System Design Patterns: From Fundamentals to Real Systems
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Write-Ahead Log

1. The Incident

Tuesday, 3:07 AM: a power failure shuts down the rack holding shard 2. The machine reboots ninety seconds later; now one of two bad mornings begins.

Bad morning A: the database refuses to start; it reports "Data files corrupted." Here is how that happens. A single UPDATE is not one disk operation; changing an order's status touches the table row and two indexes; that is three separate places on disk. The power died after the first write and before the other two. Now the files disagree with each other: an index points at a row that says something else

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