Could you explain what a write-ahead log is in storage systems?

The Write-Ahead Log (WAL) is a mechanism for ensuring data reliability and integrity in storage systems.

The idea is that before changes are applied to the main data, they are first recorded in a special log. This allows the system to recover its state in case of a crash or failure by applying or rolling back operations from the log.

Example:

• When updating a record, the new version is first written to the WAL.
• After a successful log write, the changes are applied to the main database.
• If the system crashes, upon restart, the WAL is read and incomplete operations are recovered.

This approach guarantees atomicity and durability of transactions.