Explain the principle of operation of the Write-Ahead Log and its role in ensuring data reliability.

Write-Ahead Log (WAL) is a mechanism used in database management systems and other data storage systems to ensure data reliability and integrity during failures.

Working principle:

• Before modifying data in the main database or storage, the system records information about these changes in a separate log.
• The log is written before applying changes to the main data — hence the name "write-ahead".
• If a failure occurs (e.g., crash), the system reads the log during recovery and repeats or rolls back operations to bring the data to a consistent state.

Role in ensuring reliability:

• Guarantees that no changes will be lost after operation confirmation.
• Provides the ability to recover data after failures.
• Reduces the risk of data corruption, as changes are recorded sequentially and atomically.

Example: in a database, before updating a record, the corresponding entry is first written to the WAL, and only after a successful write to the log does the main table get updated. If the system crashes, upon restart, it reads the WAL and applies all incomplete operations.