khive_storage/

graph.rs

//! Graph storage capability — edge CRUD and traversal.

use async_trait::async_trait;
use chrono::{DateTime, Utc};
use khive_types::EdgeRelation;
use uuid::Uuid;

use crate::capability::StorageCapability;
use crate::error::StorageError;
use crate::types::{
    BatchWriteSummary, DeleteMode, DirectedNeighborHit, Direction, Edge, EdgeEndpointBaseCounts,
    EdgeFilter, EdgeSeekPage, EdgeSortField, EdgeUpsertRequest, EdgeUpsertResult, GraphPath,
    GuardedBatchOutcome, GuardedEdgeBatchUpsertOutcome, GuardedEdgeUpsertOutcome,
    GuardedWriteOutcome, LinkId, NeighborCursor, NeighborHit, NeighborQuery, Page, PageRequest,
    SeekCursor, SeekPage, SortOrder, StorageResult, TraversalRequest,
};

/// The exact persisted cursor row observed by the reconciliation preview.
#[derive(Clone, Debug)]
pub struct CommitAnnotationCursorValue {
    pub value: Vec<u8>,
    pub updated_at: i64,
}

/// Preconditions carried from the selected repository and preview into the
/// graph store's writer transaction.
#[derive(Clone, Debug)]
pub struct CommitAnnotationGuard {
    pub expected_sha: String,
    pub source_identity: String,
    pub commits: CommitAnnotationCursorValue,
    pub checkpoint: CommitAnnotationCursorValue,
}

/// Result of a create-only historical commit-to-project annotation attempt.
/// The store decides this under its writer transaction; a caller-side preview
/// is never authority to replace or resurrect an edge.
#[derive(Clone, Debug)]
pub enum CommitAnnotationInsertOutcome {
    Created(Edge),
    ExistingLive,
    Tombstoned,
    SourceChanged,
    TargetChanged,
    CursorChanged,
}

/// A validated symmetric-edge patch and the exact persisted snapshot it replaces.
///
/// The caller supplies canonical endpoints for a symmetric relation. `namespace`
/// belongs to the stored record, independently of the store's routing namespace.
/// Snapshot timestamps use raw microseconds so overflow is refused without a
/// lossy timestamp conversion. Creation time, deletion state and target backend
/// are not replaced by this request.
#[derive(Clone, Debug)]
pub struct SymmetricEdgeUpdateRequest {
    pub namespace: String,
    pub id: LinkId,
    pub source_id: Uuid,
    pub target_id: Uuid,
    pub relation: EdgeRelation,
    pub weight: f64,
    pub metadata: Option<serde_json::Value>,
    pub expected_updated_at_micros: i64,
    pub expected_deleted_at_micros: Option<i64>,
}

/// Committed disposition of a guarded symmetric-edge update.
#[derive(Debug)]
pub enum SymmetricEdgeUpdateOutcome {
    /// No competing natural key existed; the requested edge was updated.
    Updated,
    /// A live or tombstoned canonical row survived unchanged and the requested
    /// duplicate was deleted. Its stored ID remains raw: parsing belongs after
    /// commit, so malformed legacy data cannot change the transaction outcome.
    Absorbed(String),
    /// The requested row disappeared or no longer matched its revision or
    /// deletion marker. Neither the update nor the conflict deletion occurred.
    Stale,
}

/// Directed edge CRUD and graph traversal over the knowledge graph.
#[async_trait]
pub trait GraphStore: Send + Sync + 'static {
    /// Return the newest live note of `kind` carrying the exact string `tag`
    /// in its properties.tags array and connected to `node_id` by a live
    /// incoming `annotates` edge in this store's namespace. Returns its UUID
    /// and creation timestamp; equal timestamps choose the smallest UUID.
    ///
    /// Apply all predicates before limiting to one result. Note lookup follows
    /// the by-ID contract; visibility is determined by the annotation edge's
    /// namespace, not by introducing a second namespace filter on the note.
    /// The note and edge must belong to this backend. Unsupported backends
    /// fail explicitly rather than scanning an arbitrary annotation window.
    async fn latest_annotating_note(
        &self,
        _node_id: Uuid,
        _kind: &str,
        _tag: &str,
    ) -> StorageResult<Option<(Uuid, i64)>> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "latest_annotating_note".into(),
            message: "this backend does not implement latest matching annotation lookup".into(),
        })
    }

    /// Like `latest_annotating_note`, but also require one exact top-level
    /// string property before selecting the newest candidate.
    async fn latest_annotating_note_with_property(
        &self,
        _node_id: Uuid,
        _kind: &str,
        _tag: &str,
        _property_key: &str,
        _property_value: &str,
    ) -> StorageResult<Option<(Uuid, i64)>> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "latest_annotating_note_with_property".into(),
            message: "this backend does not implement latest matching annotation lookup".into(),
        })
    }

    /// Insert or update a single edge.
    async fn upsert_edge(&self, edge: Edge) -> StorageResult<()>;
    /// Insert an edge only when neither its id nor natural key already
    /// exists. Returns `true` when this call inserted the row and `false`
    /// when an existing row won the race. The existing row is never updated.
    ///
    /// The default returns `Unsupported` rather than falling back to
    /// [`GraphStore::upsert_edge`], because an upsert would overwrite the
    /// winning row and violate this method's conditional-insert contract.
    async fn insert_edge_if_absent(&self, _edge: Edge) -> StorageResult<bool> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "insert_edge_if_absent".into(),
            message: "this backend does not implement conditional edge insert".into(),
        })
    }
    /// Create an `annotates` edge only while its source is a live `commit`
    /// note in `edge.namespace` with the exact SHA, its target is the live
    /// project for the selected source, the paired commit cursor rows still
    /// match the preview, and no row of the edge's natural key exists,
    /// including a tombstone. All checks and the insert occur in one writer
    /// transaction. Existing rows are immutable.
    async fn insert_commit_annotation_if_absent(
        &self,
        _edge: Edge,
        _guard: CommitAnnotationGuard,
    ) -> StorageResult<CommitAnnotationInsertOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "insert_commit_annotation_if_absent".into(),
            message: "this backend does not implement guarded commit annotation insert".into(),
        })
    }
    /// Insert or update a batch of edges.
    async fn upsert_edges(&self, edges: Vec<Edge>) -> StorageResult<BatchWriteSummary>;
    /// Insert or replace one edge and return the transaction-observed
    /// disposition plus preimage. Tombstone restoration is controlled by the
    /// request rather than being an implicit side effect of every upsert.
    async fn upsert_edge_observed(
        &self,
        _request: EdgeUpsertRequest,
    ) -> StorageResult<EdgeUpsertResult> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "upsert_edge_observed".into(),
            message: "this backend does not implement observed edge upserts".into(),
        })
    }
    /// Replace an edge only when the persisted row still matches the
    /// caller's read snapshot.
    ///
    /// `expected_updated_at` is the snapshot revision and
    /// `expected_deleted_at` closes the soft-delete race. The replacement
    /// edge's `updated_at` must be strictly greater than that persisted
    /// revision. Returns `false` when the row disappeared, changed, or was
    /// supplied a non-advancing replacement revision. This is the full-edge
    /// compare-and-swap seam used when a caller derives coupled fields from
    /// that snapshot before persistence — mirrors
    /// [`crate::NoteStore::replace_note_if_unchanged`]. The default returns
    /// `Unsupported` rather than falling back to an unguarded upsert and
    /// reintroducing the stale-snapshot race.
    async fn replace_edge_if_unchanged(
        &self,
        _edge: Edge,
        _expected_updated_at: DateTime<Utc>,
        _expected_deleted_at: Option<DateTime<Utc>>,
    ) -> StorageResult<bool> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "replace_edge_if_unchanged".into(),
            message: "this backend does not implement guarded edge replacement".into(),
        })
    }
    /// Apply a validated symmetric-edge patch in one writer transaction.
    ///
    /// First require a representable revision strictly after the snapshot, even
    /// if a competing natural key will absorb this edge. Then probe that key in
    /// the request's record namespace, including tombstones. A competing row
    /// survives unchanged; only the requested duplicate may be deleted. With no
    /// competitor, replace the requested patch fields and advance its revision.
    /// Both mutations compare the snapshot's revision and deletion marker.
    ///
    /// The default refuses rather than substituting an unguarded upsert or a
    /// caller-side read/delete sequence. Errors can report unknown settlement;
    /// callers must not infer rollback or retry an ambiguous result.
    async fn update_symmetric_edge_if_unchanged(
        &self,
        _request: SymmetricEdgeUpdateRequest,
    ) -> StorageResult<SymmetricEdgeUpdateOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "update_symmetric_edge_if_unchanged".into(),
            message: "this backend does not implement guarded symmetric-edge updates".into(),
        })
    }
    /// Insert or update a single edge, re-checking that both endpoints still
    /// exist (and are not soft-deleted) as part of the same write, not a
    /// separate prior read. Closes the TOCTOU window between an async
    /// prepare-time existence check and a later, unconditional write: a
    /// concurrent hard-delete of an endpoint that lands between the two can
    /// otherwise leave a durably dangling edge (#769).
    ///
    /// Returns [`GuardedWriteOutcome::Refused`] naming exactly which
    /// endpoint(s) were missing, determined by the guard's own in-transaction
    /// probe — never reconstructed by a caller re-reading the endpoints after
    /// the write already failed, since a concurrent write landing between the
    /// refusal and any such later read could misreport which endpoint was
    /// actually missing at write time.
    ///
    /// Default returns `StorageError::Unsupported`: a backend that does not
    /// override this method cannot honor the endpoint-existence guarantee,
    /// and silently falling back to [`GraphStore::upsert_edge`] would
    /// reintroduce the TOCTOU window this method exists to close.
    async fn upsert_edge_guarded(&self, _edge: Edge) -> StorageResult<GuardedWriteOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "upsert_edge_guarded".into(),
            message: "this backend does not implement guarded edge writes".into(),
        })
    }
    /// Observed form of [`GraphStore::upsert_edge_guarded`]. In addition to
    /// the endpoint guard, it distinguishes create, live replacement, and
    /// explicit resurrection without a caller-side read/write race.
    async fn upsert_edge_guarded_observed(
        &self,
        _request: EdgeUpsertRequest,
    ) -> StorageResult<GuardedEdgeUpsertOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "upsert_edge_guarded_observed".into(),
            message: "this backend does not implement observed guarded edge upserts".into(),
        })
    }
    /// Batch form of [`GraphStore::upsert_edge_guarded`]. All-or-nothing:
    /// if any edge's endpoints are missing at write time, no edge from the
    /// batch is persisted, `BatchWriteSummary::affected` is `0`, and
    /// `GuardedBatchOutcome::refused` names the first failing batch entry and
    /// its missing endpoint(s) — determined by the same in-transaction
    /// pre-check that aborted the batch, not a post-hoc re-read.
    /// Retain the original ordered input to enumerate every aborted write via
    /// [`GuardedBatchOutcome::refusal_page`] beyond the default bounded sample.
    ///
    /// Default returns `StorageError::Unsupported`, for the same reason as
    /// [`GraphStore::upsert_edge_guarded`]'s default.
    async fn upsert_edges_guarded(&self, _edges: Vec<Edge>) -> StorageResult<GuardedBatchOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "upsert_edges_guarded".into(),
            message: "this backend does not implement guarded edge writes".into(),
        })
    }
    /// All-or-nothing observed batch form. Implementations must perform
    /// endpoint and tombstone-policy preflight in the same write transaction
    /// before applying any row.
    async fn upsert_edges_guarded_observed(
        &self,
        _requests: Vec<EdgeUpsertRequest>,
    ) -> StorageResult<GuardedEdgeBatchUpsertOutcome> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "upsert_edges_guarded_observed".into(),
            message: "this backend does not implement observed guarded edge batches".into(),
        })
    }
    /// Fetch an edge by link ID, returning `None` if absent. Filters soft-deleted rows.
    async fn get_edge(&self, id: LinkId) -> StorageResult<Option<Edge>>;
    /// Fetch an edge by link ID including soft-deleted rows. Used by the runtime hard-delete path
    /// to locate and namespace-check an already-soft-deleted edge before purging it.
    async fn get_edge_including_deleted(&self, id: LinkId) -> StorageResult<Option<Edge>>;
    /// Fetch an edge by natural key (namespace, source, target, relation) including
    /// soft-deleted rows. Used by the atomic-apply result renderer for a symmetric-relation
    /// update whose surviving canonical row may be tombstoned (ADR-039 DO NOTHING) — the
    /// normal `query_edges`/`list_edges` path filters `deleted_at IS NULL` and would report
    /// "not found" for exactly that row.
    ///
    /// `namespace` is the natural key's own `namespace` column value (part of the
    /// `UNIQUE(namespace, source_id, target_id, relation)` constraint this method queries by)
    /// — it is passed explicitly rather than implied by whichever store instance `self` is,
    /// so a caller who resolved the record's namespace independently of its own ambient token
    /// (the atomic-apply renderer, which knows the committed edge's namespace from its prepare-
    /// time `EdgeNaturalKey`, not from the caller's token) cannot accidentally query the wrong
    /// namespace by relying on implicit store scoping.
    async fn get_edge_by_natural_key_including_deleted(
        &self,
        namespace: &str,
        source_id: Uuid,
        target_id: Uuid,
        relation: EdgeRelation,
    ) -> StorageResult<Option<Edge>>;
    /// Delete an edge by link ID using the specified delete mode.
    async fn delete_edge(&self, id: LinkId, mode: DeleteMode) -> StorageResult<bool>;
    /// Query edges with filter, sort, and pagination without an implicit
    /// exact count. Implementations should return `total: None`; callers that
    /// need a count use [`Self::count_edges`] explicitly.
    async fn query_edges(
        &self,
        filter: EdgeFilter,
        sort: Vec<SortOrder<EdgeSortField>>,
        page: PageRequest,
    ) -> StorageResult<Page<Edge>>;
    /// Query edges across the given namespaces in one deterministic query
    /// with real SQL paging. The multi-namespace analogue of
    /// [`Self::query_edges`]: a single statement with `namespace IN (...)`
    /// keeps `offset` continuation coherent, where fetching per-namespace
    /// prefixes and slicing a client-side merge floats the window between
    /// calls (silent duplicate/skip enumeration). Backends without batched
    /// namespace support retain the single-namespace path and reject
    /// multi-namespace requests explicitly. Implementations should return
    /// `total: None`; callers that need a count use
    /// [`Self::count_edges_in_namespaces`] explicitly.
    async fn query_edges_in_namespaces(
        &self,
        namespaces: &[String],
        filter: EdgeFilter,
        sort: Vec<SortOrder<EdgeSortField>>,
        page: PageRequest,
    ) -> StorageResult<Page<Edge>> {
        match namespaces.len() {
            0 => Ok(Page {
                items: Vec::new(),
                total: None,
            }),
            1 => self.query_edges(filter, sort, page).await,
            _ => Err(StorageError::Unsupported {
                capability: StorageCapability::Graph,
                operation: "query_edges_in_namespaces".into(),
                message: "this backend does not implement batched namespace edge queries".into(),
            }),
        }
    }
    /// Count edges matching the given filter.
    async fn count_edges(&self, filter: EdgeFilter) -> StorageResult<u64>;
    /// Count edges across the given namespaces in one aggregate query.
    /// Backends without batched namespace support retain the single-namespace
    /// path and reject multi-namespace requests explicitly.
    async fn count_edges_in_namespaces(
        &self,
        namespaces: &[String],
        filter: EdgeFilter,
    ) -> StorageResult<u64> {
        match namespaces.len() {
            0 => Ok(0),
            1 => self.count_edges(filter).await,
            _ => Err(StorageError::Unsupported {
                capability: StorageCapability::Graph,
                operation: "count_edges_in_namespaces".into(),
                message: "this backend does not implement batched namespace edge counts".into(),
            }),
        }
    }
    /// Count edges grouped by relation, ignoring soft-deleted rows. Cheap
    /// aggregate (`GROUP BY relation`) used to report the true per-relation
    /// population for full-graph audits (#702.3).
    async fn count_edges_by_relation(&self) -> StorageResult<Vec<(EdgeRelation, u64)>>;
    /// Count edges grouped by relation across the given namespaces in one
    /// aggregate query.
    async fn count_edges_by_relation_in_namespaces(
        &self,
        namespaces: &[String],
    ) -> StorageResult<Vec<(EdgeRelation, u64)>> {
        match namespaces.len() {
            0 => Ok(Vec::new()),
            1 => self.count_edges_by_relation().await,
            _ => Err(StorageError::Unsupported {
                capability: StorageCapability::Graph,
                operation: "count_edges_by_relation_in_namespaces".into(),
                message: "this backend does not implement batched namespace relation counts".into(),
            }),
        }
    }
    /// Count live edges grouped by the base each endpoint resolves against.
    ///
    /// A relation breakdown cannot answer this: relations do not determine
    /// endpoint bases, and the same relation appears on both sides of the
    /// structure/provenance line. One aggregate query, same shape and cost as
    /// the relation counts.
    async fn count_edges_by_endpoint_base(&self) -> StorageResult<EdgeEndpointBaseCounts> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "count_edges_by_endpoint_base".into(),
            message: "this backend does not implement endpoint-base edge counts".into(),
        })
    }
    /// Count live edges by endpoint base across the given namespaces in one
    /// aggregate query.
    async fn count_edges_by_endpoint_base_in_namespaces(
        &self,
        namespaces: &[String],
    ) -> StorageResult<EdgeEndpointBaseCounts> {
        match namespaces.len() {
            0 => Ok(EdgeEndpointBaseCounts::default()),
            1 => self.count_edges_by_endpoint_base().await,
            _ => Err(StorageError::Unsupported {
                capability: StorageCapability::Graph,
                operation: "count_edges_by_endpoint_base_in_namespaces".into(),
                message: "this backend does not implement batched namespace endpoint-base counts"
                    .into(),
            }),
        }
    }
    /// Count live edges across the given namespaces grouped by relation and,
    /// within each relation, by the base each endpoint resolves against.
    ///
    /// The live edge total, the relation counts and the endpoint-base counts
    /// describe one population, so one grouped scan answers all three:
    /// summing each relation's buckets gives its relation count, and summing
    /// the buckets across relations gives the endpoint-base counts. Asking for
    /// them as three aggregates walks the edge table and its endpoint probes
    /// three times. Backends without this aggregate report `Unsupported`, and
    /// callers fall back to the three single-purpose counts.
    async fn count_edges_by_relation_and_endpoint_base_in_namespaces(
        &self,
        namespaces: &[String],
    ) -> StorageResult<Vec<(EdgeRelation, EdgeEndpointBaseCounts)>> {
        let _ = namespaces;
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "count_edges_by_relation_and_endpoint_base_in_namespaces".into(),
            message: "this backend does not implement the combined relation and endpoint-base \
                      edge count"
                .into(),
        })
    }
    /// Seek-pagination page of edges ordered by `id` ascending, using an
    /// indexed range scan (`id > after`) against the `(namespace, id)`
    /// primary key instead of `OFFSET`. `after` is exclusive; `None` starts
    /// from the beginning of the set. This remains an efficient compatibility
    /// path for a fixed edge set, but random UUIDs inserted concurrently may
    /// sort behind an issued boundary. Public concurrent walks use
    /// [`Self::query_edges_sequence_after`] instead (#1424).
    async fn query_edges_after(
        &self,
        filter: EdgeFilter,
        after: Option<Uuid>,
        limit: u32,
    ) -> StorageResult<EdgeSeekPage>;
    /// Resolve an edge id to its immutable insertion sequence.
    async fn edge_sequence(&self, _id: Uuid) -> StorageResult<Option<i64>> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "edge_sequence".into(),
            message: "this backend does not implement edge insertion sequences".into(),
        })
    }
    /// Resolve edge ids to immutable insertion sequences. Implementations may
    /// override this to batch the lookup; the default preserves correctness.
    async fn edge_sequences(&self, ids: &[Uuid]) -> StorageResult<Vec<(Uuid, i64)>> {
        let mut resolved = Vec::with_capacity(ids.len());
        for id in ids {
            if let Some(sequence) = self.edge_sequence(*id).await? {
                resolved.push((*id, sequence));
            }
        }
        Ok(resolved)
    }
    /// Seek-pagination page ordered by immutable insertion sequence. This is
    /// the stable public-list contract for walks overlapping inserts (#1424).
    async fn query_edges_sequence_after(
        &self,
        _filter: EdgeFilter,
        _after: Option<SeekCursor>,
        _limit: u32,
    ) -> StorageResult<SeekPage<Edge>> {
        Err(StorageError::Unsupported {
            capability: StorageCapability::Graph,
            operation: "query_edges_sequence_after".into(),
            message: "this backend does not implement insertion-sequence edge pagination".into(),
        })
    }
    /// Return immediate neighbors of a graph node.
    async fn neighbors(
        &self,
        node_id: Uuid,
        query: NeighborQuery,
    ) -> StorageResult<Vec<NeighborHit>>;
    /// Return one deterministic neighbor page. `after` is exclusive and
    /// `neighbor_kinds`, when present, filters entity and note kinds before
    /// the limit is applied. Backends that do not implement kind-aware paging
    /// retain an explicit unsupported result rather than silently returning a
    /// misleading page.
    async fn neighbors_page(
        &self,
        node_id: Uuid,
        mut query: NeighborQuery,
        after: Option<NeighborCursor>,
        neighbor_kinds: Option<Vec<String>>,
    ) -> StorageResult<Vec<NeighborHit>> {
        if neighbor_kinds
            .as_ref()
            .is_some_and(|kinds| !kinds.is_empty())
        {
            return Err(StorageError::Unsupported {
                capability: StorageCapability::Graph,
                operation: "neighbors_page".into(),
                message: "this backend does not implement neighbor kind filtering".into(),
            });
        }
        let limit = query.limit;
        query.limit = None;
        let mut hits = self.neighbors(node_id, query).await?;
        if let Some(cursor) = after {
            hits.retain(|hit| cursor.is_after(hit));
        }
        if let Some(limit) = limit {
            hits.truncate(limit as usize);
        }
        Ok(hits)
    }
    /// Return neighbors in BOTH directions in a single call, each tagged with
    /// the direction (`Out`/`In`) it was found in. `query.direction` is
    /// ignored — this always fetches both directions.
    ///
    /// Exists so a caller that needs both-direction neighbors labeled by
    /// direction (e.g. the `context` verb) can do so with one storage query
    /// instead of two separate direction-scoped `neighbors` calls. The
    /// default implementation preserves the original two-call behavior for
    /// backends that don't override it; `SqlGraphStore` overrides this with a
    /// single `UNION ALL` query that projects a direction literal per arm.
    async fn neighbors_both_directions(
        &self,
        node_id: Uuid,
        query: NeighborQuery,
    ) -> StorageResult<Vec<DirectedNeighborHit>> {
        let mut out_query = query.clone();
        out_query.direction = Direction::Out;
        let mut in_query = query;
        in_query.direction = Direction::In;
        let mut result = Vec::new();
        for hit in self.neighbors(node_id, out_query).await? {
            result.push(DirectedNeighborHit {
                hit,
                direction: Direction::Out,
            });
        }
        for hit in self.neighbors(node_id, in_query).await? {
            result.push(DirectedNeighborHit {
                hit,
                direction: Direction::In,
            });
        }
        Ok(result)
    }
    /// Fetch multiple edges by their link IDs in a single round-trip.
    ///
    /// IDs that are not found (absent or soft-deleted) are silently skipped;
    /// the returned `Vec` may be shorter than `ids`. Backends that support
    /// batched `IN (...)` queries should override this; the default loops
    /// `get_edge` so non-SQLite backends keep compiling unchanged.
    ///
    /// Callers must chunk large ID lists before calling if they need a strict
    /// size bound; this method does not enforce a maximum.
    async fn get_edges(&self, ids: &[LinkId]) -> StorageResult<Vec<Edge>> {
        let mut out = Vec::with_capacity(ids.len());
        for &id in ids {
            if let Some(edge) = self.get_edge(id).await? {
                out.push(edge);
            }
        }
        Ok(out)
    }
    /// Read live edges in input order, retaining each row's decode outcome.
    ///
    /// The vector has exactly one entry per input, including duplicates and
    /// missing/soft-deleted IDs (`Ok(None)`). An inner error belongs to that
    /// input; an outer error is a statement/admission failure for the batch.
    /// Callers must bound batches and select errors in their original order.
    /// The default preserves backend support with point reads; SQLite batches.
    async fn get_edge_read_outcomes(
        &self,
        ids: &[LinkId],
    ) -> StorageResult<Vec<StorageResult<Option<Edge>>>> {
        let mut outcomes = Vec::with_capacity(ids.len());
        for &id in ids {
            outcomes.push(self.get_edge(id).await);
        }
        Ok(outcomes)
    }

    /// Return neighbors for multiple source nodes in a single round-trip,
    /// yielding `(source_id, hit)` pairs.
    ///
    /// The `query` parameters (direction, relations, min_weight) are applied
    /// uniformly to every source node. `query.limit` is applied **per source**:
    /// each source returns at most `limit` hits. Backends that support batched
    /// `source_id IN (...)` queries should override this; the default loops
    /// `neighbors` so non-SQLite backends keep compiling unchanged.
    async fn batch_neighbors(
        &self,
        sources: &[Uuid],
        query: NeighborQuery,
    ) -> StorageResult<Vec<(Uuid, NeighborHit)>> {
        let mut out = Vec::new();
        for &src in sources {
            let hits = self.neighbors(src, query.clone()).await?;
            for hit in hits {
                out.push((src, hit));
            }
        }
        Ok(out)
    }
    /// Bounded multi-hop BFS traversal from the given roots.
    ///
    /// Implementations must validate [`TraversalRequest::validate`], count
    /// adjacency rows before first-visit de-duplication against the request's
    /// shared execution budget, stop a root as soon as its effective result
    /// limit is filled, and return an error rather than partial paths when the
    /// work or time budget expires. Minimum-depth BFS selection is normative;
    /// same-depth tie ordering is not.
    async fn traverse(&self, request: TraversalRequest) -> StorageResult<Vec<GraphPath>>;
    /// Hard-delete every incident edge (source or target) for `node_id`, regardless of soft-delete
    /// state. Used during endpoint hard-delete to prevent dangling `graph_edges` rows (ADR-002
    /// no-dangling-references contract).
    async fn purge_incident_edges(&self, node_id: Uuid) -> StorageResult<u64>;
}