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transaction-correctness

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How WAL mechanics, checkpointing, concurrency rules, recovery work in tursodb

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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.

WAL 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:

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