Write-Ahead Log (WAL)
A write-ahead log, or WAL, is a technique where a database records each change to a durable log before applying it to the main data store. If the system crashes, it can replay the log to recover without losing committed data.
How a WAL protects data
The principle is simple: write down what you are about to do before you do it. By forcing the change to a durable log first, the database guarantees that even if power is lost mid-operation, it can replay the log on restart and end up in a consistent state.
This is foundational to durability and crash recovery in databases. It is also the mechanism behind many replication and change-data-capture systems, since the log is an ordered record of every change.
A WAL is itself an append-only, ordered, timestamped sequence, which is why the concept overlaps with event sourcing and streaming.
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