System Design Patterns: From Fundamentals to Real Systems

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Write-Ahead Log

1. The Incident

Tuesday, 3:07 AM: a power event takes out the rack holding shard 2. The machine reboots ninety seconds later. And now one of two bad mornings begins.

Bad morning A: the database refuses to start. "Data files corrupted." Here's how that happens: a single UPDATE isn't one disk operation. Changing an order's status touches the table row and two indexes: three separate places on disk. The power died after the first write and before the other two, and now the files disagree with each other: an index pointing at a row that says something else

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