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A retry can reach your API after the server has committed the first request but before the client receives its response. To make that retry safe, define what counts as the same logical operation, prevent concurrent copies from applying it twice, and retain its outcome for a documented period. Idempotency concerns the intended effect—not whether every response is byte-for-byte identical.
What idempotency means for an API update
RFC 9110 defines an idempotent method by its intended effect on the server: multiple identical requests should have the same intended effect as one request. The responses do not have to be identical, and incidental effects such as logging or recording a revision may still occur. See RFC 9110, Section 9.2.2.
HTTP methods establish useful defaults, but the endpoint’s actual behavior must honor the contract. PUT, DELETE, and safe methods are idempotent under HTTP semantics. POST and PATCH are not inherently idempotent in Google Cloud’s API style guidance. A method name alone does not prove that an implementation is safe to repeat.
Choose between setting state and performing an action
When the goal is to make a resource match a desired state, design the operation so repeating it converges on that state. For example, “set quantity to 4” has the same intended result when applied again; “add 1 to quantity” applies another increment each time. This is a design distinction, not a requirement that HTTP prescribe a particular endpoint shape.
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Use PUT when the operation naturally expresses an idempotent resource update. For an action with a one-time side effect—such as creating a charge or triggering a job—an application-level idempotency key can identify retries as the same logical operation. Google’s guidance discusses method semantics; Stripe’s API design article explains the application-level role of idempotency.
Define a stable idempotency-key contract
The client should create one key for a logical operation and reuse it, with the same parameters, for every network attempt. A new key on each retry defeats deduplication. The server should associate the key with the operation and reject reuse with different parameters rather than silently treating a different action as the original one.
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- Generate a collision-resistant key: Stripe recommends a V4 UUID or another random value with enough entropy. AWS warns against using a timestamp alone, since separate operations can share a time value.
- Keep the key stable: Re-send the original key and operation parameters when recovering from a timeout or lost response.
- Choose and document scope: For example, scope keys by account or tenant and endpoint. There is no universal scope prescribed by the cited sources; choose one that keeps unrelated users’ and operations’ identities distinct.
- Specify parameter matching: State which request parameters must match and what error the API returns when a key is reused with different values.
Stripe’s API reference documents parameter comparison for repeated keys. AWS Well-Architected guidance recommends reusing the same token when repeating an operation.
Coordinate concurrent duplicates
Two requests with the same key can arrive while the first is still executing. The server must coordinate them so they do not independently apply the mutation. Conceptually, record an in-progress operation before applying the side effect, then transition it to a completed outcome consistently with that mutation. The right transaction and storage design depends on the system’s database and boundaries.
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Define what a concurrent duplicate receives: the API can return a documented in-progress or conflict response, or wait for the first request’s result. Stripe documents that a conflict with a currently executing request is not saved as a completed result and can be retried. That is Stripe’s behavior, not a universal HTTP rule.
Record outcomes and replay them consistently
Once execution reaches a result the API considers recordable, retain enough information to provide a stable logical outcome to a retry. Specify the boundary between validation failures before execution, in-progress conflicts, and completed execution; clients need to know which results are safe to retry and what response to expect.
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Stripe’s implementation stores the first request’s resulting status code and body, including a 500 response, and returns that result for later requests with the same key. It does not save a result when validation fails before endpoint execution begins or while another request is executing. Those details are implementation choices, not a standard every API must copy.
Avoid promising “exactly once” as a blanket distributed-systems guarantee. AWS notes the difficulty of achieving exactly-once effects compared with at-most-once or at-least-once behavior. A more precise contract is that requests sharing an operation identity have one intended effect and receive a stable recorded outcome during the documented retention window.
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Set retry conditions and key retention
When a response is lost, the client may not know whether the server committed the operation. For a keyed operation, retry with the original key and payload while the server guarantees the idempotency record still exists. Set a retention window that covers the client’s retry horizon and explain what happens when it expires.
Stripe says keys can be pruned after they are at least 24 hours old; reusing a key after it has been pruned can create a new request. This is Stripe-specific behavior, not a general standard or a retention recommendation for every API.
For requests without an application key, follow HTTP method semantics. RFC 9110 says a client should not automatically retry a non-idempotent method unless it can establish that the operation is idempotent in practice or that the original request was never applied. An idempotent method can be retried after an uncertain communication failure when appropriate, but its implementation still needs to honor its intended-effect contract.
Document the behavior clients can rely on
Publish the details that make retry handling predictable, including:
Quick Recap
- Which operations are naturally idempotent and which require a key.
- How keys are generated, scoped, matched to parameters, and rejected when reused incorrectly.
- How concurrent duplicates are handled and which outcomes are stored.
- How long keys are retained, what happens after expiry, and the supported retry horizon.
- Which communication failures clients may retry automatically and which require application-level judgment.
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