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Two requests with the same idempotency key are meant to represent one logical operation, but a simultaneous duplicate does not have one universal outcome. The API provider decides whether the second request gets a conflict or transient error, can be retried, or receives a previously saved response. Check the contract for the specific provider and endpoint before deciding what to do.
What if both requests arrive at the same time?
The key gives the server a way to recognize requests as duplicates; it does not guarantee that every API makes the second request wait or returns the first request’s eventual response. While the first request is still executing, providers can handle the collision differently from a retry that arrives after the first operation has completed.
For example, Adyen documents that in a race, one request may be processed while the other returns a transient error. It also documents an in-progress duplicate that can return HTTP 422 or HTTP 409 with error code 704, meaning the request was already processed or is in progress. Stripe says a request that conflicts with another request executing concurrently is not saved as the idempotent result and can be retried. These are provider-specific behaviors, not general status-code rules.
How do provider behaviors differ?
| Provider or guidance | Documented behavior | What to do |
|---|---|---|
| Adyen | In a race, one request may be processed and the other may return a transient error. A duplicate arriving before the first completes can return HTTP 422 or HTTP 409 with error code 704. | Check the transient-error header. Retry later with the same key only when its value is true; Adyen recommends exponential backoff. See Adyen’s API idempotency documentation. |
| Stripe | Stripe saves a request’s status and body after endpoint execution begins. A conflicting request during concurrent execution is not saved as the idempotent result and can be retried. Reusing a key with a different endpoint or parameters causes an idempotency error. | Keep the request parameters unchanged and distinguish a concurrent conflict from a completed request’s saved outcome. See Stripe’s idempotent requests reference and Stripe’s errors reference. |
| Amazon EC2 | EC2 describes idempotency as ensuring an API request completes no more than once and explains safe repetition after successful completion. | Check the particular operation’s token scope and contract; EC2 behavior does not establish how unrelated APIs work. See AWS’s EC2 idempotency guidance. |
| Amazon Pay | Amazon Pay says it saves the first response and returns that saved result for subsequent requests with the same key. Its cited page does not establish every in-progress concurrency response. | Do not infer the in-progress behavior from the saved-response rule alone. See Amazon Pay’s idempotency documentation. |
| AWS implementation guidance | AWS recommends tracking token and operation state and maintaining consistency between recording the token and performing the mutation through suitable concurrency controls. | Use coordinated state handling, such as locks, transactions, or optimistic concurrency control, where needed. This is implementation guidance, not the response contract for every AWS API. See AWS Well-Architected REL04-BP04. |
Does the second request wait, fail, or return the first response?
There is no provider-neutral answer. The second call might receive a transient error or conflict while the first is in progress; after completion, a provider may replay a saved result. Stripe states, “We save results only after the execution of an endpoint begins.” Its documentation separately says a request conflicting with concurrent execution is not saved as the idempotent result. Amazon Pay documents saved-response replay for subsequent same-key requests, but the cited page does not spell out every in-progress collision.
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When comparing an API’s behavior, look for the second call’s response, an explicit retry signal, whether completed results are replayed, how changed parameters are treated, how long keys are retained, and whether the documented guarantee applies to your endpoint. Adyen also states that keys are not checked for duplication across multiple regional endpoints simultaneously, so a shared key should not be assumed to coordinate requests sent to different regional endpoints.
Can you retry while the first request is still processing?
Only when the provider’s contract says the particular response is retryable. For Adyen, retry with the same key when the transient-error header is true; its documentation says not to retry when the header is missing or false. Adyen recommends exponential backoff to avoid flooding the API. Stripe permits retrying a request that conflicts with another executing request, but the request must still represent the same logical operation.
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A client timeout or missing response is not proof that the mutation failed. Follow the provider’s reconciliation or retry instructions. Adyen suggests using webhooks to help track operations when a response is missing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should you use the key safely?
Create one key for one logical mutation
Generate a high-entropy key and preserve it when retrying the same operation. Stripe recommends UUID v4 or another sufficiently random string. Do not reuse a key for a changed payload or a different operation: Stripe checks endpoint and parameter consistency and returns an idempotency error for mismatches.
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Keep the key and operation state coordinated
If you implement idempotency in your own service, record the token and operation state consistently with the mutation. AWS recommends suitable concurrency controls, such as locks, transactions, or optimistic concurrency control, to maintain consistency and atomicity. As AWS notes, distributed systems make exactly-once behavior difficult; an idempotency token helps prevent duplicate records or side effects, but the token alone does not make every system action atomic.
Check retention and scope
Keys do not necessarily remain valid forever. Adyen says its keys are valid for 7 to 14 days after first submission; Stripe says keys can be pruned when they are at least 24 hours old. These are separate vendor policies, not a universal retention period. Confirm the relevant API’s key scope, retention, endpoint coverage, and regional behavior before relying on a key for delayed retries.
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