What Is a Write‑Ahead Log (WAL), and How Does It Ensure Durability?

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What Is a Write‑Ahead Log (WAL), and How Does It Ensure Durability?

A Write-Ahead Log (WAL) is a technique where all changes are written to a separate, durable log file on disk before being applied to the main database or storage system, so that if a system crash occurs, the log can be used to recover the data and thus ensure no information is lost.

Durability is the “D” in the ACID properties of transactions – it means that once a transaction is committed, its changes are permanent and will survive any system failure.

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