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An Idempotency-Key header handles one of five layers where duplicates cause harm. Database writes, queue consumers, external API calls, and saga compensation each have failure modes the HTTP key doesn't cover.
Every tutorial shows you the happy path. Here is the part they skip: the exact schema, concurrent duplicate requests, crashed workers, and the eviction query that doesn't delete the wrong keys.
A unique constraint stops the check-before-insert race but leaves concurrent in-flight requests undefined. Here is the state machine pattern — with Postgres code — that handles all six failure modes.
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