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 = 'experimental_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

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