Что такое WAL (Write-Ahead Log)?

Write-Ahead Log (WAL) is a method of ensuring data reliability and integrity in database management systems. The essence of WAL is that before modifying data in the main database, all operations are first recorded in a change log (journal). This allows the database to be restored to a consistent state after a crash or failure by applying or rolling back changes from the log.

Example: when updating a record, information about the change is first written to the WAL, then the record itself is updated. If the system crashes during the operation, upon restart, the database reads the WAL and completes or cancels incomplete transactions.

Write-Ahead Log (WAL) is a logging mechanism for database changes, where all modifications are first recorded in a log before they are applied to the main database.

Why WAL is needed:

• Ensures data reliability and integrity during failures — if the system crashes, the state can be recovered by replaying the log entries.
• Allows implementing transaction atomicity: changes are recorded in the log, and only after that are they applied to the data.
• Improves performance by sequentially writing to the log instead of random disk access.

Example: when updating a record in a table, a log entry describing the operation is first created in WAL, then the changes are applied to the table. If a failure occurs afterward, the system reads the WAL during recovery and repeats the operations to prevent data loss.