PRODUCTION SCENARIO
An order-creation call times out after the request was sent. The order service accepts an idempotency key and supports lookup by that key. The client holds the key it used and has no response receipt.
What should the client do next?
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Show answer and explanation
Answer: Look up the order by the original idempotency key and act on the result
A timeout leaves the outcome unknown, so the client resolves it by asking the service about the key it already used; that key returns the same meaning as the original response, whether the order committed or not. A fresh key makes the retry a distinct request and creates the duplicate. Matching on customer and timestamp cannot tell a duplicate from a genuine second order placed moments apart. Declaring failure reports an outcome nobody observed.