WAL Mechanics

Write Path

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

Read Path

Reader acquires read mark (mxFrame = last valid commit frame)
For each page: check WAL up to mxFrame, fall back to main DB
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:

First connection acquires exclusive lock
Replays valid commits from WAL
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

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

transaction-correctness

安装