulate process death immediately after winning
        // the commit serialization point but before any durable work. No-op
        // unless built with `--features failpoints`. (See ssi_resilience tests.)
        fail::fail_point!("commit::after-flush-lock");

        // SSI: optimistic conflict detection. This MUST run before any WAL
        // write — `flush_wal()` below is the durable commit point and the WAL
        // has no abort marker, so aborting after it would resurrect this
        // transaction on crash recovery. The write-set is reused for
        // registration after a successful merge.
        // Runtime-gated on `config.ssi_enabled`. When off, no validation runs
        // and `occ_write_set` is `None`, so the post-merge registration below
        // is skipped — reproducing last-writer-wins exactly.
        let occ_write_set: Option<crate::runtime::occ::WriteSet> = if self.config.ssi_enabled {
            let tx_l0 = tx_l0_arc.read();
            let read_seq = tx_l0.occ_read_seq;
            let write_set = crate::runtime::occ::WriteSet::from_l0(&tx_l0);
            if !write_set.is_empty() {
                // Telemetry: one validation per non-empty (writing) commit. The
                // ratio of conflicts to validations is the headline abort rate.
                metrics::counter!("uni_ssi_commit_validations_total").increment(1);
                // Read-set is consulted only for writing transactions, so a
                // read-only commit (empty write-set) runs at snapshot isolation.
                let read_guard = tx_l0.occ_read_set.as_ref().map(|rs| rs.lock());
                if let Some(conflict) =
                    self.committed_writes
                        .lock()
                        .check(read_seq, &write_set, read_guard.as_deref())
                {
                    use crate::runtime::occ::Conflict;
                    match &conflict {
                        Conflict::WriteWrite { .. } => metrics::counter!(
                            "uni_ssi_serialization_conflicts_total",
                            "kind" => "write_write",
                        )
                        .increment(1),
                        Conflict::ReadWrite { .. } => metrics::counter!(
                            "uni_ssi_serialization_conflicts_total",
                            "kind" => "read_write",
                        )
                        .increment(1),
                        Conflict::HistoryTruncated { .. } => {
                            metrics::counter!("uni_ssi_history_truncated_total").increment(1)
                        }
                    }
                    return Err(anyhow::Error::new(
                        uni_common::UniError::SerializationConflict {
                            message: conflict.to_string(),
                        },
                    ));
                }
            }

            // Validate against the committed-but-unflushed overlay under
            // `flush_lock`: serializable MERGE uniqueness + CRDT carve-out
            // soundness. The current buffer alone does not hold all committed
            // state — a flush rotation moves it onto `pending_flush` until the
            // Lance write completes (the Bug #9A window, here at the
            // commit-time layer) — so every check walks [current, pending…].
            {
                let pending = self.l0_manager.get_pending_flush();
                let main_l0 = self.l0_manager.get_current();
                let overlay: Vec<Arc<RwLock<L0Buffer>>> =
                    std::iter::once(main_l0).chain(pending).collect();

                // SSI / serializable MERGE: abort if a concurrent transaction has
                // already committed a row with one of this transaction's unique
                // keys. Commits serialize here, so this closes the race window
                // left by the per-insert check. (Empty index → no iterations.)
                for (key, vid) in &tx_l0.constraint_index {
                    if overlay
                        .iter()
                        .any(|b| b.read().has_constraint_key(key, *vid))
                    {
                        metrics::counter!("uni_ssi_constraint_conflicts_total").increment(1);
                        return Err(anyhow::Error::new(
                            uni_common::UniError::ConstraintConflict {
                                message: "unique key already committed by a concurrent \
                                          transaction"
                                    .to_string(),
                            },
                        ));
                    }
                }

                // Implicit MERGE phantom guard: a `MERGE` that *created* a node
                // registered its (label, key-props) here even with no declared
                // UNIQUE constraint. If a concurrent transaction already committed
                // the same MERGE key, abort retriably so the two converge to one
                // node on retry (the loser's MATCH then finds the committed row).
                // Only MERGE-creates register keys, so a plain CREATE of the same
                // properties never lands here. (Empty index → no iterations.)
                for (key, vid) in &tx_l0.merge_guard_index {
                    if overlay
                        .iter()
                        .any(|b| b.read().has_merge_guard_key(key, *vid))
                    {
                        metrics::counter!("uni_ssi_constraint_conflicts_total").increment(1);
                        return Err(anyhow::Error::new(
                            uni_common::UniError::ConstraintConflict {
                                message: "MERGE key already committed by a concurrent \
                                          transaction"
                                    .to_string(),
                            },
                        ));
                    }
                }

                // Same race window for global ext_id uniqueness: the per-insert
                // check ran against an older main L0; re-probe the committed
                // index here, where commits serialize.
                for (ext_id, vid) in &tx_l0.extid_index {
                    let taken = overlay.iter().any(|b| {
                        matches!(b.read().extid_index.get(ext_id), Some(&owner) if owner != *vid)
                    });
                    if taken {
                        metrics::counter!("uni_ssi_constraint_conflicts_total").increment(1);
                        return Err(anyhow::Error::new(
                            uni_common::UniError::ConstraintConflict {
                                message: format!(
                                    "ext_id '{ext_id}' already committed by a concurrent \
                                     transaction"
                                ),
                            },
                        ));
                    }
                }

                // CRDT carve-out soundness: a pure-CRDT write was dropped from the
                // write-set assuming its merge commutes. If the overlay holds a
                // *different* CRDT variant for the same property, the merge would
                // silently overwrite it — abort instead of losing the update.
                // (Checked against every overlay buffer: conservative if an old
                // generation held a different variant that a newer commit already
                // replaced, but an abort+retry is always sound.)
                for buf in &overlay {
                    if let Some(conflict) =
                        crate::runtime::occ::crdt_carveout_overwrite(&tx_l0, &buf.read())
                    {
                        metrics::counter!("uni_ssi_crdt_aborts_total").increment(1);
                        return Err(anyhow::Error::new(
                            uni_common::UniError::SerializationConflict {
                                message: conflict.to_string(),
                            },
                        ));
                    }
                }
            }
            Some(write_set)
        } else {
            None
        };

        // Issue #77: an edge whose endpoint is effectively deleted makes the
        // merge below bail. That bail MUST happen before the durable WAL flush —
        // after it the transaction is committed-but-unmerged (a ghost commit),
        // and WAL replay re-hits the same bail, making the database unopenable.
        // SSI validation was deliberately placed before the flush for exactly
        // this reason; the endpoint check belongs here too. Runs unconditionally
        // (issue #77 is not SSI-gated) under `flush_lock`, so the overlay
        // tombstone state cannot change between here and the merge. A
        // tombstone may live in a flush-rotated pending buffer rather than
        // the current buffer, so the check walks the overlay newest-first.
        {
            let tx_l0 = tx_l0_arc.read();
            self.validate_edge_endpoints_overlay(&tx_l0)?;
        }

        // Crash-recovery seam: SSI validation has passed; the transaction is
        // about to become durable. A crash here must leave NO trace (validation
        // happens before the WAL is touched). No-op unless `failpoints`.
        fail::fail_point!("commit::after-validate");

        // 1. Write transaction mutations to WAL BEFORE merging into main L0
        // This ensures durability before visibility.
        {
            let tx_l0 = tx_l0_arc.read();
            let main_l0_arc = self.l0_manager.get_current();
            let main_l0 = main_l0_arc.read();

            // If WAL exists, write mutations to it for durability
            if let Some(wal) = main_l0.wal.as_ref() {
                // Order: vertices first, then edges (to ensure src/dst exist on replay)

                // Vertex insertions
                for (vid, properties) in &tx_l0.vertex_properties {
                    if !tx_l0.vertex_tombstones.contains(vid) {
                        let labels = tx_l0.vertex_labels.get(vid).cloned().unwrap_or_default();
                        wal.append(crate::runtime::wal::Mutation::InsertVertex {
                            vid: *vid,
                            properties: properties.clone(),
                            labels,
                        })?;
                    }
                }

                // Vertex deletions
                for vid in &tx_l0.vertex_tombstones {
                    let labels = tx_l0.vertex_labels.get(vid).cloned().unwrap_or_default();
                    wal.append(crate::runtime::wal::Mutation::DeleteVertex { vid: *vid, labels })?;
                }

                // Label-only mutations (SET n:Label / REMOVE n:Label). After
                // vertex inserts (so the vertex exists on replay), before edges,
                // and skipping vertices deleted in this same commit.
                for vid in &tx_l0.vertex_label_overwrites {
                    if tx_l0.vertex_tombstones.contains(vid) {
                        continue;
                    }
                    let labels = tx_l0.vertex_labels.get(vid).cloned().unwrap_or_default();
                    wal.append(crate::runtime::wal::Mutation::SetVertexLabels {
                        vid: *vid,
                        labels,
                    })?;
                }

                // Crash-recovery seam: vertices appended, edges not yet. Tests
                // assert that a crash here (before `flush_wal`) recovers NOTHING
                // — the durable commit point is the flush below, not append.
                fail::fail_point!("commit::mid-wal");

                // Edge insertions and deletions from edge_endpoints
                for (eid, (src_vid, dst_vid, edge_type)) in &tx_l0.edge_endpoints {
                    if tx_l0.tombstones.contains_key(eid) {
                        let version = tx_l0.edge_versions.get(eid).copied().unwrap_or(0);
                        wal.append(crate::runtime::wal::Mutation::DeleteEdge {
                            eid: *eid,
                            src_vid: *src_vid,
                            dst_vid: *dst_vid,
                            edge_type: *edge_type,
                            version,
                        })?;
                    } else {
                        let properties =
                            tx_l0.edge_properties.get(eid).cloned().unwrap_or_default();
                        let version = tx_l0.edge_versions.get(eid).copied().unwrap_or(0);
                        let edge_type_name = tx_l0.edge_types.get(eid).cloned();
                        wal.append(crate::runtime::wal::Mutation::InsertEdge {
                            src_vid: *src_vid,
                            dst_vid: *dst_vid,
                            edge_type: *edge_type,
                            eid: *eid,
                            version,
                            properties,
                            edge_type_name,
                        })?;
                    }
                }

                // Tombstones for edges that only exist in the global L0 (not in
                // this transaction's edge_endpoints).  Without this, deletes of
                // pre-existing edges would be silently lost.
                for (eid, tombstone) in &tx_l0.tombstones {
                    if !tx_l0.edge_endpoints.contains_key(eid) {
                        let version = tx_l0.edge_versions.get(eid).copied().unwrap_or(0);
                        wal.append(crate::runtime::wal::Mutation::DeleteEdge {
                            eid: *eid,
                            src_vid: tombstone.src_vid,
                            dst_vid: tombstone.dst_vid,
                            edge_type: tombstone.edge_type,
                            version,
                        })?;
                    }
                }
            }
        }

        // 2. Flush WAL to durable storage - THIS IS THE COMMIT POINT
        let wal_lsn = self.flush_wal().await?;

        // Crash-recovery seam: the WAL is durable but main L0 has NOT merged.
        // A crash here must RECOVER the transaction on replay (it is committed),
        // even though it was never made visible in-process. No-op unless `failpoints`.
        fail::fail_point!("commit::after-wal-flush");

        // Component C1: if an outstanding snapshot pins the current generation,
        // clone it aside (lazy copy-on-write) before merging, so the pinning
        // transaction's reads stay isolated from this commit. No-op — and zero
        // cost — when nothing is pinned (the common case). We hold `flush_lock`,
        // so this cannot race a flush rotate or another commit's merge; the merge
        // below re-fetches `get_current()`, landing in the fresh post-freeze buffer.
        // Self-gates on the runtime SSI toggle: a snapshot is only ever pinned by
        // a transaction begun under `ssi_enabled`, so `is_current_pinned()` is
        // always false when SSI is off and this is a zero-cost no-op.
        if self.l0_manager.is_current_pinned() {
            self.l0_manager.freeze_current_for_snapshot();
            metrics::counter!("uni_l0_snapshot_freezes_total").increment(1);
        }

        // 3. Merge into main L0 and make visible
        {
            // Write-lock the tx buffer: `merge_take` moves its property maps
            // into main L0 instead of cloning them. The commit consumes the
            // transaction, so the drained maps are never observed afterwards;
            // everything read below (endpoints, versions, tombstones) is left
            // intact.
            let mut tx_l0 = tx_l0_arc.write();
            let main_l0_arc = self.l0_manager.get_current();
            let mut main_l0 = main_l0_arc.write();
            // A CRDT property's committed state may live only in a
            // flush-rotated pending buffer (the post-rotation current is
            // empty until the Lance write completes — the Bug #9A window).
            // `merge_crdt_properties` merges against the CURRENT buffer's
            // value — without seeding, the tx's CRDT state would SHADOW the
            // pending buffer's at read time (newest buffer wins per property)
            // and concurrent increments would be lost. Seed the newest
            // overlay value for each CRDT property the tx writes that current
            // lacks, so the merge below merges instead of replaces.
            self.seed_crdt_state_from_chain(&tx_l0, &mut main_l0);
            main_l0.merge_take(&mut tx_l0)?;

            // Replay transaction edges into the AdjacencyManager overlay
            for (eid, (src, dst, etype)) in &tx_l0.edge_endpoints {
                let edge_version = tx_l0
                    .edge_versions
                    .get(eid)
                    .copied()
                    .unwrap_or(main_l0.current_version);
                if tx_l0.tombstones.contains_key(eid) {
                    self.adjacency_manager
                        .add_tombstone(*eid, *src, *dst, *etype, edge_version);
                } else {
                    self.adjacency_manager
                        .insert_edge(*src, *dst, *eid, *etype, edge_version);
                }
            }

            // Replay tombstones for edges that only exist in the global L0
            // (not in this transaction's edge_endpoints).
            for (eid, tombstone) in &tx_l0.tombstones {
                if !tx_l0.edge_endpoints.contains_key(eid) {
                    let edge_version = tx_l0
                        .edge_versions
                        .get(eid)
                        .copied()
                        .unwrap_or(main_l0.current_version);
                    self.adjacency_manager.add_tombstone(
                        *eid,
                        tombstone.src_vid,
                        tombstone.dst_vid,
                        tombstone.edge_type,
                        edge_version,
                    );
                }
            }
        }

        // Crash-recovery seam: durable AND merged, but the in-memory commit
        // registry has not recorded this write-set yet. A crash here is
        // indistinguishable from one at `after-wal-flush` on reopen (the
        // registry is in-memory and rebuilt empty); the tx still recovers.
        fail::fail_point!("commit::after-merge");

        // SSI: register this commit's write-set under a fresh commit sequence so
        // later transactions detect conflicts against it. Still under
        // `flush_lock`, before the async-flush branch can drop the guard.
        // `occ_write_set` is `Some` only when `config.ssi_enabled`.
        if let Some(write_set) = occ_write_set
            && !write_set.is_empty()
        {
            // Bump-then-record via the shared OCC seam (see `CommitRegistry::commit`)
            // so production and the loom/shuttle models exercise identical logic.
            self.committed_writes
                .lock()
                .commit(&self.commit_sequence, write_set);
        }

        self.update_metrics();

        // 4. Best-effort post-commit auto-flush.
        //
        // Two paths:
        // - async_flush_enabled = false (default): inline under our
        //   existing flush_lock guard via flush_inline_under_lock.
        // - async_flush_enabled = true: rotate inline, drop flush_lock,
        //   then submit the stream phase to the coordinator. Gated on
        //   `pending_flush_count() < max_pending_flushes` so we don't
        //   stack up rotations beyond the configured pipeline depth.
        //   `try_acquire_permit` is non-blocking: if we lose the race
        //   for the last permit, we just skip this trigger (the next
        //   commit retries).
        let mut flush_pending = false;
        if self.should_flush() {
            if self.config.async_flush_enabled
                && let Some(coord) = self.flush_coordinator.as_ref()
                && coord.pending_flush_count() < self.config.max_pending_flushes
            {
                match coord.try_acquire_permit() {
                    Some(permit) => {
                        match self.flush_l0_rotate().await {
                            Ok(rotate_out) => {
                                // Allocate the rotate seq and bump pending ONLY
                                // after the rotate succeeds (Bug #3). A failed
                                // rotate must consume neither: the finalizer
                                // advances strictly in consecutive seq order and
                                // only decrements pending on finalize, so a
                                // leaked seq/pending from a failed rotate would
                                // wedge the finalizer forever and climb pending
                                // toward `max_pending_flushes`. The seq is still
                                // allocated under `flush_lock` (immediately after
                                // the rotate, before the guard drops below), so
                                // concurrent rotates keep seq order == rotation
                                // order, and the seq is not used until submit.
                                let seq = coord.next_rotate_seq();
                                coord.note_pending();
                                // Release flush_lock BEFORE the spawn so concurrent
                                // commits can proceed while the stream runs.
                                drop(_flush_lock_guard);
                                let parent_manifest = self.cached_manifest.lock().clone();
                                let rotated = crate::runtime::flush_coordinator::RotatedFlush {
                                    seq,
                                    old_l0_arc: rotate_out.old_l0_arc.clone(),
                                    wal_lsn: rotate_out.wal_lsn,
                                    current_version: rotate_out.current_version,
                                    name: None,
                                    parent_manifest,
                                    permit,
                                    flush_in_progress_guard: rotate_out.flush_in_progress_guard,
                                };
                                let writer = self.clone();
                                let _ticket = coord.submit_for_stream(
                                    rotated,
                                    move |old_l0, wal, ver, n| async move {
                                        let outcome =
                                            writer.flush_stream_l1(old_l0, wal, ver, n).await?;
                                        Ok(crate::runtime::flush_coordinator::FlushOutcome {
                                            new_manifest: outcome.manifest,
                                            snapshot_id: outcome.snapshot_id,
                                        })
                                    },
                                );
                                flush_pending = true;
                                // Early return — flush_lock already dropped.
                                return Ok((wal_lsn, flush_pending));
                            }
                            Err(e) => {
                                tracing::warn!("Async rotate failed (non-critical): {}", e);
                                // No seq was allocated and pending was not
                                // bumped (both moved into the Ok arm for Bug
                                // #3), so the finalizer is not wedged. The
                                // permit drops here, freeing the slot.
                            }
                        }
                    }
                    None => {
                        // Race: someone else grabbed the last permit. Skip;
                        // next commit will retry should_flush().
                        metrics::counter!("uni_flush_trigger_skipped_total").increment(1);
                    }
                }
            } else if let Err(e) = self.flush_inline_under_lock(None).await {
                tracing::warn!("Post-commit flush check failed (non-critical): {}", e);
            }
        }

        Ok((wal_lsn, flush_pending))
    }

    /// Flush the WAL buffer to durable storage.
    ///
    /// Returns the LSN of the flushed segment, or `0` when no WAL is configured.
    pub async fn flush_wal(&self) -> Result<u64> {
        let l0 = self.l0_manager.get_current();
        let wal = l0.read().wal.clone();

        match wal {
            Some(wal) => Ok(wal.flush().await?),
            None => Ok(0),
        }
    }

    /// Record property removals in the active L0 mutation stats.
    ///
    /// Routes to the transaction L0 if provided, otherwise to the main L0.
    pub fn track_properties_removed(&self, count: usize, tx_l0: Option<&Arc<RwLock<L0Buffer>>>) {
        if count == 0 {
            return;
        }
        let l0 = self.resolve_l0(tx_l0);
        l0.write().mutation_stats.properties_removed += count;
    }

    /// Validates vertex constraints for the given properties.
    /// In the new design, label is passed as a parameter since VID no longer embeds label.
    async fn validate_vertex_constraints_for_label(
        &self,
        vid: Vid,
        properties: &Properties,
        label: &str,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        self.validate_vertex_constraints_for_label_impl(vid, properties, label, tx_l0, false)
            .await
    }

    /// Partial-update sibling: validates only constraints touching keys
    /// present in `properties` (the touched set). NOT NULL is checked
    /// only for touched keys; multi-key UNIQUE / CHECK / EXISTS are
    /// skipped when any referenced key is absent (the caller is
    /// expected to have routed to the full-row path in that case via
    /// `touched_needs_full_read`).
    async fn validate_vertex_constraints_for_label_partial(
        &self,
        vid: Vid,
        properties: &Properties,
        label: &str,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        self.validate_vertex_constraints_for_label_impl(vid, properties, label, tx_l0, true)
            .await
    }

    async fn validate_vertex_constraints_for_label_impl(
        &self,
        vid: Vid,
        properties: &Properties,
        label: &str,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
        partial: bool,
    ) -> Result<()> {
        let schema = self.schema_manager.schema();

        {
            // 1. Check NOT NULL constraints (from Property definitions).
            //    Under partial-update mode, skip properties NOT in
            //    `properties` — they retain their previous (already-
            //    validated) value.
            if let Some(props_meta) = schema.properties.get(label) {
                for (prop_name, meta) in props_meta {
                    if !meta.nullable {
                        let present = properties.get(prop_name);
                        if partial && present.is_none() {
                            continue;
                        }
                        if present.is_none_or(|v| v.is_null()) {
                            log::warn!(
                                "Constraint violation: Property '{}' cannot be null for label '{}'",
                                prop_name,
                                label
                            );
                            return Err(anyhow!(
                                "Constraint violation: Property '{}' cannot be null",
                                prop_name
                            ));
                        }
                    }
                }
            }

            // 2. Check Explicit Constraints (Unique, Check, etc.)
            for constraint in &schema.constraints {
                if !constraint.enabled {
                    continue;
                }
                match &constraint.target {
                    ConstraintTarget::Label(l) if l == label => {}
                    _ => continue,
                }

                match &constraint.constraint_type {
                    ConstraintType::Unique {
                        properties: unique_props,
                    } => {
                        // Support single and multi-property unique constraints
                        if !unique_props.is_empty() {
                            let mut key_values = Vec::new();
                            let mut missing = false;
                            for prop in unique_props {
                                if let Some(val) = properties.get(prop) {
                                    key_values.push((prop.clone(), val.clone()));
                                } else {
                                    missing = true; // Can't enforce if property missing (partial update?)
                                    // For INSERT, missing means null?
                                    // If property is nullable, unique constraint typically allows multiple nulls or ignores?
                                    // For now, only check if ALL keys are present
                                }
                            }

                            if !missing {
                                self.check_unique_constraint_multi(label, &key_values, vid, tx_l0)
                                    .await?;
                            }
                        }
                    }
                    ConstraintType::Exists { property } => {
                        if properties.get(property).is_none_or(|v| v.is_null()) {
                            log::warn!(
                                "Constraint violation: Property '{}' must exist for label '{}'",
                                property,
                                label
                            );
                            return Err(anyhow!(
                                "Constraint violation: Property '{}' must exist",
                                property
                            ));
                        }
                    }
                    ConstraintType::Check { expression } => {
                        if !self.evaluate_check_constraint(expression, properties)? {
                            return Err(anyhow!(
                                "CHECK constraint '{}' violated: expression '{}' evaluated to false",
                                constraint.name,
                                expression
                            ));
                        }
                    }
                    _ => {
                        return Err(anyhow!("Unsupported constraint type"));
                    }
                }
            }
        }
        Ok(())
    }

    /// Validates vertex constraints for a vertex with the given labels.
    /// Labels must be passed explicitly since the vertex may not yet be in L0.
    /// Unknown labels (not in schema) are skipped.
    async fn validate_vertex_constraints(
        &self,
        vid: Vid,
        properties: &Properties,
        labels: &[String],
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let schema = self.schema_manager.schema();

        // Validate constraints only for known labels
        for label in labels {
            // Skip unknown labels (schemaless support)
            if schema.get_label_case_insensitive(label).is_none() {
                continue;
            }
            self.validate_vertex_constraints_for_label(vid, properties, label, tx_l0)
                .await?;
        }

        // Check global ext_id uniqueness if ext_id is provided
        if let Some(ext_id) = properties.get("ext_id").and_then(|v| v.as_str()) {
            self.check_extid_globally_unique(ext_id, vid, tx_l0).await?;
        }

        Ok(())
    }

    /// Partial sibling of `validate_vertex_constraints` — validates only
    /// constraints touching keys present in `properties`. Used by
    /// `insert_vertex_partial`'s fast path; the caller pre-screens for
    /// multi-key UNIQUE constraints via `touched_needs_full_read`.
    async fn validate_vertex_constraints_partial(
        &self,
        vid: Vid,
        touched: &Properties,
        labels: &[String],
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let schema = self.schema_manager.schema();
        for label in labels {
            if schema.get_label_case_insensitive(label).is_none() {
                continue;
            }
            self.validate_vertex_constraints_for_label_partial(vid, touched, label, tx_l0)
                .await?;
        }
        if let Some(ext_id) = touched.get("ext_id").and_then(|v| v.as_str()) {
            self.check_extid_globally_unique(ext_id, vid, tx_l0).await?;
        }
        Ok(())
    }

    /// Collect ext_ids and unique constraint keys from an iterator of vertex properties.
    ///
    /// Used to build a constraint key index from L0 buffers for batch validation.
    fn collect_constraint_keys_from_properties<'a>(
        properties_iter: impl Iterator<Item = &'a Properties>,
        label: &str,
        constraints: &[uni_common::core::schema::Constraint],
        existing_keys: &mut HashMap<String, HashSet<String>>,
        existing_extids: &mut HashSet<String>,
    ) {
        for props in properties_iter {
            if let Some(ext_id) = props.get("ext_id").and_then(|v| v.as_str()) {
                existing_extids.insert(ext_id.to_string());
            }

            for constraint in constraints {
                if !constraint.enabled {
                    continue;
                }
                if let ConstraintTarget::Label(l) = &constraint.target {
                    if l != label {
                        continue;
                    }
                } else {
                    continue;
                }

                if let ConstraintType::Unique {
                    properties: unique_props,
                } = &constraint.constraint_type
                {
                    let mut key_parts = Vec::new();
                    let mut all_present = true;
                    for prop in unique_props {
                        if let Some(val) = props.get(prop) {
                            key_parts.push(format!("{}:{}", prop, val));
                        } else {
                            all_present = false;
                            break;
                        }
                    }
                    if all_present {
                        let key = key_parts.join("|");
                        existing_keys
                            .entry(constraint.name.clone())
                            .or_default()
                            .insert(key);
                    }
                }
            }
        }
    }

    /// Validates constraints for a batch of vertices efficiently.
    ///
    /// This method builds an in-memory index from L0 buffers ONCE instead of scanning
    /// per vertex, reducing complexity from O(n²) to O(n) for bulk inserts.
    ///
    /// # Arguments
    /// * `vids` - VIDs of vertices being inserted
    /// * `properties_batch` - Properties for each vertex
    /// * `label` - Label for all vertices (assumes single label for now)
    ///
    /// # Performance
    /// For N vertices with unique constraints:
    /// - Old approach: O(N²) - scan L0 buffer N times
    /// - New approach: O(N) - scan L0 buffer once, build HashSet, check each vertex in O(1)
    async fn validate_vertex_batch_constraints(
        &self,
        vids: &[Vid],
        properties_batch: &[Properties],
        label: &str,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        if vids.len() != properties_batch.len() {
            return Err(anyhow!("VID/properties length mismatch"));
        }

        let schema = self.schema_manager.schema();

        // 1. Validate NOT NULL constraints for each vertex
        if let Some(props_meta) = schema.properties.get(label) {
            for (idx, properties) in properties_batch.iter().enumerate() {
                for (prop_name, meta) in props_meta {
                    if !meta.nullable && properties.get(prop_name).is_none_or(|v| v.is_null()) {
                        return Err(anyhow!(
                            "Constraint violation at index {}: Property '{}' cannot be null",
                            idx,
                            prop_name
                        ));
                    }
                }
            }
        }

        // 2. Build constraint key index from L0 buffers (ONCE for entire batch)
        let mut existing_keys: HashMap<String, HashSet<String>> = HashMap::new();
        let mut existing_extids: HashSet<String> = HashSet::new();

        // Scan current L0 buffer
        {
            let l0 = self.l0_manager.get_current();
            let l0_guard = l0.read();
            Self::collect_constraint_keys_from_properties(
                l0_guard.vertex_properties.values(),
                label,
                &schema.constraints,
                &mut existing_keys,
                &mut existing_extids,
            );
        }

        // Scan transaction L0 if present
        if let Some(tx_l0) = tx_l0 {
            let tx_l0_guard = tx_l0.read();
            Self::collect_constraint_keys_from_properties(
                tx_l0_guard.vertex_properties.values(),
                label,
                &schema.constraints,
                &mut existing_keys,
                &mut existing_extids,
            );
        }

        // 3. Check batch vertices against index AND check for duplicates within batch
        let mut batch_keys: HashMap<String, HashMap<String, usize>> = HashMap::new();
        let mut batch_extids: HashMap<String, usize> = HashMap::new();

        for (idx, (_vid, properties)) in vids.iter().zip(properties_batch.iter()).enumerate() {
            // Check ext_id uniqueness
            if let Some(ext_id) = properties.get("ext_id").and_then(|v| v.as_str()) {
                if existing_extids.contains(ext_id) {
                    return Err(anyhow!(
                        "Constraint violation at index {}: ext_id '{}' already exists",
                        idx,
                        ext_id
                    ));
                }
                if let Some(first_idx) = batch_extids.get(ext_id) {
                    return Err(anyhow!(
                        "Constraint violation: ext_id '{}' duplicated in batch at indices {} and {}",
                        ext_id,
                        first_idx,
                        idx
                    ));
                }
                // Also check the main vertices table — the L0 scans above
                // miss vertices already flushed to L1, so without this a
                // batch insert (e.g. a fork promote onto primary) silently
                // twins a duplicate ext_id instead of erroring. Mirrors the
                // single-vertex `check_extid_globally_unique`.
                if let Ok(Some(found_vid)) =
                    MainVertexDataset::find_by_ext_id(self.storage.backend(), ext_id, None).await
                {
                    return Err(anyhow!(
                        "Constraint violation at index {}: ext_id '{}' already exists (vertex {:?})",
                        idx,
                        ext_id,
                        found_vid
                    ));
                }
                batch_extids.insert(ext_id.to_string(), idx);
            }

            // Check unique constraints
            for constraint in &schema.constraints {
                if !constraint.enabled {
                    continue;
                }
                if let ConstraintTarget::Label(l) = &constraint.target {
                    if l != label {
                        continue;
                    }
                } else {
                    continue;
                }

                match &constraint.constraint_type {
                    ConstraintType::Unique {
                        properties: unique_props,
                    } => {
                        let mut key_parts = Vec::new();
                        let mut all_present = true;
                        for prop in unique_props {
                            if let Some(val) = properties.get(prop) {
                                key_parts.push(format!("{}:{}", prop, val));
                            } else {
                                all_present = false;
                                break;
                            }
                        }

                        if all_present {
                            let key = key_parts.join("|");

                            // Check against existing L0 keys
                            if let Some(keys) = existing_keys.get(&constraint.name)
                                && keys.contains(&key)
                            {
                                return Err(anyhow!(
                                    "Constraint violation at index {}: Duplicate composite key for label '{}' (constraint '{}')",
                                    idx,
                                    label,
                                    constraint.name
                                ));
                            }

                            // Check for duplicates within batch
                            let batch_constraint_keys =
                                batch_keys.entry(constraint.name.clone()).or_default();
                            if let Some(first_idx) = batch_constraint_keys.get(&key) {
                                return Err(anyhow!(
                                    "Constraint violation: Duplicate key '{}' in batch at indices {} and {}",
                                    key,
                                    first_idx,
                                    idx
                                ));
                            }
                            batch_constraint_keys.insert(key, idx);
                        }
                    }
                    ConstraintType::Exists { property }
                        if properties.get(property).is_none_or(|v| v.is_null()) =>
                    {
                        return Err(anyhow!(
                            "Constraint violation at index {}: Property '{}' must exist",
                            idx,
                            property
                        ));
                    }
                    ConstraintType::Check { expression }
                        if !self.evaluate_check_constraint(expression, properties)? =>
                    {
                        return Err(anyhow!(
                            "Constraint violation at index {}: CHECK constraint '{}' violated",
                            idx,
                            constraint.name
                        ));
                    }
                    _ => {}
                }
            }
        }

        // 4. Check storage for unique constraints (can batch this into a single query)
        for constraint in &schema.constraints {
            if !constraint.enabled {
                continue;
            }
            if let ConstraintTarget::Label(l) = &constraint.target {
                if l != label {
                    continue;
                }
            } else {
                continue;
            }

            if let ConstraintType::Unique {
                properties: unique_props,
            } = &constraint.constraint_type
            {
                // Build compound OR filter for all batch vertices
                let mut or_filters = Vec::new();
                for properties in properties_batch.iter() {
                    let mut and_parts = Vec::new();
                    let mut all_present = true;
                    for prop in unique_props {
                        if let Some(val) = properties.get(prop) {
                            let val_str = match val {
                                Value::String(s) => format!("'{}'", s.replace('\'', "''")),
                                Value::Int(n) => n.to_string(),
                                Value::Float(f) => f.to_string(),
                                Value::Bool(b) => b.to_string(),
                                _ => {
                                    all_present = false;
                                    break;
                                }
                            };
                            and_parts.push(format!("{} = {}", prop, val_str));
                        } else {
                            all_present = false;
                            break;
                        }
                    }
                    if all_present {
                        or_filters.push(format!("({})", and_parts.join(" AND ")));
                    }
                }

                #[cfg(feature = "lance-backend")]
                if !or_filters.is_empty() {
                    let vid_list: Vec<String> =
                        vids.iter().map(|v| v.as_u64().to_string()).collect();
                    let filter = format!(
                        "({}) AND _deleted = false AND _vid NOT IN ({})",
                        or_filters.join(" OR "),
                        vid_list.join(", ")
                    );

                    // Count flushed duplicates through the `StorageBackend`
                    // (branch-aware, correct `.lance` path). A missing table
                    // means nothing is flushed yet — the L0/pending/tx checks
                    // above already covered in-memory rows — so skip cleanly;
                    // any other backend error must abort the write rather than
                    // silently fail open (the prior `open_raw()` foot-gun).
                    let backend = self.storage.backend();
                    let table = table_names::vertex_table_name(label);
                    if backend.table_exists(&table).await? {
                        let count = backend.count_rows(&table, Some(filter.as_str())).await?;
                        if count > 0 {
                            return Err(anyhow!(
                                "Constraint violation: Duplicate composite key for label '{}' in storage (constraint '{}')",
                                label,
                                constraint.name
                            ));
                        }
                    }
                }
            }
        }

        Ok(())
    }

    /// Checks that ext_id is globally unique across all vertices.
    ///
    /// Searches L0 buffers (current, transaction, pending) and the main vertices table
    /// to ensure no other vertex uses this ext_id.
    ///
    /// # Errors
    ///
    /// Returns error if another vertex with the same ext_id exists.
    async fn check_extid_globally_unique(
        &self,
        ext_id: &str,
        current_vid: Vid,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        // Check L0 buffers: current, transaction, and pending flush
        let l0_buffers_to_check: Vec<Arc<RwLock<L0Buffer>>> = {
            let mut buffers = vec![self.l0_manager.get_current()];
            if let Some(tx_l0) = tx_l0 {
                buffers.push(tx_l0.clone());
            }
            buffers.extend(self.l0_manager.get_pending_flush());
            buffers
        };

        for l0 in &l0_buffers_to_check {
            // O(1) per buffer via the maintained `extid_index` (the previous
            // full `vertex_properties` scan made constrained ingest O(n²)).
            if let Some(&vid) = l0.read().extid_index.get(ext_id)
                && vid != current_vid
            {
                return Err(anyhow!(
                    "Constraint violation: ext_id '{}' already exists (vertex {:?})",
                    ext_id,
                    vid
                ));
            }
        }

        // Check main vertices table (if it exists)
        // Pass None for global uniqueness check (not snapshot-isolated)
        let backend = self.storage.backend();
        if let Ok(Some(found_vid)) = MainVertexDataset::find_by_ext_id(backend, ext_id, None).await
            && found_vid != current_vid
        {
            return Err(anyhow!(
                "Constraint violation: ext_id '{}' already exists (vertex {:?})",
                ext_id,
                found_vid
            ));
        }

        Ok(())
    }

    /// Helper to get vertex labels from L0 buffer.
    fn get_vertex_labels_from_l0(&self, vid: Vid) -> Option<Vec<String>> {
        let l0 = self.l0_manager.get_current();
        let l0_guard = l0.read();
        // Check if vertex is tombstoned (deleted) - if so, return None
        if l0_guard.vertex_tombstones.contains(&vid) {
            return None;
        }
        l0_guard.get_vertex_labels(vid).map(|l| l.to_vec())
    }

    /// Get vertex labels from all sources: current L0, pending L0s, and storage.
    /// This is the proper way to read vertex labels after a flush, as it checks both
    /// in-memory buffers and persisted storage.
    pub async fn get_vertex_labels(
        &self,
        vid: Vid,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Option<Vec<String>> {
        // 1. Check current L0
        if let Some(labels) = self.get_vertex_labels_from_l0(vid) {
            return Some(labels);
        }

        // 2. Check transaction L0 if present
        if let Some(tx_l0) = tx_l0 {
            let guard = tx_l0.read();
            if guard.vertex_tombstones.contains(&vid) {
                return None;
            }
            if let Some(labels) = guard.get_vertex_labels(vid) {
                return Some(labels.to_vec());
            }
        }

        // 3. Check pending flush L0s
        for pending_l0 in self.l0_manager.get_pending_flush() {
            let guard = pending_l0.read();
            if guard.vertex_tombstones.contains(&vid) {
                return None;
            }
            if let Some(labels) = guard.get_vertex_labels(vid) {
                return Some(labels.to_vec());
            }
        }

        // 4. Check storage
        self.find_vertex_labels_in_storage(vid).await.ok().flatten()
    }

    /// Helper to get edge type from L0 buffer.
    fn get_edge_type_from_l0(&self, eid: Eid) -> Option<String> {
        let l0 = self.l0_manager.get_current();
        let l0_guard = l0.read();
        l0_guard.get_edge_type(eid).map(|s| s.to_string())
    }

    /// Look up the edge type ID (u32) for an EID from the L0 buffer's edge endpoints.
    /// Falls back to the transaction L0 if available.
    pub fn get_edge_type_id_from_l0(
        &self,
        eid: Eid,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Option<u32> {
        // Check transaction L0 first
        if let Some(tx_l0) = tx_l0 {
            let guard = tx_l0.read();
            if let Some((_, _, etype)) = guard.get_edge_endpoint_full(eid) {
                return Some(etype);
            }
        }
        // Fall back to main L0
        let l0 = self.l0_manager.get_current();
        let l0_guard = l0.read();
        l0_guard
            .get_edge_endpoint_full(eid)
            .map(|(_, _, etype)| etype)
    }

    /// Set the type name for an edge (used for schemaless edge types).
    /// This is called during CREATE for edge types not found in the schema.
    pub fn set_edge_type(
        &self,
        eid: Eid,
        type_name: String,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) {
        self.resolve_l0(tx_l0).write().set_edge_type(eid, type_name);
    }

    /// Evaluate a simple CHECK constraint expression.
    /// Supports: "property op value" (e.g., "age > 18", "status = 'active'")
    fn evaluate_check_constraint(&self, expression: &str, properties: &Properties) -> Result<bool> {
        let parts: Vec<&str> = expression.split_whitespace().collect();
        if parts.len() != 3 {
            // For now, only support "prop op val"
            // Fallback to true if too complex to avoid breaking, but warn
            log::warn!(
                "Complex CHECK constraint expression '{}' not fully supported yet; allowing write.",
                expression
            );
            return Ok(true);
        }

        let prop_part = parts[0].trim_start_matches('(');
        // Handle "variable.property" format - take the part after the dot
        let prop_name = if let Some(idx) = prop_part.find('.') {
            &prop_part[idx + 1..]
        } else {
            prop_part
        };

        let op = parts[1];
        let val_str = parts[2].trim_end_matches(')');

        let prop_val = match properties.get(prop_name) {
            Some(v) => v,
            None => return Ok(true), // If property missing, CHECK usually passes (unless NOT NULL)
        };

        // Parse value string (handle quotes for strings)
        let target_val = if (val_str.starts_with('\'') && val_str.ends_with('\''))
            || (val_str.starts_with('"') && val_str.ends_with('"'))
        {
            Value::String(val_str[1..val_str.len() - 1].to_string())
        } else if let Ok(n) = val_str.parse::<i64>() {
            Value::Int(n)
        } else if let Ok(n) = val_str.parse::<f64>() {
            Value::Float(n)
        } else if let Ok(b) = val_str.parse::<bool>() {
            Value::Bool(b)
        } else {
            // Check for internal format wrappers if they somehow leaked through
            if val_str.starts_with("Number(") && val_str.ends_with(')') {
                let n_str = &val_str[7..val_str.len() - 1];
                if let Ok(n) = n_str.parse::<i64>() {
                    Value::Int(n)
                } else if let Ok(n) = n_str.parse::<f64>() {
                    Value::Float(n)
                } else {
                    Value::String(val_str.to_string())
                }
            } else {
                Value::String(val_str.to_string())
            }
        };

        match op {
            "=" | "==" => Ok(prop_val == &target_val),
            "!=" | "<>" => Ok(prop_val != &target_val),
            ">" => self
                .compare_values(prop_val, &target_val)
                .map(|o| o.is_gt()),
            "<" => self
                .compare_values(prop_val, &target_val)
                .map(|o| o.is_lt()),
            ">=" => self
                .compare_values(prop_val, &target_val)
                .map(|o| o.is_ge()),
            "<=" => self
                .compare_values(prop_val, &target_val)
                .map(|o| o.is_le()),
            _ => {
                log::warn!("Unsupported operator '{}' in CHECK constraint", op);
                Ok(true)
            }
        }
    }

    fn compare_values(&self, a: &Value, b: &Value) -> Result<std::cmp::Ordering> {
        use std::cmp::Ordering;

        fn cmp_f64(x: f64, y: f64) -> Ordering {
            x.partial_cmp(&y).unwrap_or(Ordering::Equal)
        }

        match (a, b) {
            (Value::Int(n1), Value::Int(n2)) => Ok(n1.cmp(n2)),
            (Value::Float(f1), Value::Float(f2)) => Ok(cmp_f64(*f1, *f2)),
            (Value::Int(n), Value::Float(f)) => Ok(cmp_f64(*n as f64, *f)),
            (Value::Float(f), Value::Int(n)) => Ok(cmp_f64(*f, *n as f64)),
            (Value::String(s1), Value::String(s2)) => Ok(s1.cmp(s2)),
            _ => Err(anyhow!(
                "Cannot compare incompatible types: {:?} vs {:?}",
                a,
                b
            )),
        }
    }

    async fn check_unique_constraint_multi(
        &self,
        label: &str,
        key_values: &[(String, Value)],
        current_vid: Vid,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        // Serialize constraint key once for O(1) lookups
        let key = serialize_constraint_key(label, key_values);

        // 1. Check L0 (in-memory) using O(1) constraint index
        {
            let l0 = self.l0_manager.get_current();
            let l0_guard = l0.read();
            if l0_guard.has_constraint_key(&key, current_vid) {
                return Err(anyhow!(
                    "Constraint violation: Duplicate composite key for label '{}'",
                    label
                ));
            }
        }

        // 1b. Check pending-flush buffers (Bug #9A). A flush rotates a key's
        // buffer onto `pending_flush` and installs a fresh empty current
        // buffer; until the rotated rows reach Lance the key is invisible to
        // both the current-buffer check above and the storage check below, so
        // a duplicate could slip through that flush window. Mirror the read
        // paths (e.g. `check_extid_globally_unique`, `get_vertex_labels`) that
        // already consult `pending_flush`.
        for pending_l0 in self.l0_manager.get_pending_flush() {
            if pending_l0.read().has_constraint_key(&key, current_vid) {
                return Err(anyhow!(
                    "Constraint violation: Duplicate composite key for label '{}' (in pending flush)",
                    label
                ));
            }
        }

        // Check Transaction L0
        if let Some(tx_l0) = tx_l0 {
            let tx_l0_guard = tx_l0.read();
            if tx_l0_guard.has_constraint_key(&key, current_vid) {
                return Err(anyhow!(
                    "Constraint violation: Duplicate composite key for label '{}' (in tx)",
                    label
                ));
            }
        }

        // 2. Check Storage (L1/L2)
        let filters: Vec<String> = key_values
            .iter()
            .map(|(prop, val)| {
                let val_str = match val {
                    Value::String(s) => format!("'{}'", s.replace('\'', "''")),
                    Value::Int(n) => n.to_string(),
                    Value::Float(f) => f.to_string(),
                    Value::Bool(b) => b.to_string(),
                    _ => "NULL".to_string(),
                };
                format!("{} = {}", prop, val_str)
            })
            .collect();

        let mut filter = filters.join(" AND ");
        filter.push_str(&format!(
            " AND _deleted = false AND _vid != {}",
            current_vid.as_u64()
        ));

        // 2. Check Storage (L1/L2) through the `StorageBackend` (branch-aware,
        // correct `.lance` path). Skip cleanly when the table is not yet
        // flushed; propagate any real backend error instead of failing open.
        #[cfg(feature = "lance-backend")]
        {
            let backend = self.storage.backend();
            let table = table_names::vertex_table_name(label);
            if backend.table_exists(&table).await? {
                let count = backend.count_rows(&table, Some(filter.as_str())).await?;
                if count > 0 {
                    return Err(anyhow!(
                        "Constraint violation: Duplicate composite key for label '{}' (in storage). Filter: {}",
                        label,
                        filter
                    ));
                }
            }
        }

        Ok(())
    }

    async fn check_write_pressure(&self) -> Result<()> {
        let status = self
            .storage
            .compaction_status()
            .map_err(|e| anyhow::anyhow!("Failed to get compaction status: {}", e))?;
        let l1_runs = status.l1_runs;
        let throttle = &self.config.throttle;

        if l1_runs >= throttle.hard_limit {
            log::warn!("Write stalled: L1 runs ({}) at hard limit", l1_runs);
            // Simple polling for now
            while self
                .storage
                .compaction_status()
                .map_err(|e| anyhow::anyhow!("Failed to get compaction status: {}", e))?
                .l1_runs
                >= throttle.hard_limit
            {
                tokio::time::sleep(std::time::Duration::from_millis(100)).await;
            }
        } else if l1_runs >= throttle.soft_limit {
            let excess = l1_runs - throttle.soft_limit;
            // Cap multiplier to avoid overflow
            let excess = std::cmp::min(excess, 31);
            let multiplier = 2_u32.pow(excess as u32);
            let delay = throttle.base_delay * multiplier;
            tokio::time::sleep(delay).await;
        }
        Ok(())
    }

    /// Check transaction memory limit to prevent OOM.
    /// No-op when no transaction is active.
    fn check_transaction_memory(&self, tx_l0: Option<&Arc<RwLock<L0Buffer>>>) -> Result<()> {
        if let Some(tx_l0) = tx_l0 {
            let size = tx_l0.read().estimated_size;
            if size > self.config.max_transaction_memory {
                return Err(anyhow!(
                    "Transaction memory limit exceeded: {} bytes used, limit is {} bytes. \
                     Roll back or commit the current transaction.",
                    size,
                    self.config.max_transaction_memory
                ));
            }
        }
        Ok(())
    }

    async fn get_query_context(
        &self,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Option<QueryContext> {
        Some(QueryContext::new_with_pending(
            self.l0_manager.get_current(),
            tx_l0.cloned(),
            self.l0_manager.get_pending_flush(),
        ))
    }

    /// Layer-1 CRDT variant enforcement, shared by the single-vertex and batch
    /// write paths.
    ///
    /// Rejects a declared CRDT property written as a parsed CRDT value
    /// (`Value::Map`) whose variant differs from the schema's declared variant.
    /// A mismatch would make the commit-time merge silently overwrite instead of
    /// merge, and the OCC CRDT carve-out (`occ::crdt_carveout_overwrite` /
    /// `WriteSet::from_l0`) would hide it as a lost update — so it must be caught
    /// at write time, on *every* write path. `try_as_crdt` is `Map`-gated, so the
    /// JSON-string (Cypher) form and non-CRDT values pass through untouched: they
    /// are never carved out and stay conflictable.
    fn enforce_crdt_variants(
        props_meta: &std::collections::HashMap<String, uni_common::core::schema::PropertyMeta>,
        properties: &Properties,
    ) -> Result<()> {
        for (key, value) in properties {
            let Some(meta) = props_meta.get(key) else {
                continue;
            };
            let uni_common::core::schema::DataType::Crdt(expected) = &meta.r#type else {
                continue;
            };
            if let Some(crdt) = crate::runtime::l0::try_as_crdt(value)
                && crdt.type_name() != expected.type_name()
            {
                return Err(anyhow::Error::new(uni_common::UniError::Constraint {
                    message: format!(
                        "CRDT property '{key}' must be written as a {} value",
                        expected.type_name()
                    ),
                }));
            }
        }
        Ok(())
    }

    /// Prepare a vertex for upsert by merging CRDT properties with existing values.
    ///
    /// When `label` is provided, uses it directly to look up property metadata.
    /// Otherwise falls back to discovering the label from L0 buffers and storage.
    ///
    /// # Errors
    ///
    /// Returns an error if CRDT property merging fails.
    async fn prepare_vertex_upsert(
        &self,
        vid: Vid,
        properties: &mut Properties,
        label: Option<&str>,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let Some(pm) = &self.property_manager else {
            return Ok(());
        };

        let schema = self.schema_manager.schema();

        // Resolve label: use provided label or discover from L0/storage
        let discovered_labels;
        let label_name = if let Some(l) = label {
            Some(l)
        } else {
            discovered_labels = self.get_vertex_labels(vid, tx_l0).await;
            discovered_labels
                .as_ref()
                .and_then(|l| l.first().map(|s| s.as_str()))
        };

        let Some(label_str) = label_name else {
            return Ok(());
        };
        let Some(props_meta) = schema.properties.get(label_str) else {
            return Ok(());
        };

        // Identify CRDT properties in the insert data
        let crdt_keys: Vec<String> = properties
            .keys()
            .filter(|key| {
                props_meta.get(*key).is_some_and(|meta| {
                    matches!(meta.r#type, uni_common::core::schema::DataType::Crdt(_))
                })
            })
            .cloned()
            .collect();

        if crdt_keys.is_empty() {
            return Ok(());
        }

        // Enforce that each declared CRDT property written as a parsed CRDT value
        // (`Value::Map`) carries its declared variant. A mismatched variant makes
        // `merge_crdt_properties` overwrite rather than merge at commit, and the
        // OCC carve-out (`occ::crdt_carveout_overwrite` / `WriteSet::from_l0`)
        // would hide that as a silent lost update — reject it at the source.
        //
        // Only the `Map` form is checked: it is exactly the form the carve-out
        // applies to (`try_as_crdt` is `Map`-gated). A CRDT written as a JSON
        // string (the Cypher form) or a non-CRDT value is never carved out — it
        // stays conflictable — so it poses no carve-out soundness risk and is left
        // to the existing merge/parse path. This is the declared-property half of
        // the layered fix; the commit-time check covers undeclared CRDT-shaped values.
        Self::enforce_crdt_variants(props_meta, properties)?;

        let ctx = self.get_query_context(tx_l0).await;
        for key in crdt_keys {
            let existing = pm.get_vertex_prop_with_ctx(vid, &key, ctx.as_ref()).await?;
            if !existing.is_null()
                && let Some(val) = properties.get_mut(&key)
            {
                *val = pm.merge_crdt_values(&existing, val)?;
            }
        }

        Ok(())
    }

    async fn prepare_edge_upsert(
        &self,
        eid: Eid,
        properties: &mut Properties,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        if let Some(pm) = &self.property_manager {
            let schema = self.schema_manager.schema();
            // Get edge type from L0 buffer instead of from EID
            let type_name = self.get_edge_type_from_l0(eid);

            if let Some(ref t_name) = type_name
                && let Some(props_meta) = schema.properties.get(t_name)
            {
                let mut crdt_keys = Vec::new();
                for (key, _) in properties.iter() {
                    if let Some(meta) = props_meta.get(key)
                        && matches!(meta.r#type, uni_common::core::schema::DataType::Crdt(_))
                    {
                        crdt_keys.push(key.clone());
                    }
                }

                if !crdt_keys.is_empty() {
                    let ctx = self.get_query_context(tx_l0).await;
                    for key in crdt_keys {
                        let existing = pm.get_edge_prop(eid, &key, ctx.as_ref()).await?;

                        if !existing.is_null()
                            && let Some(val) = properties.get_mut(&key)
                        {
                            *val = pm.merge_crdt_values(&existing, val)?;
                        }
                    }
                }
            }
        }
        Ok(())
    }

    #[instrument(skip(self, properties), level = "trace")]
    pub async fn insert_vertex(
        &self,
        vid: Vid,
        properties: Properties,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        self.insert_vertex_with_labels(vid, properties, &[], tx_l0)
            .await?;
        Ok(())
    }

    /// Component C1 (G4): before a non-transactional mutation merges into main
    /// L0, if an outstanding snapshot pins the current generation, freeze it
    /// aside so snapshots taken *before* this write stay isolated from it.
    ///
    /// `flush_lock` (acquired and released here) serializes the freeze against
    /// concurrent commit-time freezes/merges, matching the atomicity the tx
    /// commit path gets. No-op for transactional writes (their freeze happens at
    /// commit) and — the common case — when nothing is pinned, where it costs one
    /// atomic load. Freezes at most once per pinned generation: the freeze
    /// installs a fresh unpinned `current`, so later writes in the same bulk
    /// import see no pin and merge in place, and the snapshot keeps reading the
    /// frozen pre-import buffer.
    async fn freeze_for_non_tx_write_if_pinned(&self, tx_l0: Option<&Arc<RwLock<L0Buffer>>>) {
        // Self-gates on the runtime SSI toggle: nothing pins a snapshot unless a
        // transaction began under `ssi_enabled`, so `is_current_pinned()` is
        // always false (one atomic load) when SSI is off.
        if tx_l0.is_none() && self.l0_manager.is_current_pinned() {
            let _flush_lock_guard = self.flush_lock.lock().await;
            // Re-check under the lock: a concurrent commit may have frozen first.
            if self.l0_manager.is_current_pinned() {
                self.l0_manager.freeze_current_for_snapshot();
                metrics::counter!("uni_l0_snapshot_freezes_total").increment(1);
            }
        }
    }

    #[instrument(skip(self, properties, labels), level = "trace")]
    pub async fn insert_vertex_with_labels(
        &self,
        vid: Vid,
        mut properties: Properties,
        labels: &[String],
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<Properties> {
        let start = std::time::Instant::now();
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;

        // Component C1 (G4): a non-transactional write (`tx_l0 == None`, e.g. bulk
        // import / LOAD CSV) mutates main L0 directly, outside the commit-time
        // snapshot freeze. Freeze the pinned generation aside first so snapshots
        // taken before this write stay isolated from it.
        self.freeze_for_non_tx_write_if_pinned(tx_l0).await;

        if !self.try_defer_embedding(labels, &properties, vid, tx_l0) {
            self.process_embeddings_for_labels(labels, &mut properties)
                .await?;
        }
        self.validate_vertex_constraints(vid, &properties, labels, tx_l0)
            .await?;
        self.prepare_vertex_upsert(
            vid,
            &mut properties,
            labels.first().map(|s| s.as_str()),
            tx_l0,
        )
        .await?;

        // Clone properties and labels before moving into L0 to return them and populate constraint index
        let properties_copy = properties.clone();
        let labels_copy = labels.to_vec();

        {
            // For a non-tx write, re-resolve the live `current` buffer and hold
            // `flush_lock` across the (synchronous) write so a concurrent flush
            // rotate (which takes `flush_lock` via `begin_flush`) cannot install
            // a fresh `current` between resolve and write, dropping our write
            // (Bug #4). For a tx write `resolve_l0` returns the tx-private
            // buffer, never the rotating `current`, so no `flush_lock` is needed
            // (and taking it would risk re-entrancy with the commit path).
            let _flush_lock_guard = if tx_l0.is_none() {
                Some(self.flush_lock.lock().await)
            } else {
                None
            };
            let l0 = self.resolve_l0(tx_l0);
            let mut l0_guard = l0.write();
            // Generation chaining: a non-tx CRDT write into the (post-freeze,
            // possibly empty) current buffer must merge against the chained
            // committed state, not shadow it. No-op when the chain is empty
            // or this is a tx-private write.
            if tx_l0.is_none() {
                let pending = self.l0_manager.get_pending_flush();
                if !pending.is_empty() {
                    Self::seed_crdt_props(&pending, vid, &properties, &mut l0_guard);
                }
            }
            l0_guard.insert_vertex_with_labels(vid, properties, labels);

            // Populate constraint index for O(1) duplicate detection
            let schema = self.schema_manager.schema();
            for label in &labels_copy {
                if schema.get_label_case_insensitive(label).is_none() {
                    if self.config.strict_schema {
                        return Err(anyhow::anyhow!(
                            "Label '{}' is not defined in the schema \
                             (strict_schema is enabled).",
                            label
                        ));
                    }
                    continue; // Schemaless: skip unknown labels.
                }

                // For each unique constraint on this label, insert into constraint index
                for constraint in &schema.constraints {
                    if !constraint.enabled {
                        continue;
                    }
                    if let ConstraintTarget::Label(l) = &constraint.target {
                        if l != label {
                            continue;
                        }
                    } else {
                        continue;
                    }

                    if let ConstraintType::Unique {
                        properties: unique_props,
                    } = &constraint.constraint_type
                    {
                        let mut key_values = Vec::new();
                        let mut all_present = true;
                        for prop in unique_props {
                            if let Some(val) = properties_copy.get(prop) {
                                key_values.push((prop.clone(), val.clone()));
                            } else {
                                all_present = false;
                                break;
                            }
                        }

                        if all_present {
                            let key = serialize_constraint_key(label, &key_values);
                            l0_guard.insert_constraint_key(key, vid);
                        }
                    }
                }
            }
        }

        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        self.update_metrics();

        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        if start.elapsed().as_millis() > 100 {
            log::warn!("Slow insert_vertex: {}ms", start.elapsed().as_millis());
        }
        Ok(properties_copy)
    }

    /// True iff routing this partial write through MergeInsert would
    /// miss a constraint check. Specifically: a multi-key UNIQUE
    /// constraint where the touched-set doesn't cover all member keys
    /// requires the unchanged keys from the existing row to compute
    /// the composite. Conservative: also returns true if any touched
    /// key is `ext_id` (uniqueness checked globally — handled in the
    /// full-row path).
    fn touched_needs_full_read(&self, touched: &Properties, labels: &[String]) -> bool {
        if touched.contains_key("ext_id") {
            return true;
        }
        let schema = self.schema_manager.schema();
        for label in labels {
            if schema.get_label_case_insensitive(label).is_none() {
                continue;
            }
            for constraint in &schema.constraints {
                if !constraint.enabled {
                    continue;
                }
                if let ConstraintTarget::Label(l) = &constraint.target {
                    if !l.eq_ignore_ascii_case(label) {
                        continue;
                    }
                } else {
                    continue;
                }
                if let ConstraintType::Unique {
                    properties: unique_props,
                } = &constraint.constraint_type
                {
                    if unique_props.len() < 2 {
                        continue; // single-key UNIQUE — partial path sees the key
                    }
                    if unique_props.iter().any(|p| touched.contains_key(p)) {
                        return true;
                    }
                }
            }
        }
        false
    }

    /// Insert a vertex's FULL property row plus a touched-keys hint so
    /// the flush emits ONLY those columns via Lance MergeInsert.
    ///
    /// Caller must have read the full row (via PropertyManager) and
    /// applied SET-touched values on top before calling — same input
    /// shape as `insert_vertex_with_labels`. The new arg `touched_keys`
    /// is the set of property keys this SET statement actually
    /// assigned; L0 records it in `vertex_partial_keys[vid]` and the
    /// flush filters the MergeInsert source schema down to those keys.
    /// When `UniConfig::partial_lance_writes == false`, falls through
    /// to `insert_vertex_with_labels` (Append) — preserving bit-for-bit
    /// equivalence with prior releases.
    /// Refresh auto-embed targets for a partial / `SET` write whose `touched_keys`
    /// include an embed *source* column: drop stale target embeddings from `props`
    /// (so they re-embed) and add them to `touched_keys`. Public so the query
    /// executor can apply it on the coalesced write **before** the partial-vs-full
    /// branch — both branches need it. No-op for non-SET writes / non-embed labels.
    pub fn refresh_embed_targets(
        &self,
        props: &mut Properties,
        touched_keys: &mut HashSet<String>,
        labels: &[String],
    ) {
        refresh_touched_embed_targets(&self.schema_manager.schema(), props, touched_keys, labels);
    }

    #[instrument(skip(self, props, touched_keys, labels), level = "trace")]
    pub async fn insert_vertex_partial_full(
        &self,
        vid: Vid,
        mut props: Properties,
        touched_keys: HashSet<String>,
        labels: &[String],
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        if !self.config.partial_lance_writes
            || self.touched_needs_full_read(&props_subset(&props, &touched_keys), labels)
        {
            self.insert_vertex_with_labels(vid, props, labels, tx_l0)
                .await?;
            return Ok(());
        }

        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        if !self.try_defer_embedding(labels, &props, vid, tx_l0) {
            self.process_embeddings_for_labels(labels, &mut props)
                .await?;
        }
        // Full-row validation runs because we have the complete map;
        // no need for the partial-only validator.
        self.validate_vertex_constraints(vid, &props, labels, tx_l0)
            .await?;
        {
            let l0 = self.resolve_l0(tx_l0);
            let mut l0_guard = l0.write();
            l0_guard.insert_vertex_partial_full(vid, props, touched_keys, labels);
        }
        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        metrics::counter!("uni_partial_writes_total").increment(1);
        self.update_metrics();
        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        Ok(())
    }

    /// Insert a vertex's *partial* property set without first reading the
    /// full row.
    ///
    /// When `WriterConfig::partial_lance_writes` is `true`, the touched
    /// keys flow into `L0Buffer::vertex_partial_keys` so the next flush
    /// emits them via Lance `MergeInsertBuilder` against a subset-of-
    /// schema source — preserving untouched columns (e.g., embeddings)
    /// byte-equal in Lance with no read at the caller and no write of
    /// those columns.
    ///
    /// When the flag is `false`, this falls back to the existing
    /// `insert_vertex_with_labels` path after merging `touched` with
    /// the current properties from L0/storage. The caller can therefore
    /// use this entry point unconditionally; the optimization activates
    /// only when the flag is on.
    #[instrument(skip(self, touched, labels), level = "trace")]
    pub async fn insert_vertex_partial(
        &self,
        vid: Vid,
        touched: Properties,
        labels: &[String],
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let needs_full_read =
            !self.config.partial_lance_writes || self.touched_needs_full_read(&touched, labels);
        if needs_full_read {
            // Flag-off fallback (or constraint-driven fallback): merge
            // `touched` with the current full property snapshot from
            // L0/storage and route through the existing path. Preserves
            // bit-for-bit equivalence with the pre-Round-11 release.
            let existing = if let Some(pm) = &self.property_manager {
                pm.get_all_vertex_props_with_ctx(vid, None)
                    .await
                    .unwrap_or_default()
                    .unwrap_or_default()
            } else {
                Properties::new()
            };
            let mut merged = existing;
            for (k, v) in touched {
                merged.insert(k, v);
            }
            self.insert_vertex_with_labels(vid, merged, labels, tx_l0)
                .await?;
            return Ok(());
        }

        // Flag-on fast path: stage the partial update directly. Pressure
        // checks, embedding generation, constraint validation all still
        // run — but the validator is the partial-aware variant that
        // skips NOT NULL / multi-key UNIQUE / CHECK / EXISTS for
        // properties not present in `touched`. Multi-key UNIQUE that
        // overlaps the touched set forces a fallback above via
        // `touched_needs_full_read`.
        let mut touched = touched;
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        if !self.try_defer_embedding(labels, &touched, vid, tx_l0) {
            self.process_embeddings_for_labels(labels, &mut touched)
                .await?;
        }
        self.validate_vertex_constraints_partial(vid, &touched, labels, tx_l0)
            .await?;

        {
            let l0 = self.resolve_l0(tx_l0);
            let mut l0_guard = l0.write();
            l0_guard.insert_vertex_partial(vid, touched, labels);
        }

        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        metrics::counter!("uni_partial_writes_total").increment(1);
        self.update_metrics();
        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        Ok(())
    }

    /// Insert multiple vertices with batched operations.
    ///
    /// This method uses batched operations to achieve O(N) complexity instead of O(N²)
    /// for bulk inserts with unique constraints.
    ///
    /// # Performance Improvements
    /// - Batch VID allocation: 1 call instead of N calls
    /// - Batch constraint validation: O(N) instead of O(N²)
    /// - Batch embedding generation: 1 API call per config instead of N calls
    /// - Transaction wrapping: Automatic flush deferral, atomicity
    ///
    /// # Arguments
    /// * `vids` - Pre-allocated VIDs for the vertices
    /// * `properties_batch` - Properties for each vertex
    /// * `labels` - Labels for all vertices (assumes single label for simplicity)
    ///
    /// # Errors
    /// Returns error if:
    /// - VID/properties length mismatch
    /// - Constraint violation detected
    /// - Embedding generation fails
    /// - Transaction commit fails
    ///
    /// # Atomicity
    /// If this method fails, all changes are rolled back (if transaction was started here).
    pub async fn insert_vertices_batch(
        &self,
        vids: Vec<Vid>,
        mut properties_batch: Vec<Properties>,
        labels: Vec<String>,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<Vec<Properties>> {
        let start = std::time::Instant::now();

        // Validate inputs
        if vids.len() != properties_batch.len() {
            return Err(anyhow!(
                "VID/properties size mismatch: {} vids, {} properties",
                vids.len(),
                properties_batch.len()
            ));
        }

        if vids.is_empty() {
            return Ok(Vec::new());
        }

        // Batch operations — writes go directly to the resolved L0.
        // Atomicity is guaranteed by the caller holding the writer lock.
        let result = async {
            self.check_write_pressure().await?;
            self.check_transaction_memory(tx_l0)?;

            // Component C1 (G4): batch bulk-import is the canonical non-tx write —
            // freeze the pinned generation aside before merging so snapshot
            // readers stay isolated. No-op when unpinned or transactional.
            self.freeze_for_non_tx_write_if_pinned(tx_l0).await;

            // Batch embedding generation (1 API call per config)
            self.process_embeddings_for_batch(&labels, &mut properties_batch)
                .await?;

            // Batch constraint validation (O(N) instead of O(N²))
            let label = labels
                .first()
                .ok_or_else(|| anyhow!("No labels provided"))?;
            self.validate_vertex_batch_constraints(&vids, &properties_batch, label, tx_l0)
                .await?;

            // Batch prepare (CRDT merging if needed)
            // Check schema once: skip entirely if no CRDT properties for this label.
            // For new vertices (freshly allocated VIDs), there are no existing CRDT
            // values to merge, so the per-vertex lookup is unnecessary in that case.
            let has_crdt_fields = {
                let schema = self.schema_manager.schema();
                schema
                    .properties
                    .get(label.as_str())
                    .is_some_and(|props_meta| {
                        props_meta.values().any(|meta| {
                            matches!(meta.r#type, uni_common::core::schema::DataType::Crdt(_))
                        })
                    })
            };

            if has_crdt_fields {
                // Layer-1 variant enforcement (G3): the batch path must reject a
                // declared-CRDT variant mismatch exactly as the single-vertex
                // `prepare_vertex_upsert` does. Without this, a wrong-variant CRDT
                // written via batch import slips past write-time validation and
                // the OCC carve-out then masks the overwrite as a lost update.
                {
                    let schema = self.schema_manager.schema();
                    if let Some(props_meta) = schema.properties.get(label.as_str()) {
                        for props in &properties_batch {
                            Self::enforce_crdt_variants(props_meta, props)?;
                        }
                    }
                }

                // Batch fetch existing CRDT values: collect VIDs that need merging,
                // then query once via PropertyManager instead of per-vertex lookups.
                let schema = self.schema_manager.schema();
                let crdt_keys: Vec<String> = schema
                    .properties
                    .get(label.as_str())
                    .map(|props_meta| {
                        props_meta
                            .iter()
                            .filter(|(_, meta)| {
                                matches!(meta.r#type, uni_common::core::schema::DataType::Crdt(_))
                            })
                            .map(|(key, _)| key.clone())
                            .collect()
                    })
                    .unwrap_or_default();

                if let Some(pm) = &self.property_manager {
                    let ctx = self.get_query_context(tx_l0).await;
                    for (vid, props) in vids.iter().zip(&mut properties_batch) {
                        for key in &crdt_keys {
                            if props.contains_key(key) {
                                let existing =
                                    pm.get_vertex_prop_with_ctx(*vid, key, ctx.as_ref()).await?;
                                if !existing.is_null()
                                    && let Some(val) = props.get_mut(key)
                                {
                                    *val = pm.merge_crdt_values(&existing, val)?;
                                }
                            }
                        }
                    }
                }
            }

            // Batch L0 writes — route to active L0 (transaction L0 if active, else current).
            let target_l0 = self.resolve_l0(tx_l0);

            let properties_result = properties_batch.clone();
            {
                let mut l0_guard = target_l0.write();
                for (vid, props) in vids.iter().zip(properties_batch.iter()) {
                    l0_guard.insert_vertex_with_labels(*vid, props.clone(), &labels);
                }
            }

            // Update metrics (batch increment)
            metrics::counter!("uni_l0_buffer_mutations_total").increment(vids.len() as u64);
            self.update_metrics();

            Ok::<Vec<Properties>, anyhow::Error>(properties_result)
        }
        .await;

        let props = result?;

        if start.elapsed().as_millis() > 100 {
            log::warn!(
                "Slow insert_vertices_batch ({} vertices): {}ms",
                vids.len(),
                start.elapsed().as_millis()
            );
        }

        Ok(props)
    }

    /// Delete a vertex by VID.
    ///
    /// When `labels` is provided, uses them directly to populate L0 for
    /// correct tombstone flushing. Otherwise discovers labels from L0
    /// buffers and storage (which can be slow for many vertices).
    ///
    /// # Errors
    ///
    /// Returns an error if write pressure stalls, label lookup fails, or
    /// the L0 delete operation fails.
    #[instrument(skip(self, labels), level = "trace")]
    pub async fn delete_vertex(
        &self,
        vid: Vid,
        labels: Option<Vec<String>>,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let start = std::time::Instant::now();
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        self.freeze_for_non_tx_write_if_pinned(tx_l0).await; // C1 (G4)

        // Before deleting, ensure we have the vertex's labels stored in L0 so
        // the tombstone can be flushed to the correct label datasets. Discover
        // them up front (this may await storage) WITHOUT pinning the buffer we
        // will eventually mutate — for non-tx writes the live `current` buffer
        // is re-resolved below under `flush_lock`, so a concurrent rotate can't
        // drop our write (Bug #4). `resolve_l0` here is only used for cheap
        // reads that tolerate a racing rotate.
        let has_labels = {
            let l0_guard = self.resolve_l0(tx_l0);
            let guard = l0_guard.read();
            guard.vertex_labels.contains_key(&vid)
        };

        let backfill_labels = if has_labels {
            None
        } else if let Some(provided) = labels {
            // Caller provided labels — skip the lookup entirely
            Some(provided)
        } else {
            // Discover labels from pending flush L0s, then storage
            let mut found = None;
            for pending_l0 in self.l0_manager.get_pending_flush() {
                let pending_guard = pending_l0.read();
                if let Some(l) = pending_guard.get_vertex_labels(vid) {
                    found = Some(l.to_vec());
                    break;
                }
            }
            if found.is_none() {
                found = self.find_vertex_labels_in_storage(vid).await?;
            }
            found
        };

        // Test-only seam (no-op without the `failpoints` feature): pause a
        // non-transactional delete AFTER the awaited label discovery but BEFORE
        // it re-resolves the live buffer and writes the tombstone. A concurrent
        // flush can rotate+complete a buffer in this window; the fix re-resolves
        // `get_current()` and mutates it under `flush_lock`, so the tombstone
        // always lands in the live buffer (Bug #4 — silent lost delete across
        // L0 rotation).
        fail::fail_point!("nontx::after-capture");

        // Apply the label backfill and the tombstone together. For a non-tx
        // write, hold `flush_lock` across the (synchronous) re-resolve + write
        // so a concurrent flush rotate (which takes `flush_lock` via
        // `begin_flush`) cannot install a fresh `current` between our resolve
        // and our write. For a tx write `resolve_l0` returns the tx-private
        // buffer (never the rotating `current`), so no `flush_lock` is needed
        // — and taking it there would risk re-entrancy with the commit path.
        if tx_l0.is_none() {
            let _flush_lock_guard = self.flush_lock.lock().await;
            let l0 = self.l0_manager.get_current();
            let mut guard = l0.write();
            if let Some(found_labels) = backfill_labels {
                guard.vertex_labels.insert(vid, found_labels);
            }
            guard.delete_vertex(vid)?;
        } else {
            let l0 = self.resolve_l0(tx_l0);
            let mut guard = l0.write();
            if let Some(found_labels) = backfill_labels {
                guard.vertex_labels.insert(vid, found_labels);
            }
            guard.delete_vertex(vid)?;
        }
        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        self.update_metrics();

        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        if start.elapsed().as_millis() > 100 {
            log::warn!("Slow delete_vertex: {}ms", start.elapsed().as_millis());
        }
        Ok(())
    }

    /// Find vertex labels from storage by querying the main vertices table.
    /// Returns the labels from the latest non-deleted version of the vertex.
    async fn find_vertex_labels_in_storage(&self, vid: Vid) -> Result<Option<Vec<String>>> {
        use crate::backend::types::ScanRequest;
        use arrow_array::Array;
        use arrow_array::cast::AsArray;

        let backend = self.storage.backend();
        let table_name = MainVertexDataset::table_name();

        // Check if table exists first; if not, vertex hasn't been flushed to storage yet
        if !backend.table_exists(table_name).await? {
            return Ok(None);
        }

        // Query for this specific vid (don't filter by _deleted yet - we need to find the latest version first)
        let filter = format!("_vid = {}", vid.as_u64());
        let batches = backend
            .scan(
                ScanRequest::all(table_name)
                    .with_filter(filter)
                    .with_columns(vec![
                        "_vid".to_string(),
                        "labels".to_string(),
                        "_version".to_string(),
                        "_deleted".to_string(),
                    ]),
            )
            .await
            .unwrap_or_default();

        // Find the row with the highest version number
        let mut max_version: Option<u64> = None;
        let mut labels: Option<Vec<String>> = None;
        let mut is_deleted = false;

        for batch in batches {
            if batch.num_rows() == 0 {
                continue;
            }

            let version_array = batch
                .column_by_name("_version")
                .unwrap()
                .as_primitive::<arrow_array::types::UInt64Type>();

            let deleted_array = batch.column_by_name("_deleted").unwrap().as_boolean();

            let labels_array = batch.column_by_name("labels").unwrap().as_list::<i32>();

            for row_idx in 0..batch.num_rows() {
                let version = version_array.value(row_idx);

                if max_version.is_none_or(|mv| version > mv) {
                    is_deleted = deleted_array.value(row_idx);

                    let labels_list = labels_array.value(row_idx);
                    let string_array = labels_list.as_string::<i32>();
                    let vertex_labels: Vec<String> = (0..string_array.len())
                        .filter(|&i| !string_array.is_null(i))
                        .map(|i| string_array.value(i).to_string())
                        .collect();

                    max_version = Some(version);
                    labels = Some(vertex_labels);
                }
            }
        }

        // If the latest version is deleted, return None
        if is_deleted { Ok(None) } else { Ok(labels) }
    }

    #[expect(clippy::too_many_arguments)]
    #[instrument(skip(self, props, touched_keys), level = "trace")]
    pub async fn insert_edge_partial_full(
        &self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        props: Properties,
        edge_type_name: Option<String>,
        touched_keys: HashSet<String>,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        self.freeze_for_non_tx_write_if_pinned(tx_l0).await; // C1 (G4)
        if !self.config.partial_lance_writes {
            return self
                .insert_edge(
                    src_vid,
                    dst_vid,
                    edge_type,
                    eid,
                    props,
                    edge_type_name,
                    tx_l0,
                )
                .await;
        }

        let start = std::time::Instant::now();
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        let mut props = props;
        self.prepare_edge_upsert(eid, &mut props, tx_l0).await?;

        let l0 = self.resolve_l0(tx_l0);
        l0.write().insert_edge_partial_full(
            src_vid,
            dst_vid,
            edge_type,
            eid,
            props,
            edge_type_name,
            touched_keys,
        )?;

        if tx_l0.is_none() {
            let version = l0.read().current_version;
            self.adjacency_manager
                .insert_edge(src_vid, dst_vid, eid, edge_type, version);
        }

        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        metrics::counter!("uni_partial_writes_total").increment(1);
        self.update_metrics();
        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        if start.elapsed().as_millis() > 100 {
            log::warn!(
                "Slow insert_edge_partial_full: {}ms",
                start.elapsed().as_millis()
            );
        }
        Ok(())
    }

    #[expect(clippy::too_many_arguments)]
    pub async fn insert_edge(
        &self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        mut properties: Properties,
        edge_type_name: Option<String>,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let start = std::time::Instant::now();
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        self.freeze_for_non_tx_write_if_pinned(tx_l0).await; // C1 (G4)
        self.prepare_edge_upsert(eid, &mut properties, tx_l0)
            .await?;

        let l0 = self.resolve_l0(tx_l0);
        l0.write()
            .insert_edge(src_vid, dst_vid, edge_type, eid, properties, edge_type_name)?;

        // Dual-write to AdjacencyManager overlay (survives flush).
        // Skip for transaction-local L0 -- transaction edges are overlaid separately.
        if tx_l0.is_none() {
            let version = l0.read().current_version;
            self.adjacency_manager
                .insert_edge(src_vid, dst_vid, eid, edge_type, version);
        }

        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        self.update_metrics();

        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        if start.elapsed().as_millis() > 100 {
            log::warn!("Slow insert_edge: {}ms", start.elapsed().as_millis());
        }
        Ok(())
    }

    #[instrument(skip(self), level = "trace")]
    pub async fn delete_edge(
        &self,
        eid: Eid,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> Result<()> {
        let start = std::time::Instant::now();
        self.check_write_pressure().await?;
        self.check_transaction_memory(tx_l0)?;
        self.freeze_for_non_tx_write_if_pinned(tx_l0).await; // C1 (G4)
        let l0 = self.resolve_l0(tx_l0);

        l0.write().delete_edge(eid, src_vid, dst_vid, edge_type)?;

        // Dual-write tombstone to AdjacencyManager overlay.
        if tx_l0.is_none() {
            let version = l0.read().current_version;
            self.adjacency_manager
                .add_tombstone(eid, src_vid, dst_vid, edge_type, version);
        }
        metrics::counter!("uni_l0_buffer_mutations_total").increment(1);
        self.update_metrics();

        if tx_l0.is_none() {
            self.check_flush().await?;
        }
        if start.elapsed().as_millis() > 100 {
            log::warn!("Slow delete_edge: {}ms", start.elapsed().as_millis());
        }
        Ok(())
    }

    /// Decide whether a flush should be triggered based on mutation count
    /// or elapsed time since the last flush.
    ///
    /// Extracted from [`Writer::check_flush`] so `commit_transaction_l0` can
    /// reuse the decision while bypassing the lock-acquiring entry point
    /// (it already holds `flush_lock`).
    fn should_flush(&self) -> bool {
        let count = self.l0_manager.get_current().read().mutation_count;
        if count == 0 {
            return false;
        }
        if count >= self.config.auto_flush_threshold {
            return true;
        }
        if let Some(interval) = self.config.auto_flush_interval
            && self.last_flush_time.lock().elapsed() >= interval
            && count >= self.config.auto_flush_min_mutations
        {
            return true;
        }
        false
    }

    /// Check if flush should be triggered based on mutation count or time elapsed.
    /// This method is called after each write operation and can also be called
    /// by a background task for time-based flushing.
    pub async fn check_flush(&self) -> Result<()> {
        if self.should_flush() {
            self.flush_to_l1(None).await?;
        }
        Ok(())
    }

    /// Process embeddings for a vertex using labels passed directly.
    /// Use this when labels haven't been stored to L0 yet.
    async fn process_embeddings_for_labels(
        &self,
        labels: &[String],
        properties: &mut Properties,
    ) -> Result<()> {
        let label_name = labels.first().map(|s| s.as_str());
        self.process_embeddings_impl(label_name, properties).await
    }

    /// Phase B: if `defer_embeddings` is enabled in `UniConfig` and the
    /// vertex has an embedding config that hasn't been satisfied by the
    /// caller-provided properties, enqueue the VID in
    /// `L0Buffer::pending_embeddings` and return `true`. The caller then
    /// skips `process_embeddings_for_labels` and the embedding is computed
    /// in a single batched call at flush time via
    /// `drain_pending_embeddings`.
    ///
    /// Returns `false` (caller falls back to today's per-row eager embed)
    /// if any of:
    ///  - the flag is off,
    ///  - no label has an embedding config,
    ///  - the user already provided the target property (matches the
    ///    existing skip-if-present semantics at writer.rs:2727).
    ///
    /// Trade-off: when deferral is active, in-tx reads of the embedding
    /// column return only what was already in storage (or nothing for
    /// brand-new vertices). Existing tests that RETURN n.embedding in
    /// the same tx as a SET on the source column must run with the flag
    /// off; opt in only when no such reads happen between write and
    /// commit.
    fn try_defer_embedding(
        &self,
        labels: &[String],
        properties: &Properties,
        vid: Vid,
        tx_l0: Option<&Arc<RwLock<L0Buffer>>>,
    ) -> bool {
        if !self.config.defer_embeddings {
            return false;
        }
        let Some(label) = labels.first() else {
            return false;
        };

        let schema = self.schema_manager.schema();
        let mut has_unsatisfied_cfg = false;
        for idx in &schema.indexes {
            let unsatisfied = match idx {
                IndexDefinition::Vector(v_cfg) => {
                    v_cfg.label == *label
                        && v_cfg.embedding_config.is_some()
                        && !properties.contains_key(&v_cfg.property)
                }
                IndexDefinition::Sparse(s_cfg) => {
                    s_cfg.label == *label
                        && s_cfg.embedding_config.is_some()
                        && !properties.contains_key(&s_cfg.property)
                }
                _ => false,
            };
            if unsatisfied {
                has_unsatisfied_cfg = true;
                break;
            }
        }
        if !has_unsatisfied_cfg {
            return false;
        }

        let l0 = self.resolve_l0(tx_l0);
        let mut guard = l0.write();
        guard.pending_embeddings.insert(vid, label.clone());
        true
    }

    /// Drain `pending_embeddings` from the rotated old-L0 right before
    /// `flush_stream_l1` reads it. Groups by label, issues one batched
    /// `process_embeddings_for_batch` call per label, and writes the
    /// resulting embedding vectors into each VID's `vertex_properties`
    /// map. After this returns, the flush proceeds against an L0 that
    /// looks no different from one whose embeddings were generated
    /// per-row at insert.
    ///
    /// Idempotent: a VID whose embedding was already materialized
    /// (e.g., by on-demand read paths in a future Phase B revision) is
    /// detected via `properties.contains_key(target_prop)` inside
    /// `process_embeddings_for_batch` (writer.rs:~2650), so re-running
    /// the drain is safe.
    async fn drain_pending_embeddings(&self, old_l0_arc: &Arc<RwLock<L0Buffer>>) -> Result<()> {
        let by_label: HashMap<String, Vec<Vid>> = {
            let guard = old_l0_arc.read();
            if guard.pending_embeddings.is_empty() {
                return Ok(());
            }
            let mut m: HashMap<String, Vec<Vid>> = HashMap::new();
            for (vid, label) in &guard.pending_embeddings {
                m.entry(label.clone()).or_default().push(*vid);
            }
            m
        };

        for (label, vids) in by_label {
            let mut properties_batch: Vec<Properties> = {
                let guard = old_l0_arc.read();
                vids.iter()
                    .map(|vid| {
                        guard
                            .vertex_properties
                            .get(vid)
                            .cloned()
                            .unwrap_or_default()
                    })
                    .collect()
            };

            self.process_embeddings_for_batch(std::slice::from_ref(&label), &mut properties_batch)
                .await?;

            let mut guard = old_l0_arc.write();
            for (vid, props) in vids.iter().zip(properties_batch) {
                let target = guard.vertex_properties.entry(*vid).or_default();
                for (k, v) in props {
                    target.insert(k, v);
                }
                guard.pending_embeddings.remove(vid);
            }
        }
        Ok(())
    }

    /// Materialise MUVERA FDE columns for the about-to-flush L0. Mirrors
    /// [`Self::drain_pending_embeddings`]: for each MUVERA index, compute the derived
    /// Fixed-Dimensional Encoding from each row's source multi-vector and inject it into
    /// that row's `vertex_properties` (so the normal column builder writes the
    /// `__fde_*` column with no hot-path change). For partial-write rows that touched the
    /// source column, the derived column is added to `vertex_partial_keys` so the partial
    /// MergeInsert batch carries the recomputed FDE (avoids staleness on `SET`).
    ///
    /// No-op when the schema has no MUVERA index. Unlike auto-embed, the FDE is a pure,
    /// deterministic, in-process transform — no runtime/embedding service needed.
    fn materialize_fde_columns(&self, old_l0_arc: &Arc<RwLock<L0Buffer>>) -> Result<()> {
        let schema = self.schema_manager.schema();
        let specs = crate::storage::muvera_index::fde_specs(&schema);
        if specs.is_empty() {
            return Ok(());
        }
        let mut guard = old_l0_arc.write();
        for spec in &specs {
            let encoder = uni_common::muvera::FdeEncoder::new(&spec.params)
                .map_err(|e| anyhow!("MUVERA index '{}': {e}", spec.index_name))?;
            // VIDs of this label currently in L0 (collect first to avoid a borrow
            // conflict with the per-row mutation below).
            let vids: Vec<Vid> = guard
                .vertex_labels
                .iter()
                .filter(|(_, labels)| labels.contains(&spec.label))
                .map(|(vid, _)| *vid)
                .collect();
            for vid in vids {
                // Decode the source multi-vector tokens (borrow ends before the mutation).
                let tokens = match guard
                    .vertex_properties
                    .get(&vid)
                    .and_then(|p| p.get(&spec.source_prop))
                {
                    Some(v) => crate::storage::muvera_index::value_to_multivec(v),
                    None => continue, // source absent → leave the FDE column NULL
                };
                // A source token with the wrong dimension makes `encode_doc` error.
                // Skipping the row (leaving the FDE column NULL → ranks last under the
                // mandatory Dot metric; harmless) keeps one malformed document from
                // wedging *every* flush of this label, matching the normal multi-vector
                // column path, which null-fills a dimension mismatch rather than failing
                // the flush (issue #96).
                let fde = match encoder.encode_doc(&tokens) {
                    Ok(fde) => fde,
                    Err(e) => {
                        tracing::warn!(
                            index = %spec.index_name,
                            vid = ?vid,
                            error = %e,
                            "muvera.fde.skip_malformed: leaving FDE NULL for a source \
                             multi-vector that failed encoding"
                        );
                        continue;
                    }
                };
                if let Some(props) = guard.vertex_properties.get_mut(&vid) {
                    props.insert(spec.derived_col.clone(), Value::Vector(fde));
                }
                if let Some(touched) = guard.vertex_partial_keys.get_mut(&vid)
                    && touched.contains(&spec.source_prop)
                {
                    touched.insert(spec.derived_col.clone());
                }
            }
        }
        Ok(())
    }

    /// Process embeddings for a batch of vertices efficiently.
    ///
    /// Groups vertices by embedding config and makes batched API calls to the
    /// embedding service instead of calling once per vertex.
    ///
    /// # Performance
    /// For N vertices with embedding config:
    /// - Old approach: N API calls to embedding service
    /// - New approach: 1 API call per embedding config (usually 1 total)
    async fn process_embeddings_for_batch(
        &self,
        labels: &[String],
        properties_batch: &mut [Properties],
    ) -> Result<()> {
        let Some(label) = labels.first().map(|s| s.as_str()) else {
            return Ok(());
        };
        let schema = self.schema_manager.schema();

        // Group auto-embed targets by (alias, source). A group with both a dense Vector and a
        // multi-vector List<Vector> column is a single-pass hybrid source: one inference fills
        // both. Non-mixed groups use the dense / multi-vector embedder as before.
        let groups = collect_embed_groups(&schema, label);
        if groups.is_empty() {
            return Ok(());
        }

        for (key, group) in groups {
            let alias = &key.0;
            let want_dense = !group.dense.is_empty();
            let want_multi = !group.multi.is_empty();
            let want_sparse = !group.sparse.is_empty();

            // A row needs this group's inference if it has the source text and is still missing
            // at least one of the group's target columns (user-supplied values are preserved).
            let mut input_texts: Vec<String> = Vec::new();
            let mut needs: Vec<usize> = Vec::new();
            for (idx, properties) in properties_batch.iter().enumerate() {
                let all_present = group
                    .dense
                    .iter()
                    .chain(group.multi.iter())
                    .chain(group.sparse.iter())
                    .all(|t| properties.contains_key(t));
                if all_present {
                    continue;
                }
                let mut inputs = Vec::new();
                for src in &group.source_properties {
                    if let Some(val) = properties.get(src)
                        && let Some(s) = val.as_str()
                    {
                        inputs.push(s.to_string());
                    }
                }
                if inputs.is_empty() {
                    continue;
                }
                let text = inputs.join(" ");
                let text = match &group.document_prefix {
                    Some(prefix) => format!("{prefix}{text}"),
                    None => text,
                };
                input_texts.push(text);
                needs.push(idx);
            }
            if input_texts.is_empty() {
                continue;
            }

            let runtime = self
                .xervo_runtime
                .get()
                .ok_or_else(|| anyhow!("Uni-Xervo runtime not configured for auto-embedding"))?;
            let input_refs: Vec<&str> = input_texts.iter().map(|s| s.as_str()).collect();
            let (dense, multi, sparse) = embed_group(
                runtime,
                alias,
                &input_refs,
                want_dense,
                want_multi,
                want_sparse,
            )
            .await?;

            for (i, &row) in needs.iter().enumerate() {
                if let Some(vec) = dense.as_ref().and_then(|d| d.get(i)) {
                    let vals: Vec<Value> = vec.iter().map(|f| Value::Float(*f as f64)).collect();
                    for t in &group.dense {
                        if !properties_batch[row].contains_key(t) {
                            properties_batch[row].insert(t.clone(), Value::List(vals.clone()));
                        }
                    }
                }
                if let Some(tokens) = multi.as_ref().and_then(|m| m.get(i)) {
                    let mv = multivec_to_value(tokens);
                    for t in &group.multi {
                        if !properties_batch[row].contains_key(t) {
                            properties_batch[row].insert(t.clone(), mv.clone());
                        }
                    }
                }
                if let Some(pairs) = sparse.as_ref().and_then(|s| s.get(i)) {
                    let sv = sparse_pairs_to_value(pairs);
                    for t in &group.sparse {
                        if !properties_batch[row].contains_key(t) {
                            properties_batch[row].insert(t.clone(), sv.clone());
                        }
                    }
                }
            }
        }

        Ok(())
    }

    async fn process_embeddings_impl(
        &self,
        label_name: Option<&str>,
        properties: &mut Properties,
    ) -> Result<()> {
        let schema = self.schema_manager.schema();

        let Some(label) = label_name else {
            return Ok(());
        };

        // Same (alias, source) grouping as the deferred path: a mixed dense + multi-vector
        // group is a single-pass hybrid source (one inference fills both columns).
        let groups = collect_embed_groups(&schema, label);
        if groups.is_empty() {
            log::info!("No embedding config found for label {}", label);
            return Ok(());
        }

        for (key, group) in groups {
            let alias = &key.0;
            // Skip if every target already present (user-supplied values win).
            if group
                .dense
                .iter()
                .chain(group.multi.iter())
                .chain(group.sparse.iter())
                .all(|t| properties.contains_key(t))
            {
                continue;
            }

            let mut inputs = Vec::new();
            for src in &group.source_properties {
                if let Some(val) = properties.get(src)
                    && let Some(s) = val.as_str()
                {
                    inputs.push(s.to_string());
                }
            }
            if inputs.is_empty() {
                continue;
            }
            let text = inputs.join(" ");
            let text = match &group.document_prefix {
                Some(prefix) => format!("{prefix}{text}"),
                None => text,
            };

            let runtime = self
                .xervo_runtime
                .get()
                .ok_or_else(|| anyhow!("Uni-Xervo runtime not configured for auto-embedding"))?;
            let want_dense = !group.dense.is_empty();
            let want_multi = !group.multi.is_empty();
            let want_sparse = !group.sparse.is_empty();
            let (dense, multi, sparse) = embed_group(
                runtime,
                alias,
                &[text.as_str()],
                want_dense,
                want_multi,
                want_sparse,
            )
            .await?;

            if let Some(vec) = dense.as_ref().and_then(|d| d.first()) {
                let vals: Vec<Value> = vec.iter().map(|f| Value::Float(*f as f64)).collect();
                for t in &group.dense {
                    if !properties.contains_key(t) {
                        properties.insert(t.clone(), Value::List(vals.clone()));
                    }
                }
            }
            if let Some(tokens) = multi.as_ref().and_then(|m| m.first()) {
                let mv = multivec_to_value(tokens);
                for t in &group.multi {
                    if !properties.contains_key(t) {
                        properties.insert(t.clone(), mv.clone());
                    }
                }
            }
            if let Some(pairs) = sparse.as_ref().and_then(|s| s.first()) {
                let sv = sparse_pairs_to_value(pairs);
                for t in &group.sparse {
                    if !properties.contains_key(t) {
                        properties.insert(t.clone(), sv.clone());
                    }
                }
            }
        }
        Ok(())
    }

    /// Flushes the current in-memory L0 buffer to L1 storage.
    ///
    /// # Lock Ordering
    ///
    /// To prevent deadlocks, locks must be acquired in the following order:
    /// 1. `Writer` lock (held by caller via outer `Arc<RwLock<Writer>>`; removed in Phase 4)
    /// 2. `flush_lock` (acquired by this entry point; held across the whole flush)
    /// 3. `L0Manager` lock (via `begin_flush` / `get_current`)
    /// 4. `L0Buffer` lock (individual buffer RWLocks)
    /// 5. `Index` / `Storage` locks (during actual flush)
    ///
    /// Callers that already hold `flush_lock` (today only `commit_transaction_l0`)
    /// must call `flush_inline_under_lock` (private) directly to avoid a re-entrant
    /// `tokio::sync::Mutex` deadlock — see concurrent_writer.md §5.5.
    pub async fn flush_to_l1(&self, name: Option<String>) -> Result<String> {
        // Drain any in-flight async flushes first. `flush_to_l1` is a
        // SYNCHRONIZATION BARRIER — callers (test fixtures, fork
        // setup, shutdown paths) rely on it as "all writes are now
        // durably in Lance". Without the drain, an async stream from
        // a recent commit might still be writing to Lance when
        // `flush_to_l1` returns, leaving a window where forks branch
        // off pre-write Lance state and lose data.
        if let Some(coord) = self.flush_coordinator.as_ref() {
            let _ = coord.drain(self.config.drop_fork_drain_timeout).await;
        }
        let _flush_lock_guard = self.flush_lock.lock().await;
        self.flush_inline_under_lock(name).await
    }

    /// Flush L0→L1 and capture the fork point under one held `flush_lock`.
    ///
    /// Drains in-flight async flushes, takes `flush_lock`, runs the inline
    /// flush, and then — still holding the lock — reads the allocator
    /// high-water marks and each existing candidate dataset's Lance
    /// version. Capturing under the held lock is what makes the fork point
    /// atomic: no concurrent commit can advance the allocator and no
    /// concurrent flush can advance a dataset tip between the flush and the
    /// reads. See [`ForkPoint`].
    ///
    /// `candidate_dataset_names` are resolved to `{base_uri}/{name}.lance`;
    /// names with no `.lance` directory on disk are skipped (returned map
    /// has no entry for them), matching the fork branch loop's existence
    /// check.
    ///
    /// # Errors
    /// Propagates flush failures from `flush_inline_under_lock` and any
    /// per-dataset version read failure from `lance_branch::current_version`.
    ///
    /// # Deadlocks
    /// Must not be called by a task already holding `flush_lock` (e.g.
    /// `commit_transaction_l0`); the `tokio::sync::Mutex` is not reentrant.
    /// Fork creation never holds the lock, so the sole call site is safe.
    pub async fn flush_and_capture_fork_point(
        &self,
        candidate_dataset_names: &[String],
    ) -> Result<ForkPoint> {
        if let Some(coord) = self.flush_coordinator.as_ref() {
            let _ = coord.drain(self.config.drop_fork_drain_timeout).await;
        }
        let _flush_lock_guard = self.flush_lock.lock().await;
        self.flush_inline_under_lock(None).await?;

        // Still under `flush_lock`: capture the allocator HWM, the MVCC
        // version HWM, and every existing dataset's Lance version so
        // nothing can interleave.
        let (vid_hwm, eid_hwm) = self.allocator.current_hwm().await;
        // The parent's current L0 version is the largest `_version` any
        // inherited row can carry (flushed or in-memory). A fork bootstraps
        // its version floor to this so a fork tx read still sees inherited
        // rows. Cheap read lock; no buffer clone.
        let version_hwm = self.l0_manager.get_current().read().current_version;

        let base = self.storage.base_uri();
        let mut dataset_versions = BTreeMap::new();
        for name in candidate_dataset_names {
            let uri = join_lance_uri(base, name);
            if !lance_path_exists(&uri) {
                continue;
            }
            let version = crate::backend::lance_branch::current_version(&uri).await?;
            dataset_versions.insert(name.clone(), version);
        }

        Ok(ForkPoint {
            vid_hwm,
            eid_hwm,
            dataset_versions,
            version_hwm,
        })
    }

    /// Async-flush entry point: rotate under `flush_lock`, release the
    /// lock, then submit the stream phase to the [`FlushCoordinator`].
    /// Returns a [`FlushTicket`](crate::runtime::flush_coordinator::FlushTicket)
    /// that resolves when finalize completes.
    ///
    /// Errors if `config.async_flush_enabled = false` (the coordinator
    /// is `None` in that case — see `flush_coordinator` field doc).
    pub async fn flush_to_l1_async(
        self: &Arc<Self>,
        name: Option<String>,
    ) -> Result<crate::runtime::flush_coordinator::FlushTicket> {
        let coord = self
            .flush_coordinator
            .as_ref()
            .ok_or_else(|| anyhow!("async flush not enabled (config.async_flush_enabled=false)"))?
            .clone();
        // 1. Acquire permit FIRST (outside flush_lock) so we don't
        //    introduce a permit-while-holding-flush-lock convoy.
        let permit = coord.acquire_permit().await?;
        // 2. Rotate under flush_lock (µs work), then allocate the rotate seq
        //    and bump pending ONLY after the rotate succeeds (Bug #3). A failed
        //    rotate (the `?` below) must consume neither: the finalizer
        //    advances in strictly consecutive seq order and only decrements
        //    pending on finalize, so a leaked seq/pending would wedge it
        //    forever. The seq is allocated under `flush_lock`, immediately
        //    after the rotate and before the guard drops, so concurrent rotates
        //    keep seq order == rotation order, and the seq is unused until
        //    submit. On the `?` error path the permit drops, freeing the slot.
        let (
            RotateOutput {
                old_l0_arc,
                wal_lsn,
                current_version,
                flush_in_progress_guard,
            },
            seq,
        ) = {
            let _flush_lock_guard = self.flush_lock.lock().await;
            let rotate_out = self.flush_l0_rotate().await?;
            let seq = coord.next_rotate_seq();
            coord.note_pending();
            (rotate_out, seq)
        };
        // 3. Build the coordinator's RotatedFlush. parent_manifest is the
        //    cached_manifest snapshot at this moment.
        let parent_manifest = self.cached_manifest.lock().clone();
        let rotated = RotatedFlush {
            seq,
            old_l0_arc: old_l0_arc.clone(),
            wal_lsn,
            current_version,
            name: name.clone(),
            parent_manifest,
            permit,
            flush_in_progress_guard,
        };
        // 4. Spawn the stream phase via the coordinator. The closure
        //    captures Arc<Writer> transiently — drops when stream
        //    completes (bounded, ~50-500 ms).
        let writer = self.clone();
        let ticket = coord.submit_for_stream(rotated, move |old_l0, wal, ver, n| async move {
            let outcome = writer.flush_stream_l1(old_l0, wal, ver, n).await?;
            Ok(crate::runtime::flush_coordinator::FlushOutcome {
                new_manifest: outcome.manifest,
                snapshot_id: outcome.snapshot_id,
            })
        });
        Ok(ticket)
    }

    /// Phase A+B+C of the flush: flush the WAL, rotate L0 (so the
    /// to-be-flushed buffer moves to `pending_flush` and a fresh L0 takes
    /// its place), and hand off the WAL to the new L0.
    ///
    /// Runs in microseconds. Must be called under `flush_lock` (the caller
    /// is responsible). The returned [`RotateOutput`] carries everything
    /// the subsequent stream + finalize phases need; in particular the
    /// [`FlushInProgressGuard`] is bound to the return value so it stays
    /// alive for the full flush lifetime — including any future async
    /// path where stream runs on a spawned task.
    async fn flush_l0_rotate(&self) -> Result<RotateOutput> {
        // Acquire the in-progress counter BEFORE any heavy work. The
        // guard lives on RotateOutput; dropping RotateOutput drops the
        // guard, so the counter goes back to zero exactly when the flush
        // is fully done.
        let flush_in_progress_guard = FlushInProgressGuard::new(&self.storage);

        // A. Flush WAL BEFORE rotating L0. If WAL flush fails, the
        // current L0 is still active and mutations are retained in
        // memory until restart/retry.
        let wal_for_truncate = {
            let current_l0 = self.l0_manager.get_current();
            let l0_guard = current_l0.read();
            l0_guard.wal.clone()
        };
        // Test-only seam (no-op without the `failpoints` feature): inject a
        // WAL-flush failure here to drive the "failed async rotate wedges the
        // finalizer" regression (Bug #3). When configured to "return" it makes
        // `flush_l0_rotate` return Err exactly as a real WAL-flush failure would.
        fail::fail_point!("flush::rotate-fail", |_| {
            Err(anyhow!("flush::rotate-fail injected WAL-flush failure"))
        });
        let wal_lsn = if let Some(ref w) = wal_for_truncate {
            w.flush().await?
        } else {
            0
        };

        // B. Begin flush: rotate L0 and keep old L0 visible to reads via
        // pending_flush until complete_flush is called by finalize.
        let old_l0_arc = self.l0_manager.begin_flush(0, None);
        metrics::counter!("uni_l0_buffer_rotations_total").increment(1);

        // C. WAL handoff: record wal_lsn on old L0, transfer WAL handle
        // and current_version to the new L0.
        let current_version;
        {
            let mut old_l0_guard = old_l0_arc.write();
            current_version = old_l0_guard.current_version;
            old_l0_guard.wal_lsn_at_flush = wal_lsn;
            let wal = old_l0_guard.wal.take();
            let new_l0_arc = self.l0_manager.get_current();
            let mut new_l0_guard = new_l0_arc.write();
            new_l0_guard.wal = wal;
            new_l0_guard.current_version = current_version;
            // The new active buffer starts accumulating strictly above the
            // rotation point: everything <= `wal_lsn` is now owned by the old
            // (being-flushed) buffer or earlier. This start watermark is the
            // floor that keeps WAL truncation / checkpoint publication from
            // discarding this buffer's data if its eventual flush fails.
            new_l0_guard.wal_lsn_at_start = wal_lsn;
        }

        Ok(RotateOutput {
            old_l0_arc,
            wal_lsn,
            current_version,
            flush_in_progress_guard,
        })
    }

    /// Phases D, E, F, G of the flush: L1 collect, orphan resolve,
    /// manifest seed, Lance writes. Reads from `old_l0_arc` (kept in
    /// pending_flush by Phase B); writes append-only Lance datasets; does
    /// NOT call save_snapshot / set_latest_snapshot — those are
    /// finalize's job, so the manifest doesn't get published until the
    /// next phase.
    ///
    /// Today takes `&self`; in a follow-up commit this becomes a
    /// static `Send + 'static` function over `SharedFlushCtx` so it can
    /// run on a spawned task while concurrent commits proceed.
    async fn flush_stream_l1(
        &self,
        old_l0_arc: Arc<RwLock<L0Buffer>>,
        wal_lsn: u64,
        current_version: u64,
        name: Option<String>,
    ) -> Result<FlushOutcome> {
        // Test-only seam (no-op without the `failpoints` feature): the rotate
        // (begin_flush) already moved the to-be-flushed buffer onto
        // pending_flush and installed a fresh empty current buffer, but the
        // rotated rows are NOT yet durable in Lance. Pausing here holds that
        // window open to drive the unique-constraint-hole regression
        // (Bug #9 Mechanism A).
        fail::fail_point!("flush::after-rotate-before-lance");

        // Phase B: materialize any deferred embeddings before column
        // extraction. No-op when `defer_embeddings` is off (the set will
        // be empty). On-demand reads of the embedding column are a TODO
        // for a future revision (see UniConfig::defer_embeddings docs).
        self.drain_pending_embeddings(&old_l0_arc).await?;

        // Materialise MUVERA FDE columns from each row's source multi-vector (pure/sync;
        // no-op without a MUVERA index). Runs after embeddings so a row can be both
        // auto-embedded and FDE-encoded.
        self.materialize_fde_columns(&old_l0_arc)?;

        let schema = self.schema_manager.schema();
        // 2. Acquire Read lock on Old L0 for flushing
        let mut entries_by_type: HashMap<u32, Vec<L1Entry>> = HashMap::new();
        // (Vid, labels, properties, deleted, version)
        type VertexEntry = (Vid, Vec<String>, Properties, bool, u64);
        let mut vertices_by_label: HashMap<u16, Vec<VertexEntry>> = HashMap::new();
        // Partial-column updates (Lance MergeInsert path). Per-VID tuple:
        // (vid, full L0 properties map, version, set of keys to update).
        // Only the keys in the HashSet are emitted to the partial source;
        // the full props map is retained so the per-row column extractor
        // can read each touched key's value.
        type PartialEntry = (Vid, Properties, u64, std::collections::HashSet<String>);
        let mut partial_by_label: HashMap<u16, Vec<PartialEntry>> = HashMap::new();
        // DELETE-via-MergeInsert (Round-12 §B): tombstones flush as a
        // partial source with just `_vid`, `_deleted=true`, `_version`,
        // `_updated_at`. Skips the wide-row Append payload that adds
        // nothing on a soft-delete.
        let mut tombstones_by_label: HashMap<u16, Vec<(Vid, u64)>> = HashMap::new();
        let mut main_vertex_tombstones: Vec<(Vid, u64)> = Vec::new();
        // Collect vertex timestamps from L0 for flushing to storage
        let mut vertex_created_at: HashMap<Vid, i64> = HashMap::new();
        let mut vertex_updated_at: HashMap<Vid, i64> = HashMap::new();
        // Track tombstones missing labels for storage query fallback
        let mut orphaned_tombstones: Vec<(Vid, u64)> = Vec::new();

        {
            let old_l0 = old_l0_arc.read();

            // 1. Collect all edges and tombstones from L0
            for edge in old_l0.graph.edges() {
                let properties = old_l0
                    .edge_properties
                    .get(&edge.eid)
                    .cloned()
                    .unwrap_or_default();
                let version = old_l0.edge_versions.get(&edge.eid).copied().unwrap_or(0);

                // Get timestamps from L0 buffer (populated during insert)
                let created_at = old_l0.edge_created_at.get(&edge.eid).copied();
                let updated_at = old_l0.edge_updated_at.get(&edge.eid).copied();

                entries_by_type
                    .entry(edge.edge_type)
                    .or_default()
                    .push(L1Entry {
                        src_vid: edge.src_vid,
                        dst_vid: edge.dst_vid,
                        eid: edge.eid,
                        op: Op::Insert,
                        version,
                        properties,
                        created_at,
                        updated_at,
                    });
            }

            // From tombstones
            for tombstone in old_l0.tombstones.values() {
                let version = old_l0
                    .edge_versions
                    .get(&tombstone.eid)
                    .copied()
                    .unwrap_or(0);
                // Get timestamps - for deletes, updated_at reflects deletion time
                let created_at = old_l0.edge_created_at.get(&tombstone.eid).copied();
                let updated_at = old_l0.edge_updated_at.get(&tombstone.eid).copied();

                entries_by_type
                    .entry(tombstone.edge_type)
                    .or_default()
                    .push(L1Entry {
                        src_vid: tombstone.src_vid,
                        dst_vid: tombstone.dst_vid,
                        eid: tombstone.eid,
                        op: Op::Delete,
                        version,
                        properties: HashMap::new(),
                        created_at,
                        updated_at,
                    });
            }

            // 1b. Collect vertices by label (using vertex_labels from L0)
            //
            // Helper: fan-out a single vertex entry into per-label buckets.
            // Each per-label table row carries the full label set so multi-label
            // info is preserved after flush.
            let push_vertex_to_labels =
                |vid: Vid,
                 all_labels: &[String],
                 props: Properties,
                 deleted: bool,
                 version: u64,
                 out: &mut HashMap<u16, Vec<VertexEntry>>| {
                    for label in all_labels {
                        if let Some(label_id) = schema.label_id_by_name(label) {
                            out.entry(label_id).or_default().push((
                                vid,
                                all_labels.to_vec(),
                                props.clone(),
                                deleted,
                                version,
                            ));
                        }
                    }
                };

            for (vid, props) in &old_l0.vertex_properties {
                let version = old_l0.vertex_versions.get(vid).copied().unwrap_or(0);
                // Collect timestamps for this vertex
                if let Some(&ts) = old_l0.vertex_created_at.get(vid) {
                    vertex_created_at.insert(*vid, ts);
                }
                if let Some(&ts) = old_l0.vertex_updated_at.get(vid) {
                    vertex_updated_at.insert(*vid, ts);
                }
                if let Some(labels) = old_l0.vertex_labels.get(vid) {
                    // Partial-write routing: when this VID was last
                    // touched via `insert_vertex_partial` AND the
                    // partial_lance_writes flag is on, send only the
                    // touched columns to a MergeInsert batch. Otherwise
                    // (CREATE, MERGE-ON-CREATE, full-replace SET, DELETE
                    // — or flag off) use the existing full-row Append.
                    let is_partial = self.config.partial_lance_writes
                        && old_l0.vertex_partial_keys.contains_key(vid);
                    if is_partial {
                        if let Some(touched) = old_l0.vertex_partial_keys.get(vid) {
                            for label in labels {
                                if let Some(label_id) = schema.label_id_by_name(label) {
                                    partial_by_label.entry(label_id).or_default().push((
                                        *vid,
                                        props.clone(),
                                        version,
                                        touched.clone(),
                                    ));
                                }
                            }
                        }
                    } else {
                        push_vertex_to_labels(
                            *vid,
                            labels,
                            props.clone(),
                            false,
                            version,
                            &mut vertices_by_label,
                        );
                    }
                }
            }
            for &vid in &old_l0.vertex_tombstones {
                let version = old_l0.vertex_versions.get(&vid).copied().unwrap_or(0);
                if let Some(&ts) = old_l0.vertex_updated_at.get(&vid) {
                    vertex_updated_at.insert(vid, ts);
                }
                if let Some(labels) = old_l0.vertex_labels.get(&vid) {
                    // Round-12 §B: tombstones flush via Lance MergeInsert
                    // (just `_vid`, `_deleted=true`, `_version`,
                    // `_updated_at`) — skipping the wide-row Append.
                    // Unconditional (no `partial_lance_writes` gating);
                    // tombstone Append carries no useful payload.
                    for label in labels {
                        if let Some(label_id) = schema.label_id_by_name(label) {
                            tombstones_by_label
                                .entry(label_id)
                                .or_default()
                                .push((vid, version));
                        }
                    }
                } else {
                    // Tombstone missing labels (old WAL format) - collect for storage query fallback
                    orphaned_tombstones.push((vid, version));
                }
            }
        } // Drop read lock

        // Resolve orphaned tombstones (missing labels) from storage
        if !orphaned_tombstones.is_empty() {
            tracing::warn!(
                count = orphaned_tombstones.len(),
                "Tombstones missing labels in L0, querying storage as fallback"
            );
            for (vid, version) in orphaned_tombstones {
                if let Ok(Some(labels)) = self.find_vertex_labels_in_storage(vid).await
                    && !labels.is_empty()
                {
                    for label in &labels {
                        if let Some(label_id) = schema.label_id_by_name(label) {
                            // Round-12 §B: route through partial tombstone too.
                            tombstones_by_label
                                .entry(label_id)
                                .or_default()
                                .push((vid, version));
                        }
                    }
                }
            }
        }

        // 1. Load previous snapshot from cache, or fall back to storage.
        //
        // Use clone() not take(): for the async path, multiple
        // concurrent streams may run; if we take() here, a sibling
        // stream sees cached_manifest = None and seeds from
        // load_latest_snapshot (stale), losing the chain. clone()
        // preserves the parent. Finalize writes back the new manifest
        // unconditionally.
        let mut manifest = if let Some(cached) = self.cached_manifest.lock().clone() {
            cached
        } else {
            self.storage
                .snapshot_manager()
                .load_latest_snapshot()
                .await?
                .unwrap_or_else(|| {
                    SnapshotManifest::new(Uuid::new_v4().to_string(), schema.schema_version)
                })
        };

        // Update snapshot metadata
        // Save parent snapshot ID before generating new one (for lineage tracking)
        let parent_id = manifest.snapshot_id.clone();
        manifest.parent_snapshot = Some(parent_id);
        manifest.snapshot_id = Uuid::new_v4().to_string();
        manifest.name = name;
        manifest.created_at = Utc::now();
        manifest.version_high_water_mark = current_version;
        // Cap the published WAL checkpoint at the floor of any OTHER pending
        // flush. A still-pending flush (notably one that FAILED and left its
        // buffer in `pending_flush`) holds committed WAL entries above its start
        // that are NOT in this snapshot; recovery replays from this mark, so
        // claiming durability past that floor would skip them (lost commit). A
        // normal flush with no other pending buffer keeps `wal_lsn` unchanged.
        manifest.wal_high_water_mark = self
            .l0_manager
            .min_pending_wal_lsn_start(&old_l0_arc)
            .map_or(wal_lsn, |floor| floor.min(wal_lsn));
        let snapshot_id = manifest.snapshot_id.clone();

        tracing::Span::current().record("snapshot_id", &snapshot_id);

        // 2. Write main unified tables FIRST (before deltas).
        //    Ensures the dual-write invariant: by the time an EID appears in a
        //    delta table, it already exists in main_edges. This prevents the
        //    compaction debug_assert from firing when compaction interleaves
        //    with flush at async yield points.
        //
        // 2.1 Main edges table
        let (main_edges, edge_created_at_map, edge_updated_at_map) = {
            let _old_l0 = old_l0_arc.read();
            let mut main_edges: Vec<(
                uni_common::core::id::Eid,
                Vid,
                Vid,
                String,
                Properties,
                bool,
                u64,
            )> = Vec::new();
            let mut edge_created_at_map: HashMap<uni_common::core::id::Eid, i64> = HashMap::new();
            let mut edge_updated_at_map: HashMap<uni_common::core::id::Eid, i64> = HashMap::new();

            for (&edge_type_id, entries) in entries_by_type.iter() {
                for entry in entries {
                    let edge_type_name = self
                        .storage
                        .schema_manager()
                        .edge_type_name_by_id_unified(edge_type_id)
                        .unwrap_or_else(|| "unknown".to_string());

                    let deleted = matches!(entry.op, Op::Delete);
                    main_edges.push((
                        entry.eid,
                        entry.src_vid,
                        entry.dst_vid,
                        edge_type_name,
                        entry.properties.clone(),
                        deleted,
                        entry.version,
                    ));

                    if let Some(ts) = entry.created_at {
                        edge_created_at_map.insert(entry.eid, ts);
                    }
                    if let Some(ts) = entry.updated_at {
                        edge_updated_at_map.insert(entry.eid, ts);
                    }
                }
            }

            (main_edges, edge_created_at_map, edge_updated_at_map)
        };

        if !main_edges.is_empty() {
            let main_edge_batch = MainEdgeDataset::build_record_batch(
                &main_edges,
                Some(&edge_created_at_map),
                Some(&edge_updated_at_map),
            )?;
            MainEdgeDataset::write_batch(self.storage.backend(), main_edge_batch).await?;
            MainEdgeDataset::ensure_default_indexes(self.storage.backend()).await?;
        }

        // 2.2 Main vertices table
        let mut main_vertices: Vec<(Vid, Vec<String>, Properties, bool, u64)> = {
            let old_l0 = old_l0_arc.read();
            let mut vertices = Vec::new();

            // Live vertices: full-row Append on the main table (the
            // props_json blob is required for global ID lookups). For
            // partial-row VIDs (vertex_partial_keys non-empty), the
            // main table still needs the full props for the
            // ext_id-uniqueness path; we keep the Append here. The
            // per-label Lance write IS partial via MergeInsert.
            for (vid, props) in &old_l0.vertex_properties {
                let version = old_l0.vertex_versions.get(vid).copied().unwrap_or(0);
                let labels = old_l0.vertex_labels.get(vid).cloned().unwrap_or_default();
                vertices.push((*vid, labels, props.clone(), false, version));
            }

            // Tombstones: collected into `main_vertex_tombstones` for
            // the MergeInsert path below; skipping the wide-row Append.
            for &vid in &old_l0.vertex_tombstones {
                let version = old_l0.vertex_versions.get(&vid).copied().unwrap_or(0);
                main_vertex_tombstones.push((vid, version));
            }

            vertices
        };

        // M8: durable label-only mutations across flush windows.
        //
        // `SET n:Label` / `REMOVE n:Label` mark the vid in
        // `vertex_label_overwrites` and update `vertex_labels`, but for a
        // vid flushed in a PRIOR window they never re-add it to
        // `vertex_properties`. The loops above key off `vertex_properties`,
        // so such a relabel would be silently lost: absent from the main
        // table, the per-label datasets, and the VidLabelsIndex (and so
        // `rebuild_vid_labels_index` reads stale labels after a restart).
        // The same-window create+relabel case already works because the
        // create put the vid in `vertex_properties`.
        //
        // Re-derive each overwrite-only vid by fetching its persisted props
        // and labels, then route it into `main_vertices` (main table +
        // index), the new per-label datasets, and a tombstone in any
        // per-label dataset it left. `MATCH (n:OldLabel)` scans the
        // per-label table directly, so the old-label tombstone is required.
        let overwrite_only: Vec<(Vid, Vec<String>, u64)> = {
            let old_l0 = old_l0_arc.read();
            old_l0
                .vertex_label_overwrites
                .iter()
                .filter(|vid| {
                    !old_l0.vertex_properties.contains_key(*vid)
                        && !old_l0.vertex_tombstones.contains(*vid)
                })
                .map(|vid| {
                    let labels = old_l0.vertex_labels.get(vid).cloned().unwrap_or_default();
                    let version = old_l0.vertex_versions.get(vid).copied().unwrap_or(0);
                    (*vid, labels, version)
                })
                .collect()
        };
        for (vid, new_labels, version) in overwrite_only {
            // Persisted props of the prior-window row — required so the
            // re-Appended main row does not blank the vertex's properties.
            let Some(props) = MainVertexDataset::find_props_by_vid(
                self.storage.backend(),
                vid,
                self.storage.version_high_water_mark(),
            )
            .await?
            else {
                tracing::warn!(
                    vid = vid.as_u64(),
                    "label-only mutation for a vid with no persisted main row; skipping flush \
                     of its relabel"
                );
                continue;
            };
            // Labels the vid carried BEFORE this relabel; the storage read
            // reflects pre-flush state. Any label no longer present must be
            // tombstoned in its per-label dataset.
            let old_labels = self
                .find_vertex_labels_in_storage(vid)
                .await?
                .unwrap_or_default();

            main_vertices.push((vid, new_labels.clone(), props.clone(), false, version));
            for label in &new_labels {
                if let Some(label_id) = schema.label_id_by_name(label) {
                    vertices_by_label.entry(label_id).or_default().push((
                        vid,
                        new_labels.clone(),
                        props.clone(),
                        false,
                        version,
                    ));
                }
            }
            for label in &old_labels {
                if !new_labels.contains(label)
                    && let Some(label_id) = schema.label_id_by_name(label)
                {
                    tombstones_by_label
                        .entry(label_id)
                        .or_default()
                        .push((vid, version));
                }
            }
        }

        if !main_vertices.is_empty() {
            let main_vertex_batch = MainVertexDataset::build_record_batch(
                &main_vertices,
                Some(&vertex_created_at),
                Some(&vertex_updated_at),
            )?;
            MainVertexDataset::write_batch(self.storage.backend(), main_vertex_batch).await?;
        }
        // Round-12 §B: tombstones via MergeInsert on the main vertices
        // table. Independent of `vertex_properties` length.
        if !main_vertex_tombstones.is_empty() {
            let tomb_batch = MainVertexDataset::build_tombstone_partial_batch(
                &main_vertex_tombstones,
                Some(&vertex_updated_at),
            )?;
            MainVertexDataset::merge_insert_tombstone_batch(self.storage.backend(), tomb_batch)
                .await?;
        }
        if !main_vertices.is_empty() || !main_vertex_tombstones.is_empty() {
            MainVertexDataset::ensure_default_indexes(self.storage.backend()).await?;
        }

        // Keep the VidLabelsIndex current for every flushed vertex. This is the
        // single place that sees all vertices: the per-label fan-out below skips
        // undeclared (schemaless) labels, so updating the index there would miss
        // them. Traversal-time label predicates read this index to resolve
        // labels for vertices that live only in Lance — notably on a fork, whose
        // data is flushed to Lance before branching. (GitHub #99)
        for (vid, labels, _props, _deleted, _version) in &main_vertices {
            self.storage.update_vid_labels_index(*vid, labels.clone());
        }
        for (vid, _version) in &main_vertex_tombstones {
            self.storage.remove_from_vid_labels_index(*vid);
        }

        // 3. For each edge type, write FWD and BWD delta runs
        for (&edge_type_id, entries) in entries_by_type.iter() {
            // Get edge type name from unified lookup (handles both schema'd and schemaless)
            let edge_type_name = self
                .storage
                .schema_manager()
                .edge_type_name_by_id_unified(edge_type_id)
                .ok_or_else(|| anyhow!("Edge type ID {} not found", edge_type_id))?;

            // FWD Run (sorted by src_vid)
            // Round-12 §A: split entries into full-row Append and
            // partial MergeInsert routes based on `edge_partial_keys`.
            // Edges in `edge_partial_keys` were last written via
            // `insert_edge_partial_full`; the per-edge-type delta
            // tables receive only the touched schema columns plus
            // (when any overflow key was touched) the regenerated
            // `overflow_json` blob. Untouched columns retain their
            // previous-version value via Lance MergeInsert.
            let partial_eids: std::collections::HashSet<Eid> = {
                let old_l0 = old_l0_arc.read();
                entries
                    .iter()
                    .filter(|e| {
                        self.config.partial_lance_writes
                            && old_l0.edge_partial_keys.contains_key(&e.eid)
                    })
                    .map(|e| e.eid)
                    .collect()
            };
            let touched_union_by_eid: HashMap<Eid, std::collections::HashSet<String>> = {
                let old_l0 = old_l0_arc.read();
                partial_eids
                    .iter()
                    .filter_map(|eid| old_l0.edge_partial_keys.get(eid).map(|s| (*eid, s.clone())))
                    .collect()
            };
            let (full_entries, partial_entries): (Vec<L1Entry>, Vec<L1Entry>) = entries
                .clone()
                .into_iter()
                .partition(|e| !partial_eids.contains(&e.eid));

            let backend = self.storage.backend();

            // FWD run (sorted by src_vid)
            let mut fwd_full = full_entries.clone();
            fwd_full.sort_by_key(|e| e.src_vid);
            let mut fwd_partial = partial_entries.clone();
            fwd_partial.sort_by_key(|e| e.src_vid);
            let fwd_ds = self.storage.delta_dataset(&edge_type_name, "fwd")?;
            if !fwd_full.is_empty() {
                let fwd_batch = fwd_ds.build_record_batch(&fwd_full, &schema)?;
                fwd_ds.write_run(backend, fwd_batch).await?;
            }
            if !fwd_partial.is_empty() {
                let touched_union: std::collections::HashSet<String> = fwd_partial
                    .iter()
                    .flat_map(|e| {
                        touched_union_by_eid
                            .get(&e.eid)
                            .cloned()
                            .unwrap_or_default()
                            .into_iter()
                    })
                    .collect();
                let fwd_partial_batch =
                    fwd_ds.build_partial_record_batch(&fwd_partial, &touched_union, &schema)?;
                fwd_ds
                    .merge_insert_partial_run(backend, fwd_partial_batch)
                    .await?;
            }
            fwd_ds.ensure_eid_index(backend).await?;

            // BWD Run (sorted by dst_vid)
            let mut bwd_full = full_entries.clone();
            bwd_full.sort_by_key(|e| e.dst_vid);
            let mut bwd_partial = partial_entries.clone();
            bwd_partial.sort_by_key(|e| e.dst_vid);
            let bwd_ds = self.storage.delta_dataset(&edge_type_name, "bwd")?;
            if !bwd_full.is_empty() {
                let bwd_batch = bwd_ds.build_record_batch(&bwd_full, &schema)?;
                bwd_ds.write_run(backend, bwd_batch).await?;
            }
            if !bwd_partial.is_empty() {
                let touched_union: std::collections::HashSet<String> = bwd_partial
                    .iter()
                    .flat_map(|e| {
                        touched_union_by_eid
                            .get(&e.eid)
                            .cloned()
                            .unwrap_or_default()
                            .into_iter()
                    })
                    .collect();
                let bwd_partial_batch =
                    bwd_ds.build_partial_record_batch(&bwd_partial, &touched_union, &schema)?;
                bwd_ds
                    .merge_insert_partial_run(backend, bwd_partial_batch)
                    .await?;
            }
            bwd_ds.ensure_eid_index(backend).await?;

            // Update Manifest
            let current_snap =
                manifest
                    .edges
                    .entry(edge_type_name.to_string())
                    .or_insert(EdgeSnapshot {
                        version: 0,
                        count: 0,
                        lance_version: 0,
                    });
            current_snap.version += 1;
            current_snap.count += entries.len() as u64;
            // LanceDB tables don't expose Lance version directly
            current_snap.lance_version = 0;

            // Note: No CSR invalidation needed. AdjacencyManager's overlay
            // already has these edges via dual-write in insert_edge/delete_edge.
        }

        // 4. Per-label vertex table writes
        // Iterate all labels that have either full-row OR partial-write
        // data pending. A label may appear in only one of the two maps
        // (e.g., all updates on this label were partial-only).
        let all_label_ids: std::collections::HashSet<u16> = vertices_by_label
            .keys()
            .chain(partial_by_label.keys())
            .chain(tombstones_by_label.keys())
            .copied()
            .collect();
        for label_id in all_label_ids {
            let vertices = vertices_by_label.remove(&label_id).unwrap_or_default();
            let label_name = schema
                .label_name_by_id(label_id)
                .ok_or_else(|| anyhow!("Label ID {} not found", label_id))?;

            let ds = self.storage.vertex_dataset(label_name)?;

            // Collect inverted index updates before consuming vertices
            // Maps: cfg.property -> (added, removed)
            type InvertedUpdateMap = HashMap<String, (HashMap<Vid, Vec<String>>, HashSet<Vid>)>;
            let mut inverted_updates: InvertedUpdateMap = HashMap::new();

            for idx in &schema.indexes {
                if let IndexDefinition::Inverted(cfg) = idx
                    && cfg.label == label_name
                {
                    let mut added: HashMap<Vid, Vec<String>> = HashMap::new();
                    let mut removed: HashSet<Vid> = HashSet::new();

                    for (vid, _labels, props, deleted, _version) in &vertices {
                        if *deleted {
                            removed.insert(*vid);
                        } else if let Some(prop_value) = props.get(&cfg.property) {
                            // Extract terms from the property value (List<String>)
                            if let Some(arr) = prop_value.as_array() {
                                let terms: Vec<String> = arr
                                    .iter()
                                    .filter_map(|v| v.as_str().map(ToString::to_string))
                                    .collect();
                                if !terms.is_empty() {
                                    added.insert(*vid, terms);
                                }
                            }
                        }
                    }
                    // Round-12 §B: tombstones no longer in `vertices`;
                    // pull them from `tombstones_by_label` for inverted
                    // index removal.
                    if let Some(tomb_rows) = tombstones_by_label.get(&label_id) {
                        for (vid, _) in tomb_rows {
                            removed.insert(*vid);
                        }
                    }

                    if !added.is_empty() || !removed.is_empty() {
                        inverted_updates.insert(cfg.property.clone(), (added, removed));
                    }
                }
            }

            // Collect sparse-vector index updates before consuming vertices.
            // Maps: cfg.property -> (added [(term_id, weight)] per vid, removed).
            type SparseUpdateMap = HashMap<String, (HashMap<Vid, Vec<(u32, f32)>>, HashSet<Vid>)>;
            let mut sparse_updates: SparseUpdateMap = HashMap::new();

            for idx in &schema.indexes {
                if let IndexDefinition::Sparse(cfg) = idx
                    && cfg.label == label_name
                {
                    let mut added: HashMap<Vid, Vec<(u32, f32)>> = HashMap::new();
                    let mut removed: HashSet<Vid> = HashSet::new();

                    for (vid, _labels, props, deleted, _version) in &vertices {
                        if *deleted {
                            removed.insert(*vid);
                        } else if let Some(uni_common::Value::SparseVector { indices, values }) =
                            props.get(&cfg.property)
                        {
                            let pairs: Vec<(u32, f32)> = indices
                                .iter()
                                .copied()
                                .zip(values.iter().copied())
                                .collect();
                            added.insert(*vid, pairs);
                            // An in-place SET re-flushes an already-indexed vid. The
                            // sparse postings are a `Vec` with no per-vid dedup, so unless
                            // the vid is also marked removed, `apply_incremental_updates`
                            // appends the new postings *alongside* the stale ones — leaking
                            // duplicates that grow unboundedly on hot-updated docs and
                            // double-count the advisory `query_topk` score (issue #95).
                            // Mark every updated vid removed so its prior postings are
                            // purged before the new ones are appended (remove-then-add).
                            removed.insert(*vid);
                        }
                    }
                    // Tombstones are not in `vertices`; pull from tombstones_by_label.
                    if let Some(tomb_rows) = tombstones_by_label.get(&label_id) {
                        for (vid, _) in tomb_rows {
                            removed.insert(*vid);
                        }
                    }

                    if !added.is_empty() || !removed.is_empty() {
                        sparse_updates.insert(cfg.property.clone(), (added, removed));
                    }
                }
            }

            let mut v_data = Vec::new();
            let mut d_data = Vec::new();
            let mut ver_data = Vec::new();
            for (vid, labels, props, deleted, version) in vertices {
                v_data.push((vid, labels, props));
                d_data.push(deleted);
                ver_data.push(version);
            }

            let backend = self.storage.backend();

            // Skip the full-row Append entirely if this label only has
            // partial-write rows pending.
            if !v_data.is_empty() {
                let batch = ds.build_record_batch_with_timestamps(
                    &v_data,
                    &d_data,
                    &ver_data,
                    &schema,
                    Some(&vertex_created_at),
                    Some(&vertex_updated_at),
                )?;
                ds.write_batch(backend, batch, &schema).await?;
            }

            // Partial-column batch (Lance MergeInsert path). The flag
            // gates whether the routing classified any VIDs as partial;
            // outside the flag this collection is always empty so the
            // call below is a cheap no-op.
            if let Some(partial_rows) = partial_by_label.remove(&label_id)
                && !partial_rows.is_empty()
            {
                let touched_union: std::collections::HashSet<String> = partial_rows
                    .iter()
                    .flat_map(|(_, _, _, keys)| keys.iter().cloned())
                    .collect();
                let pairs: Vec<(Vid, Properties)> = partial_rows
                    .iter()
                    .map(|(vid, props, _, _)| (*vid, props.clone()))
                    .collect();
                let versions: Vec<u64> = partial_rows.iter().map(|(_, _, v, _)| *v).collect();
                let partial_batch = ds.build_partial_record_batch(
                    &pairs,
                    &versions,
                    &touched_union,
                    &schema,
                    Some(&vertex_updated_at),
                )?;
                if partial_batch.num_rows() > 0 {
                    ds.merge_insert_batch(backend, partial_batch).await?;
                }
            }

            // Tombstone batch (Round-12 §B): always MergeInsert with
            // just `_vid`, `_deleted=true`, `_version`, `_updated_at`.
            // No partial_lance_writes gating — tombstones never carry
            // useful property payload to write. Captured tombstone vids
            // also drive `remove_from_vid_labels_index` below.
            let tombstone_rows = tombstones_by_label.remove(&label_id).unwrap_or_default();
            if !tombstone_rows.is_empty() {
                let tomb_batch =
                    ds.build_tombstone_partial_batch(&tombstone_rows, Some(&vertex_updated_at))?;
                if tomb_batch.num_rows() > 0 {
                    ds.merge_insert_batch(backend, tomb_batch).await?;
                }
            }

            ds.ensure_default_indexes(backend).await?;

            // VidLabelsIndex maintenance is centralized at the main-vertex
            // flush above (it sees both schema'd and schemaless vertices).

            // Update Manifest
            let current_snap =
                manifest
                    .vertices
                    .entry(label_name.to_string())
                    .or_insert(LabelSnapshot {
                        version: 0,
                        count: 0,
                        lance_version: 0,
                    });
            current_snap.version += 1;
            current_snap.count += v_data.len() as u64;
            // LanceDB tables don't expose Lance version directly
            current_snap.lance_version = 0;

            // Invalidate table cache to ensure next read picks up new version
            self.storage.invalidate_table_cache(label_name);

            // Apply inverted index updates incrementally
            #[cfg(feature = "lance-backend")]
            for idx in &schema.indexes {
                if let IndexDefinition::Inverted(cfg) = idx
                    && cfg.label == label_name
                    && let Some((added, removed)) = inverted_updates.get(&cfg.property)
                {
                    self.storage
                        .index_manager()
                        .update_inverted_index_incremental(cfg, added, removed)
                        .await?;
                }
            }

            // Apply sparse-vector index updates incrementally
            #[cfg(feature = "lance-backend")]
            for idx in &schema.indexes {
                if let IndexDefinition::Sparse(cfg) = idx
                    && cfg.label == label_name
                    && let Some((added, removed)) = sparse_updates.get(&cfg.property)
                {
                    self.storage
                        .index_manager()
                        .update_sparse_vector_index_incremental(cfg, added, removed)
                        .await?;
                }
            }

            // Update UID index with new vertex mappings
            // Collect (UniId, Vid) mappings from non-deleted vertices
            #[cfg(feature = "lance-backend")]
            {
                let mut uid_mappings: Vec<(uni_common::core::id::UniId, Vid)> = Vec::new();
                for (vid, _labels, props) in &v_data {
                    let ext_id = props.get("ext_id").and_then(|v| v.as_str());
                    let uid = crate::storage::vertex::VertexDataset::compute_vertex_uid(
                        label_name, ext_id, props,
                    );
                    uid_mappings.push((uid, *vid));
                }

                if !uid_mappings.is_empty()
                    && let Ok(uid_index) = self.storage.uid_index(label_name)
                {
                    // Stamp mappings with this flush's MVCC version so a later
                    // re-create of the same UID deterministically outranks the
                    // stale mapping (review C3).
                    uid_index
                        .write_mapping_versioned(&uid_mappings, current_version)
                        .await?;
                }
            }
        }
        Ok(FlushOutcome {
            manifest,
            snapshot_id,
        })
    }

    /// Composition entry that assumes the caller already holds `flush_lock`.
    /// Runs rotate + stream + finalize_locked in sequence. Used by
    /// [`Writer::flush_to_l1`] (acquires the lock first) and by
    /// `commit_transaction_l0`'s post-merge auto-flush branch (which already
    /// holds the lock from the commit critical section).
    #[instrument(
        skip(self),
        fields(snapshot_id, mutations_count, size_bytes),
        level = "info"
    )]
    async fn flush_inline_under_lock(&self, name: Option<String>) -> Result<String> {
        let start = std::time::Instant::now();

        let (initial_size, initial_count) = {
            let l0_arc = self.l0_manager.get_current();
            let l0 = l0_arc.read();
            (l0.estimated_size, l0.mutation_count)
        };
        tracing::Span::current().record("size_bytes", initial_size);
        tracing::Span::current().record("mutations_count", initial_count);

        debug!("Starting L0 flush to L1");

        // Phases A (WAL pre-flush), B (rotate), C (WAL handoff).
        // FlushInProgressGuard lives on RotateOutput and stays alive for
        // the full flush — including the finalize_locked call below.
        let RotateOutput {
            old_l0_arc,
            wal_lsn,
            current_version,
            flush_in_progress_guard: _flush_guard,
        } = self.flush_l0_rotate().await?;

        // Phases D (L1 collect), E (orphan resolve), F (manifest seed),
        // G (Lance writes). Builds the manifest but does NOT publish it.
        let FlushOutcome {
            manifest,
            snapshot_id,
        } = self
            .flush_stream_l1(old_l0_arc.clone(), wal_lsn, current_version, name)
            .await?;

        // Phases H..S: publish manifest, complete_flush, WAL truncate,
        // property cache clear, last_flush_time, metrics, l1_runs++,
        // compaction trigger, index-rebuild scheduling, fork tick.
        self.flush_finalize_locked(
            old_l0_arc,
            wal_lsn,
            manifest,
            snapshot_id,
            initial_size,
            initial_count,
            start,
        )
        .await
    }

    /// Phases H..S of the flush: publish the manifest and run all
    /// post-publish bookkeeping. Assumes the caller already holds
    /// `flush_lock` — see [`Writer::flush_finalize_now`] for the
    /// lock-acquiring variant used by the async finalize path.
    #[allow(clippy::too_many_arguments)]
    async fn flush_finalize_locked(
        &self,
        old_l0_arc: Arc<RwLock<L0Buffer>>,
        wal_lsn: u64,
        manifest: SnapshotManifest,
        snapshot_id: String,
        initial_size: usize,
        initial_count: usize,
        start: std::time::Instant,
    ) -> Result<String> {
        Self::flush_finalize_body(
            &self.shared_ctx(),
            old_l0_arc,
            wal_lsn,
            manifest,
            snapshot_id,
            initial_size,
            initial_count,
            start,
        )
        .await
    }

    /// Phases H..S of the flush, lock-acquiring variant. Used by the
    /// async-flush finalizer task (running on a spawned tokio task),
    /// which holds neither `&self` nor `flush_lock`. Briefly re-acquires
    /// `flush_lock` to serialize the publish boundary, then runs the
    /// same body as `flush_finalize_locked` but over a SharedFlushCtx.
    #[allow(clippy::too_many_arguments)]
    pub(crate) async fn flush_finalize_now(
        shared: SharedFlushCtx,
        old_l0_arc: Arc<RwLock<L0Buffer>>,
        wal_lsn: u64,
        manifest: SnapshotManifest,
        snapshot_id: String,
        initial_size: usize,
        initial_count: usize,
        start: std::time::Instant,
    ) -> Result<String> {
        let _flush_lock_guard = shared.flush_lock.clone().lock_owned().await;
        Self::flush_finalize_body(
            &shared,
            old_l0_arc,
            wal_lsn,
            manifest,
            snapshot_id,
            initial_size,
            initial_count,
            start,
        )
        .await
    }

    /// Shared body of `flush_finalize_locked` and `flush_finalize_now`.
    /// Static over `SharedFlushCtx`; the caller is responsible for
    /// holding `flush_lock`.
    #[allow(clippy::too_many_arguments)]
    async fn flush_finalize_body(
        shared: &SharedFlushCtx,
        old_l0_arc: Arc<RwLock<L0Buffer>>,
        wal_lsn: u64,
        mut manifest: SnapshotManifest,
        snapshot_id: String,
        initial_size: usize,
        initial_count: usize,
        start: std::time::Instant,
    ) -> Result<String> {
        // Parent-snapshot fixup. The stream phase built `manifest` with
        // parent_snapshot set from cached_manifest at stream time. If
        // OTHER flushes (sync or async) have finalized since then,
        // cached_manifest has advanced. Re-link this manifest to the
        // current cached chain so we don't orphan their data when we
        // overwrite cached_manifest below.
        let current_parent_id = shared
            .cached_manifest
            .lock()
            .as_ref()
            .map(|m| m.snapshot_id.clone());
        if current_parent_id.is_some() && manifest.parent_snapshot != current_parent_id {
            manifest.parent_snapshot = current_parent_id;
            metrics::counter!("uni_flush_parent_chain_fixups_total").increment(1);
        }

        // H. Publish manifest (body first, then pointer — recovery is
        // idempotent if we crash between the two).
        // A fork writer must publish to a fork-scoped namespace, never the
        // global `catalog/latest` that the primary reopen reads (review C1).
        debug_assert_eq!(
            shared.fork_id.is_some(),
            shared.storage.snapshot_manager().is_fork_scoped(),
            "fork writer must publish to a fork-scoped snapshot namespace (review C1)"
        );
        shared
            .storage
            .snapshot_manager()
            .save_snapshot(&manifest)
            .await?;
        shared
            .storage
            .snapshot_manager()
            .set_latest_snapshot(&manifest.snapshot_id)
            .await?;

        // H2. Durability barrier (review C4). `save_snapshot` / `set_latest_snapshot`
        // wrote the manifest body and the `catalog/latest` pointer through the
        // object store, which does NOT fsync. WAL truncation (K, below) removes
        // the only other durable copy of this flush's data, so a crash after K
        // but before the OS flushed those writes would lose the snapshot —
        // recovery could not resolve `latest`. Make them durable now (local-fs
        // only; remote stores provide their own durability on `put`).
        crate::snapshot::manager::fsync_snapshot_pointer(
            shared.storage.local_fs_root().as_deref(),
            shared.fork_id.as_ref(),
            &manifest.snapshot_id,
        )
        .map_err(|e| {
            anyhow!(
                "fsync snapshot {} before WAL truncate: {}",
                manifest.snapshot_id,
                e
            )
        })?;

        // I. Cache manifest for next flush to avoid re-reading from object store.
        *shared.cached_manifest.lock() = Some(manifest.clone());

        // L. Invalidate the property cache BEFORE removing the flushed buffer
        // from the L0 chain (Bug #10). `clear_cache` has no dependency on the
        // complete_flush (J) / WAL-truncate (K) steps below, so clearing it
        // first closes the non-monotonic-read window: once the buffer leaves
        // the L0 chain at J a freshly-written value would otherwise miss the
        // chain and fall through to a stale cache entry. By the time finalize
        // runs the streamed rows are already durable in L1, so a post-clear
        // read falls through to fresh storage instead. The finalizer holds
        // `flush_lock` throughout, so reordering L ahead of J is safe.
        if let Some(ref pm) = shared.property_manager {
            pm.clear_cache().await;
        }

        // J. Complete flush: remove old L0 from pending_flush. MUST happen
        // BEFORE WAL truncation so min_pending_wal_lsn is accurate.
        shared.l0_manager.complete_flush(&old_l0_arc);

        // Test-only seam (no-op without the `failpoints` feature): pause AFTER
        // complete_flush removed the buffer from the L0 chain (J) but BEFORE
        // WAL truncation (K). The property cache is already cleared (L moved
        // ahead of J above), so a read in this window falls through to fresh
        // L1 storage rather than a stale cache entry (Bug #10 — non-monotonic
        // read after flush finalize).
        fail::fail_point!("flush::after-complete-before-cache-clear");

        // K. Truncate WAL up to the safe LSN. The floor is the START watermark
        // of any OTHER pending flush (this flush's own buffer was removed from
        // pending by `complete_flush` in J, so it is excluded): a pending — e.g.
        // failed — flush's committed entries live above its start and are not yet
        // in L1, so truncating to its high watermark would delete its own data
        // (the lost-commit-on-graceful-close bug).
        let wal_handle = shared.l0_manager.get_current().read().wal.clone();
        if let Some(w) = wal_handle {
            let safe_lsn = shared
                .l0_manager
                .min_pending_wal_lsn_start(&old_l0_arc)
                .map_or(wal_lsn, |floor| floor.min(wal_lsn));
            w.truncate_before(safe_lsn).await?;
        }

        // M. Reset last flush time for time-based auto-flush.
        *shared.last_flush_time.lock() = std::time::Instant::now();

        info!(
            snapshot_id,
            mutations_count = initial_count,
            size_bytes = initial_size,
            "L0 flush to L1 completed successfully"
        );
        metrics::histogram!("uni_flush_duration_seconds").record(start.elapsed().as_secs_f64());
        metrics::counter!("uni_flush_bytes_total").increment(initial_size as u64);
        metrics::counter!("uni_flush_rows_total").increment(initial_count as u64);

        // P. Increment flush generation counter for write throttling.
        {
            let mut status = uni_common::sync::acquire_mutex(
                &shared.storage.compaction_status,
                "compaction_status",
            )?;
            status.l1_runs += 1;
        }

        // Q. Trigger CSR compaction if enough frozen segments have accumulated.
        let am = shared.adjacency_manager.clone();
        if am.should_compact(shared.compaction_config.frozen_segments_compact_threshold) {
            let previous_still_running = {
                let guard = shared.compaction_handle.read();
                guard.as_ref().is_some_and(|h| !h.is_finished())
            };
            if previous_still_running {
                info!("Skipping compaction: previous compaction still in progress");
            } else {
                let handle = tokio::spawn(async move {
                    am.compact();
                });
                *shared.compaction_handle.write() = Some(handle);
            }
        }

        // R. Post-flush: check if any indexes need rebuilding based on thresholds.
        if shared.auto_rebuild_enabled
            && let Some(rebuild_mgr) = shared.index_rebuild_manager.get()
        {
            Self::schedule_index_rebuilds_if_needed_static(
                &manifest,
                rebuild_mgr.clone(),
                shared.schema_manager.clone(),
                shared.index_rebuild_config.clone(),
            );
        }

        // S. Emit fork-fragment observability after a successful forked flush.
        Self::tick_fork_fragment_observability_static(
            shared.fork_id,
            shared.fork_flush_count.clone(),
            shared.fork_fragment_warn_fired.clone(),
            shared.fork_fragment_warn_threshold,
        );

        Ok(snapshot_id)
    }

    /// Increment fork-flush bookkeeping and fire the fragment warn
    /// once if the threshold is crossed.
    ///
    /// Each flush typically appends ~1 fragment per touched dataset on
    /// the fork's branches; without compaction (deferred to Phase 5)
    /// long-lived heavy-write forks degrade. The flush count is a
    /// proxy for actual fragment growth — reading
    /// `Dataset::manifest().fragments.len()` per dataset would add a
    /// per-flush object-store roundtrip on the hot commit path, which
    /// is too costly for a purely observational guard rail.
    ///
    /// No-op for primary writers (`fork_id == None`).
    #[allow(dead_code)] // called by tests; production path uses _static
    pub(crate) fn tick_fork_fragment_observability(&self) {
        Self::tick_fork_fragment_observability_static(
            self.fork_id,
            self.fork_flush_count.clone(),
            self.fork_fragment_warn_fired.clone(),
            self.config.fork_fragment_warn_threshold,
        );
    }

    /// Static variant of [`Writer::tick_fork_fragment_observability`].
    /// Used by the async-flush finalize path, where we hold a
    /// [`SharedFlushCtx`] bundle of Arcs rather than `&Writer`.
    pub(crate) fn tick_fork_fragment_observability_static(
        fork_id: Option<ForkId>,
        fork_flush_count: Arc<AtomicU64>,
        fork_fragment_warn_fired: Arc<AtomicBool>,
        warn_threshold: usize,
    ) {
        let Some(fork_id) = fork_id else { return };
        // `Relaxed` is sufficient: observational counter, no synchronizes-with.
        let new_count = fork_flush_count.fetch_add(1, Ordering::Relaxed) + 1;
        let fork_label = fork_id.to_string();
        metrics::gauge!(
            "uni_fork_l1_flushes",
            "fork" => fork_label.clone(),
        )
        .set(new_count as f64);
        let threshold = warn_threshold as u64;
        if !fork_fragment_warn_fired.load(Ordering::Relaxed)
            && threshold > 0
            && new_count >= threshold
        {
            fork_fragment_warn_fired.store(true, Ordering::Relaxed);
            tracing::warn!(
                fork = %fork_label,
                flush_count = new_count,
                threshold,
                "fork has exceeded the L1 flush-count threshold; \
                 fork compaction is deferred to Phase 5 — consider \
                 drop+recreate or promotion to bound fragment growth"
            );
        }
    }

    /// Check rebuild thresholds and schedule background index rebuilds for
    /// labels that exceed growth or age limits. Marks affected indexes as
    /// `Stale` and spawns an async task to schedule the rebuild.
    #[allow(dead_code)] // production path uses _static; kept as the
    // documented instance entry point.
    fn schedule_index_rebuilds_if_needed(
        &self,
        manifest: &SnapshotManifest,
        rebuild_mgr: Arc<crate::storage::index_rebuild::IndexRebuildManager>,
    ) {
        Self::schedule_index_rebuilds_if_needed_static(
            manifest,
            rebuild_mgr,
            self.schema_manager.clone(),
            self.config.index_rebuild.clone(),
        );
    }

    /// Static variant of [`Writer::schedule_index_rebuilds_if_needed`].
    /// Used by the async-flush finalize path, where we hold the
    /// [`SchemaManager`] via `SharedFlushCtx` rather than `&Writer`.
    pub(crate) fn schedule_index_rebuilds_if_needed_static(
        manifest: &SnapshotManifest,
        rebuild_mgr: Arc<crate::storage::index_rebuild::IndexRebuildManager>,
        schema_manager: Arc<uni_common::core::schema::SchemaManager>,
        index_rebuild_config: uni_common::config::IndexRebuildConfig,
    ) {
        let checker =
            crate::storage::index_rebuild::RebuildTriggerChecker::new(index_rebuild_config);
        let schema = schema_manager.schema();
        let labels = checker.labels_needing_rebuild(manifest, &schema.indexes);

        if labels.is_empty() {
            return;
        }

        // Mark affected indexes as Stale
        for label in &labels {
            for idx in &schema.indexes {
                if idx.label() == label {
                    let _ = schema_manager.update_index_metadata(idx.name(), |m| {
                        m.status = uni_common::core::schema::IndexStatus::Stale;
                    });
                }
            }
        }

        tokio::spawn(async move {
            if let Err(e) = rebuild_mgr.schedule(labels).await {
                tracing::warn!("Failed to schedule index rebuild: {e}");
            }
        });
    }
}

/// `FinalizeFn` implementation that the `FlushCoordinator` invokes from
/// its single-task finalizer loop. Unit struct on purpose: it must NOT
/// hold `Arc<Writer>` (that would create a reference cycle Writer ->
/// FlushCoordinator -> Arc<dyn FinalizeFn> -> Writer). All state needed
/// for finalize travels in via `SharedFlushCtx`.
pub(crate) struct WriterFinalizer;

impl FinalizeFn for WriterFinalizer {
    fn finalize<'a>(
        &'a self,
        rotated: RotatedFlush,
        outcome: AsyncFlushOutcome,
        shared: SharedFlushCtx,
    ) -> std::pin::Pin<Box<dyn std::future::Future<Output = Result<String>> + Send + 'a>> {
        Box::pin(async move {
            // Read initial_size / initial_count from the rotated L0 so
            // we don't have to plumb them through the coordinator
            // submission. The buffer is still alive in pending_flush
            // until `complete_flush` (J) below pops it.
            let (initial_size, initial_count) = {
                let l0 = rotated.old_l0_arc.read();
                (l0.estimated_size, l0.mutation_count)
            };
            let result = Writer::flush_finalize_now(
                shared,
                rotated.old_l0_arc.clone(),
                rotated.wal_lsn,
                outcome.new_manifest,
                outcome.snapshot_id,
                initial_size,
                initial_count,
                std::time::Instant::now(),
            )
            .await;
            // `rotated` (permit + flush_in_progress_guard) drops here.
            drop(rotated.permit);
            result
        })
    }

    fn finalize_failure<'a>(
        &'a self,
        rotated: RotatedFlush,
        err: anyhow::Error,
        _shared: SharedFlushCtx,
    ) -> std::pin::Pin<Box<dyn std::future::Future<Output = anyhow::Error> + Send + 'a>> {
        Box::pin(async move {
            tracing::warn!(
                error = %err,
                seq = rotated.seq,
                "async flush stream failed; old L0 remains in pending_flush, \
                 WAL retains its data, recovery via WAL replay on restart"
            );
            metrics::counter!("uni_flush_failures_total").increment(1);
            // Permit + guard drop here so back-pressure releases even on
            // failure.
            drop(rotated.permit);
            err
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use tempfile::tempdir;

    /// Test that commit_transaction writes mutations to WAL before merging to main L0.
    /// This verifies fix for issue #137 (transaction commit atomicity).
    #[tokio::test]
    async fn test_commit_transaction_wal_before_merge() -> Result<()> {
        use crate::runtime::wal::WriteAheadLog;
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let _label_id = schema_manager.add_label("Test")?;
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        // Create WAL for main L0
        let wal_path = ObjectStorePath::from("wal");
        let wal = Arc::new(WriteAheadLog::new(store.clone(), wal_path));

        let writer = Writer::new_with_config(
            storage.clone(),
            schema_manager.clone(),
            1,
            UniConfig::default(),
            Some(wal),
            None,
        )
        .await?;

        // Begin transaction — create a transaction L0
        let tx_l0 = writer.create_transaction_l0();

        // Insert data in transaction
        let vid_a = writer.next_vid().await?;
        let vid_b = writer.next_vid().await?;

        let mut props = std::collections::HashMap::new();
        props.insert("test".to_string(), Value::String("data".to_string()));

        writer
            .insert_vertex_with_labels(vid_a, props.clone(), &["Test".to_string()], Some(&tx_l0))
            .await?;
        writer
            .insert_vertex_with_labels(
                vid_b,
                std::collections::HashMap::new(),
                &["Test".to_string()],
                Some(&tx_l0),
            )
            .await?;

        let eid = writer.next_eid(1).await?;
        writer
            .insert_edge(
                vid_a,
                vid_b,
                1,
                eid,
                std::collections::HashMap::new(),
                None,
                Some(&tx_l0),
            )
            .await?;

        // Get WAL before commit
        let l0 = writer.l0_manager.get_current();
        let wal = l0.read().wal.clone().expect("Main L0 should have WAL");
        let mutations_before = wal.replay().await?;
        let count_before = mutations_before.len();

        // Commit transaction - this should write to WAL first
        let writer = Arc::new(writer);
        writer.commit_transaction_l0(tx_l0).await?;

        // Verify WAL has the new mutations
        let mutations_after = wal.replay().await?;
        assert!(
            mutations_after.len() > count_before,
            "WAL should contain transaction mutations after commit"
        );

        // Verify mutations are in correct order: vertices first, then edges
        let new_mutations: Vec<_> = mutations_after.into_iter().skip(count_before).collect();

        let mut saw_vertex_a = false;
        let mut saw_vertex_b = false;
        let mut saw_edge = false;

        for mutation in &new_mutations {
            match mutation {
                crate::runtime::wal::Mutation::InsertVertex { vid, .. } => {
                    if *vid == vid_a {
                        saw_vertex_a = true;
                    }
                    if *vid == vid_b {
                        saw_vertex_b = true;
                    }
                    // Vertices should come before edges
                    assert!(!saw_edge, "Vertices should be logged to WAL before edges");
                }
                crate::runtime::wal::Mutation::InsertEdge { eid: e, .. } => {
                    if *e == eid {
                        saw_edge = true;
                    }
                    // Edges should come after vertices
                    assert!(
                        saw_vertex_a && saw_vertex_b,
                        "Edge should be logged after both vertices"
                    );
                }
                _ => {}
            }
        }

        assert!(saw_vertex_a, "Vertex A should be in WAL");
        assert!(saw_vertex_b, "Vertex B should be in WAL");
        assert!(saw_edge, "Edge should be in WAL");

        // Verify data is also in main L0
        let l0_read = l0.read();
        assert!(
            l0_read.vertex_properties.contains_key(&vid_a),
            "Vertex A should be in main L0"
        );
        assert!(
            l0_read.vertex_properties.contains_key(&vid_b),
            "Vertex B should be in main L0"
        );
        assert!(
            l0_read.edge_endpoints.contains_key(&eid),
            "Edge should be in main L0"
        );

        Ok(())
    }

    /// Test that failed WAL flush leaves transaction intact for retry or rollback.
    #[tokio::test]
    async fn test_commit_transaction_wal_failure_rollback() -> Result<()> {
        use crate::runtime::wal::WriteAheadLog;
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let _label_id = schema_manager.add_label("Test")?;
        let _baseline_label_id = schema_manager.add_label("Baseline")?;
        let _txdata_label_id = schema_manager.add_label("TxData")?;
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        // Create WAL for main L0
        let wal_path = ObjectStorePath::from("wal");
        let wal = Arc::new(WriteAheadLog::new(store.clone(), wal_path));

        let writer = Writer::new_with_config(
            storage.clone(),
            schema_manager.clone(),
            1,
            UniConfig::default(),
            Some(wal),
            None,
        )
        .await?;

        // Insert baseline data (outside transaction)
        let baseline_vid = writer.next_vid().await?;
        writer
            .insert_vertex_with_labels(
                baseline_vid,
                [("baseline".to_string(), Value::Bool(true))]
                    .into_iter()
                    .collect(),
                &["Baseline".to_string()],
                None,
            )
            .await?;

        // Begin transaction — create a transaction L0
        let tx_l0 = writer.create_transaction_l0();

        // Insert data in transaction
        let tx_vid = writer.next_vid().await?;
        writer
            .insert_vertex_with_labels(
                tx_vid,
                [("tx_data".to_string(), Value::Bool(true))]
                    .into_iter()
                    .collect(),
                &["TxData".to_string()],
                Some(&tx_l0),
            )
            .await?;

        // Capture main L0 state before rollback
        let l0 = writer.l0_manager.get_current();
        let vertex_count_before = l0.read().vertex_properties.len();

        // Rollback transaction (simulating what would happen after WAL flush failure)
        drop(tx_l0);

        // Verify main L0 is unchanged
        let vertex_count_after = l0.read().vertex_properties.len();
        assert_eq!(
            vertex_count_before, vertex_count_after,
            "Main L0 should not change after rollback"
        );

        // Baseline should still be present
        assert!(
            l0.read().vertex_properties.contains_key(&baseline_vid),
            "Baseline data should remain"
        );

        // Transaction data should NOT be in main L0
        assert!(
            !l0.read().vertex_properties.contains_key(&tx_vid),
            "Transaction data should not be in main L0 after rollback"
        );

        Ok(())
    }

    /// Test that batch insert with shared labels does not clone labels per vertex.
    /// This verifies fix for issue #161 (redundant label cloning).
    #[tokio::test]
    async fn test_batch_insert_shared_labels() -> Result<()> {
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let _label_id = schema_manager.add_label("Person")?;
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        let writer = Writer::new(storage.clone(), schema_manager.clone(), 1).await?;

        // Shared labels - should not be cloned per vertex
        let labels = &["Person".to_string()];

        // Insert batch of vertices with same labels
        let mut vids = Vec::new();
        for i in 0..100 {
            let vid = writer.next_vid().await?;
            let mut props = std::collections::HashMap::new();
            props.insert("id".to_string(), Value::Int(i));
            writer
                .insert_vertex_with_labels(vid, props, labels, None)
                .await?;
            vids.push(vid);
        }

        // Verify all vertices have the correct labels
        let l0 = writer.l0_manager.get_current();
        for vid in vids {
            let l0_guard = l0.read();
            let vertex_labels = l0_guard.vertex_labels.get(&vid);
            assert!(vertex_labels.is_some(), "Vertex should have labels");
            assert_eq!(
                vertex_labels.unwrap(),
                &vec!["Person".to_string()],
                "Labels should match"
            );
        }

        Ok(())
    }

    /// Test that estimated_size tracks mutations correctly and approximates size_bytes().
    /// This verifies fix for issue #147 (O(V+E) size_bytes() in metrics).
    #[tokio::test]
    async fn test_estimated_size_tracks_mutations() -> Result<()> {
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let _label_id = schema_manager.add_label("Test")?;
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        let writer = Writer::new(storage.clone(), schema_manager.clone(), 1).await?;

        let l0 = writer.l0_manager.get_current();

        // Initial state should be empty
        let initial_estimated = l0.read().estimated_size;
        let initial_actual = l0.read().size_bytes();
        assert_eq!(initial_estimated, 0, "Initial estimated_size should be 0");
        assert_eq!(initial_actual, 0, "Initial size_bytes should be 0");

        // Insert vertices with properties
        let mut vids = Vec::new();
        for i in 0..10 {
            let vid = writer.next_vid().await?;
            let mut props = std::collections::HashMap::new();
            props.insert("name".to_string(), Value::String(format!("vertex_{}", i)));
            props.insert("index".to_string(), Value::Int(i));
            writer
                .insert_vertex_with_labels(vid, props, &[], None)
                .await?;
            vids.push(vid);
        }

        // Verify estimated_size grew
        let after_vertices_estimated = l0.read().estimated_size;
        let after_vertices_actual = l0.read().size_bytes();
        assert!(
            after_vertices_estimated > 0,
            "estimated_size should grow after insertions"
        );

        // Verify estimated_size is within reasonable bounds of actual size (within 2x)
        let ratio = after_vertices_estimated as f64 / after_vertices_actual as f64;
        assert!(
            (0.5..=2.0).contains(&ratio),
            "estimated_size ({}) should be within 2x of size_bytes ({}), ratio: {}",
            after_vertices_estimated,
            after_vertices_actual,
            ratio
        );

        // Insert edges with a simple edge type
        let edge_type = 1u32;
        for i in 0..9 {
            let eid = writer.next_eid(edge_type).await?;
            writer
                .insert_edge(
                    vids[i],
                    vids[i + 1],
                    edge_type,
                    eid,
                    std::collections::HashMap::new(),
                    Some("NEXT".to_string()),
                    None,
                )
                .await?;
        }

        // Verify estimated_size grew further
        let after_edges_estimated = l0.read().estimated_size;
        let after_edges_actual = l0.read().size_bytes();
        assert!(
            after_edges_estimated > after_vertices_estimated,
            "estimated_size should grow after edge insertions"
        );

        // Verify still within reasonable bounds
        let ratio = after_edges_estimated as f64 / after_edges_actual as f64;
        assert!(
            (0.5..=2.0).contains(&ratio),
            "estimated_size ({}) should be within 2x of size_bytes ({}), ratio: {}",
            after_edges_estimated,
            after_edges_actual,
            ratio
        );

        Ok(())
    }

    /// Test that flushing WAL on a writer with no mutations succeeds cleanly.
    #[tokio::test]
    async fn test_flush_wal_empty_l0_is_noop() -> Result<()> {
        use crate::runtime::wal::WriteAheadLog;
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        let wal_path = ObjectStorePath::from("wal");
        let wal = Arc::new(WriteAheadLog::new(store.clone(), wal_path));

        let writer = Writer::new_with_config(
            storage.clone(),
            schema_manager.clone(),
            1,
            UniConfig::default(),
            Some(wal.clone()),
            None,
        )
        .await?;

        // Flush with no mutations — should succeed cleanly
        let lsn = writer.flush_wal().await?;
        // LSN should be 0 or 1 (no real mutations flushed)
        assert!(lsn <= 1, "Empty flush should produce low LSN, got {}", lsn);

        Ok(())
    }

    /// Test that transaction data does not leak into main L0 without commit.
    #[tokio::test]
    async fn test_transaction_isolation_without_commit() -> Result<()> {
        use crate::runtime::wal::WriteAheadLog;
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let path = dir.path().to_str().unwrap();
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");

        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let _label_id = schema_manager.add_label("Person")?;
        schema_manager.save().await?;

        let storage = Arc::new(StorageManager::new(path, schema_manager.clone()).await?);

        let wal_path = ObjectStorePath::from("wal");
        let wal = Arc::new(WriteAheadLog::new(store.clone(), wal_path));

        let writer = Writer::new_with_config(
            storage.clone(),
            schema_manager.clone(),
            1,
            UniConfig::default(),
            Some(wal),
            None,
        )
        .await?;

        // Create transaction L0
        let tx_l0 = writer.create_transaction_l0();

        // Insert vertex into transaction L0
        let vid = writer.next_vid().await?;
        writer
            .insert_vertex_with_labels(
                vid,
                [("name".to_string(), Value::String("Ghost".to_string()))]
                    .into_iter()
                    .collect(),
                &["Person".to_string()],
                Some(&tx_l0),
            )
            .await?;

        // Verify data is in transaction L0
        assert!(
            tx_l0.read().vertex_properties.contains_key(&vid),
            "Transaction L0 should contain the vertex"
        );

        // Verify data is NOT in main L0
        let main_l0 = writer.l0_manager.get_current();
        assert!(
            !main_l0.read().vertex_properties.contains_key(&vid),
            "Main L0 should NOT contain uncommitted transaction data"
        );

        // Drop transaction without committing — data should be lost
        drop(tx_l0);

        // Main L0 still should not have it
        assert!(
            !main_l0.read().vertex_properties.contains_key(&vid),
            "Main L0 should remain clean after dropped transaction"
        );

        Ok(())
    }

    /// Phase 2 Day 12: the fork-fragment warn fires exactly once when
    /// the flush count crosses the configured threshold and stays
    /// silent on subsequent flushes for the lifetime of the writer.
    /// Primary writers (`fork_id == None`) never fire it.
    ///
    /// Tested directly against `tick_fork_fragment_observability` so
    /// the contract is locked in independently of the broader
    /// `flush_to_l1` path (the end-to-end fork-flush path is blocked
    /// on Day 10's on-the-fly schema overlay growth).
    #[tokio::test]
    async fn fork_fragment_warn_fires_once_then_silences() -> Result<()> {
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::fork::ForkId;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");
        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        let storage = Arc::new(
            StorageManager::new(dir.path().to_str().unwrap(), schema_manager.clone()).await?,
        );

        let config = UniConfig {
            fork_fragment_warn_threshold: 3,
            ..Default::default()
        };
        let mut writer =
            Writer::new_with_config(storage, schema_manager, 1, config, None, None).await?;

        // Primary path: never fires.
        for _ in 0..10 {
            writer.tick_fork_fragment_observability();
        }
        assert!(!writer.fork_fragment_warn_fired.load(Ordering::Relaxed));
        assert_eq!(writer.fork_flush_count.load(Ordering::Relaxed), 0);

        // Fork path: tag and tick. Below threshold → no fire.
        writer.fork_id = Some(ForkId::new());
        writer.tick_fork_fragment_observability();
        writer.tick_fork_fragment_observability();
        assert!(!writer.fork_fragment_warn_fired.load(Ordering::Relaxed));
        assert_eq!(writer.fork_flush_count.load(Ordering::Relaxed), 2);

        // Crossing threshold → fires once.
        writer.tick_fork_fragment_observability();
        assert!(writer.fork_fragment_warn_fired.load(Ordering::Relaxed));
        assert_eq!(writer.fork_flush_count.load(Ordering::Relaxed), 3);

        // Subsequent ticks bump the gauge but do not re-fire.
        let fired_after = writer.fork_fragment_warn_fired.load(Ordering::Relaxed);
        for _ in 0..5 {
            writer.tick_fork_fragment_observability();
        }
        assert_eq!(writer.fork_flush_count.load(Ordering::Relaxed), 8);
        assert_eq!(
            writer.fork_fragment_warn_fired.load(Ordering::Relaxed),
            fired_after
        );

        Ok(())
    }

    /// The hot-path mutators must not write to any `Writer` struct field.
    /// Phase 2 of the refactor
    /// gave them `&self` receivers, which the compiler enforces against
    /// direct `self.x = y` assignment — but interior-mutable writes
    /// (Mutex/Atomic/OnceLock) still compile. This regression test snapshots
    /// every potentially-writable field, calls each hot-path mutator, and
    /// asserts no field changed.
    ///
    /// Cold-path methods (`flush_to_l1`, `commit_transaction_l0`,
    /// `tick_fork_fragment_observability`) DO mutate fields by design and
    /// are intentionally out of scope here.
    #[tokio::test]
    async fn hot_path_mutators_do_not_change_writer_fields() -> Result<()> {
        use crate::storage::manager::StorageManager;
        use object_store::local::LocalFileSystem;
        use object_store::path::Path as ObjectStorePath;
        use uni_common::core::schema::SchemaManager;

        let dir = tempdir()?;
        let store = Arc::new(LocalFileSystem::new_with_prefix(dir.path())?);
        let schema_path = ObjectStorePath::from("schema.json");
        let schema_manager =
            Arc::new(SchemaManager::load_from_store(store.clone(), &schema_path).await?);
        schema_manager.add_label("Person")?;
        schema_manager.save().await?;
        let storage = Arc::new(
            StorageManager::new(dir.path().to_str().unwrap(), schema_manager.clone()).await?,
        );

        let writer =
            Writer::new_with_config(storage, schema_manager, 1, UniConfig::default(), None, None)
                .await?;

        /// Captures every `Writer` field that *could* be written by a
        /// hot-path mutator (i.e., every non-Arc, non-immutable-after-
        /// construction field). Arc'd substructures (`l0_manager`,
        /// `storage`, etc.) are intentionally not checked — they are
        /// re-pointed only at construction.
        #[derive(Debug, PartialEq)]
        struct Snapshot {
            last_flush_time: std::time::Instant,
            cached_manifest_some: bool,
            fork_flush_count: u64,
            fork_fragment_warn_fired: bool,
            xervo_runtime_some: bool,
            index_rebuild_manager_some: bool,
            fork_id: Option<ForkId>,
        }

        fn snap(w: &Writer) -> Snapshot {
            Snapshot {
                last_flush_time: *w.last_flush_time.lock(),
                cached_manifest_some: w.cached_manifest.lock().is_some(),
                fork_flush_count: w.fork_flush_count.load(Ordering::Relaxed),
                fork_fragment_warn_fired: w.fork_fragment_warn_fired.load(Ordering::Relaxed),
                xervo_runtime_some: w.xervo_runtime.get().is_some(),
                index_rebuild_manager_some: w.index_rebuild_manager.get().is_some(),
                fork_id: w.fork_id,
            }
        }

        // 1. insert_vertex_with_labels
        let before = snap(&writer);
        let vid = writer.next_vid().await?;
        writer
            .insert_vertex_with_labels(vid, Properties::new(), &["Person".to_string()], None)
            .await?;
        assert_eq!(
            snap(&writer),
            before,
            "insert_vertex_with_labels mutated a Writer field"
        );

        // 2. insert_vertices_batch
        let before = snap(&writer);
        let vids = writer.allocate_vids(2).await?;
        writer
            .insert_vertices_batch(
                vids,
                vec![Properties::new(), Properties::new()],
                vec!["Person".into()],
                None,
            )
            .await?;
        assert_eq!(
            snap(&writer),
            before,
            "insert_vertices_batch mutated a Writer field"
        );

        // 3. delete_vertex
        let before = snap(&writer);
        writer.delete_vertex(vid, None, None).await?;
        assert_eq!(
            snap(&writer),
            before,
            "delete_vertex mutated a Writer field"
        );

        // (insert_edge / delete_edge are skipped here: their fixture cost is
        // disproportionate to the audit's marginal value, and the same
        // structural argument plus the compiler-enforced `&self` covers them.)

        Ok(())
    }
}