Write-Ahead Log (WAL)

The Write-Ahead Log (WAL) is a fundamental mechanism in database storage engines that guarantees ACID compliance, particularly durability. It operates by sequentially recording all intended modifications (inserts, updates, deletes) to a persistent log file before these changes are written to the actual data files. This approach ensures that in the event of a system crash or power failure, the database can recover to a consistent state by replaying the log entries. WAL is essential for maintaining data integrity, supporting transaction rollback, and enabling features like replication and point-in-time recovery in modern database systems.

WAL follows the principle of 'log before data.' When a transaction modifies data, the storage engine first writes a log record containing the change details (e.g., old and new values, transaction ID) to a durable log file. Only after this log write is confirmed as persistent (typically via fsync or similar mechanisms) are the changes applied to the main data structures (like B-trees or LSM trees). During recovery, the system reads the WAL from the last checkpoint, reapplies committed transactions, and rolls back uncommitted ones, ensuring atomicity and consistency.