MVCC Guide (Experimental) Skill

MVCC Guide (Experimental)

Multi-Version Concurrency Control. Work in progress, not production-ready.

CRITICAL: Ignore MVCC when debugging unless the bug is MVCC-specific.

Enabling MVCC

PRAGMA journal_mode = 'mvcc';

Runtime configuration, not a compile-time feature flag. Per-database setting.

How It Works

Standard WAL: single version per page, readers see snapshot at read mark time.

MVCC: multiple row versions, snapshot isolation. Each transaction sees consistent snapshot at begin time.

Key Differences from WAL

| Aspect | WAL | MVCC | |--------|-----|------| | Write granularity | Every commit writes full pages | Affected rows only | Readers/Writers | Don't block each other | Don't block each other | | Persistence | .db-wal | .db-log (logical log) | | Isolation | Snapshot (page-level) | Snapshot (row-level) |

Versioning

Each row version tracks:

begin - timestamp when visible
end - timestamp when deleted/replaced
btree_resident - existed before MVCC enabled

Architecture

Database
  └─ mv_store: MvStore
      ├─ rows: SkipMap<RowID, Vec<RowVersion>>
      ├─ txs: SkipMap<TxID, Transaction>
      ├─ Storage (.db-log file)
      └─ CheckpointStateMachine

Per-connection: mv_tx tracks current MVCC transaction.

Shared: MvStore with lock-free crossbeam_skiplist structures.

Key Files

core/mvcc/mod.rs - Module overview
core/mvcc/database/mod.rs - Main implementation (~3000 lines)
core/mvcc/cursor.rs - Merged MVCC + B-tree cursor
core/mvcc/persistent_storage/logical_log.rs - Disk format
core/mvcc/database/checkpoint_state_machine.rs - Checkpoint logic

Checkpointing

Flushes row versions to B-tree periodically.

PRAGMA mvcc_checkpoint_threshold = <pages>;

Process: acquire lock → begin pager txn → write rows → commit → truncate log → fsync → release.

Current Limitations

Not implemented:

Garbage collection (old versions accumulate)
Recovery from logical log on restart

Known issues:

Checkpoint blocks other transactions, even reads!
No spilling to disk; memory use concerns

Testing

# Run MVCC-specific tests
cargo test mvcc

# TCL tests with MVCC
make test-mvcc

Use #[turso_macros::test(mvcc)] attribute for MVCC-enabled tests.

#[turso_macros::test(mvcc)]
fn test_something() {
    // runs with MVCC enabled
}

References

core/mvcc/mod.rs documents data anomalies (dirty reads, lost updates, etc.)
Snapshot isolation vs serializability: MVCC provides the former, not the latter

Agent Skills: MVCC Guide (Experimental)

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