Write-Ahead Logging (WAL) in SQL Server

Write-Ahead Logging (WAL) is a core SQL Server recovery mechanism that ensures transaction log records are written to stable storage before the corresponding data pages are written to the database files.

The WAL rule is: "Log First, Data Later."

This guarantees that SQL Server can recover committed transactions and roll back uncommitted transactions after a crash. WAL is the foundation for ACID properties, crash recovery, backups, Availability Groups, replication, and transaction durability.

Write-Ahead Logging (WAL) is the foundational recovery protocol used by SQL Server. WAL requires that all transaction log records describing a change be flushed to stable storage before the associated data pages are written to the database files. Additionally, before a COMMIT succeeds, SQL Server must flush all relevant log records, including the COMMIT record, to the transaction log. This design allows SQL Server to guarantee durability, support crash recovery, and maintain ACID properties. During a server failure, SQL Server uses the transaction log during the Analysis, Redo, and Undo phases of recovery to reconstruct committed transactions and reverse uncommitted ones. WAL is therefore the backbone of transaction processing, recovery, backups, replication, Always On Availability Groups, and nearly every durability mechanism in SQL Server.

 WAL = "SQL Server never trusts the data page; it trusts the transaction log."