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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11A sequential retry only tells you what happens after the first request has finished. It does not show whether an API safely handles a duplicate that arrives while the original request is still running. To test that race, hold the first request open, send a second request with the same idempotency key and payload, then check both responses and the final observable effect.
What an idempotency key is—and what a passing retry proves
An idempotency key is a client-generated value that lets a resource recognize retries of the same request. It is commonly used to make operations sent with methods such as POST or PATCH fault-tolerant. The IETF describes this use in its Idempotency-Key HTTP Header Field draft.
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For a completed retry, send the same operation again with the same key and identical payload. If the API contract says the resource replays the original result, check that the response matches that contract and that the externally visible effect remains consistent with one operation. This establishes behavior after completion; it does not test an overlapping request.
Test completed retries and in-flight duplicates separately
| Case | When the duplicate arrives | Key and payload | What to observe |
|---|---|---|---|
| Completed retry | After the original request completes | Same key, same payload | Whether the API returns the original operation’s result as documented, and whether the visible effect remains consistent with one operation. |
| Concurrent duplicate | While the original request is still outstanding | Same key, same payload | The second response’s documented in-progress or conflict behavior, and whether the visible effect indicates duplicate execution. |
| Changed-payload reuse | After or during the original request, according to the API contract | Same key, different payload | Whether the API rejects reuse of the key for a different request, and the response specified by its documentation. |
The IETF draft treats the first two timings as distinct. It says a completed duplicate can receive the earlier result, while a retry received before the original completes should receive a resource-conflict error. Its example status codes are 409 Conflict for the in-flight duplicate and 422 for reusing a key with a different payload. These are draft recommendations, not universal requirements: assert the behavior documented by the API you are testing.
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Set up a black-box race test
You need a way to keep the first operation outstanding long enough for the duplicate request to overlap it. Use a controllable slow operation or a test barrier exposed by the system under test. Do not infer that a race was exercised merely because two client calls were started close together: confirm from timing or instrumentation available at the interface that the second request arrived before the first completed.
- Establish a baseline. Send one request with a fresh key. Record its status and body, then inspect the externally visible result of the operation.
- Check the completed retry. Once the first request has completed, resend the identical operation and payload with the same key. Compare the response with the documented replay behavior and inspect the visible effect for consistency with a single operation.
- Hold a new first request open. Start a fresh operation with a new key and use the slow operation or barrier to keep it in progress.
- Send the overlapping duplicate. Before the original settles, send the same operation with the same key and identical payload. Capture the second response’s status and body; let the first request finish and capture its response too.
- Inspect the final effect. Check the resource or other observable outcome after both requests settle. Determine whether it is consistent with one operation rather than two.
- Test changed-payload reuse separately. Reuse a key with a different payload and compare the result with the API’s documented rejection behavior. Do not treat
422as mandatory unless the API contract requires it. - Repeat across timing variations. If practical, vary when the duplicate arrives and run the concurrent case repeatedly. A single run that does not reproduce a problem cannot establish that the race is absent.
Choose assertions from the API contract
For each case, record the timing, key equality, payload equality, status, response body, and final observable effect. Keep these observations distinct: a second response that looks correct does not by itself establish that the underlying operation ran only once, and a single visible effect does not establish that the response followed the documented contract.
- Completed retry: Assert the documented response for a retry after completion and verify the externally visible outcome.
- In-flight duplicate: Assert the documented conflict or in-progress response for an overlapping duplicate, then verify the final outcome after both requests settle.
- Changed payload: Assert the API’s documented behavior when a key is reused for a different request.
- Expiry boundary: Test key expiration only if the API documents an expiry policy. The draft says a resource may define expiry and should document it when applicable.
Account for the draft’s status
The cited document is draft-ietf-httpapi-idempotency-key-header-07, an Internet-Draft published on 2025-10-15. The IETF Datatracker page lists it as expired on 2026-04-18 and archived. Internet-Drafts are working documents that may be updated, replaced, or obsoleted; this document is not a finalized RFC. Use it as draft guidance, and base pass/fail status-code assertions on the target API’s current contract.
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