Can you explain what Write-Ahead Logging (WAL) is and how it ensures data integrity?

Write-Ahead Logging (WAL) is a method of ensuring data integrity in database management systems and file systems. The idea is that before making changes directly to the main data storage, these changes are first recorded in a special log.

This allows the database to recover its state after a system crash or power failure by applying the log entries, which guarantee that all operations were either fully completed or fully rolled back. Thus, WAL ensures atomicity and durability of transactions.

Example:

• When updating a record, a WAL entry describing the changes is first created.
• After successfully writing to the WAL, the changes are applied to the main database.
• If the system crashes before applying the changes, upon restart, the system reads the WAL and completes the operations.

This approach minimizes the risk of data loss and database corruption.