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mvcc — Claude Code skill: what it does, how to install

Architectural reference for experimental MVCC in Turso: row versioning, snapshot isolation, and known memory limits. Useful when working on database internals.

Install into Claude Code · Skills CLI (Vercel) All methods and platforms ↓ 
npx skills add tursodatabase/turso --skill mvcc -a claude-code
Open on GitHub

What mvcc does

This skill gives the coding agent an architectural overview of experimental Multi-Version Concurrency Control in Turso. It explains how row-level snapshot isolation works compared to WAL page-level snapshots, and how writes append to the .db-log logical log instead of full pages. The core implementation lives in core/mvcc/database/mod.rs at roughly 3000 lines of code.

MVCC is enabled per database at runtime with PRAGMA journal_mode = 'mvcc'. The skill notes this subsystem is strictly experimental and unready for production. Garbage collection is not implemented, checkpointing blocks read transactions, and memory usage remains a concern because rows do not spill to disk.

The guide also details how to run tests with cargo test mvcc or make test-mvcc, and how to use the #[turso_macros::test(mvcc)] test macro. The instructions explicitly direct the agent to ignore MVCC during general debugging unless the issue is MVCC-specific.

What it helps with

  • Understand row versioning and the MvStore architecture
  • Configure checkpoint thresholds with PRAGMA mvcc_checkpoint_threshold
  • Run MVCC unit and integration tests via cargo test mvcc
  • Identify concurrency limits and blocking behavior during checkpoints

Example prompts for the agent

  • Explain the difference between WAL and MVCC row isolation in Turso
  • How do I write a test with the #[turso_macros::test(mvcc)] attribute?
  • Show the checkpoint process and key files in core/mvcc

How to install mvcc

1Skills CLI (Vercel)

npx skills add tursodatabase/turso --skill mvcc -a claude-code

2Manually: put the files in place

~/.claude/skills/mvcc/SKILL.md

Copy the whole skill folder; for one project use .claude/skills/mvcc/

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Questions about mvcc

How to install mvcc in Claude Code?

Run npx skills add tursodatabase/turso --skill mvcc -a claude-code. This adds the MVCC reference to your Claude Code environment.

What does the mvcc skill do?

It gives the agent a reference guide to Turso's experimental MVCC implementation, including the MvStore memory structure, .db-log storage, and test workflows.

Is Turso MVCC ready for production?

No, the feature is work in progress. It lacks garbage collection and crash recovery from the logical log, and checkpoints block concurrent reads.

Source text

SKILL.md · MIT 
---
name: mvcc
description: Overview of Experimental MVCC feature - snapshot isolation, versioning, limitations
---
# 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

```sql
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.

```sql
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

```bash
# Run MVCC-specific tests
cargo test mvcc

# TCL tests with MVCC
make test-mvcc
```

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

```rust
#[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

© tursodatabase · MIT · tursodatabase/turso

See also

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