Crate mnem_backend_redb 

mnem-backend-redb

Production Blockstore + OpHeadsStore backed by redb - a pure-Rust embedded ACID key-value store.

A single .redb file holds two tables:

• objects - CID bytes → object bytes. Every Node, Edge, Tree chunk, Commit, View, Operation is a row.
• op_heads - set-of-CIDs modelled as CID bytes → () with presence-as-truth. The current op-heads set is the key-set of this table.

Writes are atomic per-call: each put / delete / update opens a write transaction, mutates, and commits (which fsyncs). No cross-call batching yet; a future refinement can expose a “batch mode” that holds a transaction open across multiple puts to reduce fsync overhead on the mnem_core::repo::Transaction::commit hot path.

Concurrency

redb: single-writer, many-reader per database file. Within a process, Arc<Database> is safe to share across threads - redb serialises writers internally. Across processes, redb’s filesystem-locking protects the file format; concurrent write transactions from two processes will serialise or one will fail.

Durability

tx.commit() in redb calls fsync on the database file. The Simple backend’s atomic-rename story is effectively what redb does internally for each commit - one transaction = one durable state transition.

Backend choice notes

redb is the production embedded backend for exactly these reasons: pure Rust, small dep tree, mmap reads, ACID writes, single-file persistence. Note redb majors break on-disk format (1 → 2 → 3 → 4). We pin the major in the crate’s Cargo dependency and document the migration path when we bump.

Re-exports

pub use blockstore::RedbBlockstore;

pub use knn_edges_store::KNN_EDGES_TABLE;

pub use knn_edges_store::load_knn_edges;

pub use knn_edges_store::store_knn_edges;

pub use op_heads::RedbOpHeadsStore;

Modules

blockstore

RedbBlockstore - redb-backed Blockstore implementation.

knn_edges_store

redb persistence for mnem-ann::KnnEdgeIndex (experiment E0).

op_heads

RedbOpHeadsStore - redb-backed OpHeadsStore.

Functions

open_or_init

Open or create the redb database at path and return wired-up blockstore + op-heads stores.