5. Write-ahead Log

A machine can crash in the middle of changing data. When it restarts, something has to tell it exactly what it was doing.

Background

Machines fail and restart at any time. A program can be in the middle of changing data. Then the machine loses power. On restart, it needs to know the last thing it was doing. It must redo, undo, or finish that change. Which one depends on what atomicity and durability require. Without a record of that work in progress, the program has no way to know what it was doing. It cannot tell what happened before the crash.