Databases Advanced

What is Write-Ahead Log (WAL)?

A technique where changes are first written to a log before being applied to the database, ensuring crash recovery and data integrity.

Write-Ahead Logging is fundamental to database reliability. Before any data modification is written to the actual data files, it is first recorded in a sequential log. If the database crashes, it can replay the WAL to recover committed transactions and undo incomplete ones. PostgreSQL uses WAL extensively — it powers crash recovery, point-in-time recovery (PITR), streaming replication, and logical replication. WAL settings like wal_level, max_wal_size, and checkpoint_timeout directly impact performance and recovery capabilities.

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