# Write-Ahead Log (WAL)

> An append-only record written durably before the corresponding data changes, allowing a system to recover committed state after failure.

- HTML version: https://robbiepalmer.me/ideas/write-ahead-log
- Source: https://www.postgresql.org/docs/current/wal-intro.html

A write-ahead log records a change before the system writes that change to its main data files. A
transaction can be reported as committed once its log records are durable. The database may flush
the affected table and index pages later.

After a crash, the system starts from a checkpoint and replays the log to redo committed changes
that had not reached the data files. Sequential log writes are also cheaper than forcing scattered
data pages to disk for every commit. The ordering rule is the important part: the log record must be
safe before the data page it describes can be written.

A WAL is not automatically an audit trail or an event-sourcing interface. It is usually an internal
recovery format that may contain physical page changes, be recycled after checkpoints, and change
between storage-engine versions. Replication and point-in-time recovery can build on it, but those
uses need explicit retention and decoding rules.

## Questions it prompts

* What must reach durable storage before success is reported?
* Where is the recovery checkpoint, and which records must replay after it?
* Can a torn or corrupt log record be detected safely?
* Is a consumer relying on an internal recovery log as though it were a stable event contract?

## Related ideas

- [Commit Log](https://robbiepalmer.me/ideas/commit-log.md): An ordered, append-only record of accepted state changes that can be replicated or replayed to reconstruct derived state.
- [Stream/Table Duality](https://robbiepalmer.me/ideas/stream-table-duality.md): A stream can describe the changes to a table, while a table represents the latest state produced by those changes.

## Where it appears

- Technology: [Kafka](https://robbiepalmer.me/technologies/kafka.md)
- Technology: [Neon](https://robbiepalmer.me/technologies/neon.md)
- Technology: [PostgreSQL](https://robbiepalmer.me/technologies/postgresql.md)

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