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name: transaction-correctness description: How WAL mechanics, checkpointing, concurrency rules, recovery work in tursodb

Transaction Correctness Guide

Turso uses WAL (Write-Ahead Logging) mode exclusively.

Files: .db, .db-wal (no .db-shm - Turso uses in-memory WAL index)

WAL Mechanics

Write Path

1. Writer appends frames (page data) to WAL file (sequential I/O)
2. COMMIT = frame with non-zero db_size in header (marks transaction end)
3. Original DB unchanged until checkpoint

Read Path

1. Reader acquires read mark (mxFrame = last valid commit frame)
2. For each page: check WAL up to mxFrame, fall back to main DB
3. Reader sees consistent snapshot at its read mark

Checkpointing

Transfers WAL content back to main DB.

CopyWAL grows → checkpoint triggered (default: 1000 pages) → pages copied to DB → WAL reused

Checkpoint types:

• PASSIVE: Non-blocking, stops at pages needed by active readers
• FULL: Waits for readers, checkpoints everything
• RESTART: Like FULL, also resets WAL to beginning
• TRUNCATE: Like RESTART, also truncates WAL file to zero length

WAL-Index

SQLite uses a shared memory file (-shm) for WAL index. Turso does not - it uses in-memory data structures (frame_cache hashmap, atomic read marks) since multi-process access is not supported.

Concurrency Rules

• One writer at a time
• Readers don't block writer, writer doesn't block readers
• Checkpoint must stop at pages needed by active readers

Recovery

On crash:

1. First connection acquires exclusive lock
2. Replays valid commits from WAL
3. Releases lock, normal operation resumes

Turso Implementation

Key files:

• WAL implementation - WAL implementation
• Page management, transactions

Connection-Private vs Shared

Per-Connection (private):

• Pager - page cache, dirty pages, savepoints, commit state
• WalFile - connection's snapshot view:
  • max_frame / min_frame - frame range for this connection's snapshot
  • max_frame_read_lock_index - which read lock slot this connection holds
  • last_checksum - rolling checksum state

Shared across connections:

• WalFileShared - global WAL state:
  • frame_cache - page-to-frame index (replaces .shm file)
  • max_frame / nbackfills - global WAL progress
  • read_locks[5] - read mark slots (TursoRwLock with embedded frame values)
  • write_lock - exclusive writer lock
  • checkpoint_lock - checkpoint serialization
  • file - WAL file handle
• DatabaseStorage - main .db file
• BufferPool - shared memory allocation

Correctness Invariants

1. Durability: COMMIT record must be fsynced before returning success
2. Atomicity: Partial transactions never visible to readers
3. Isolation: Each reader sees consistent snapshot
4. No lost updates: Checkpoint can't overwrite uncommitted changes

References

• SQLite WAL
• WAL File Format