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What idempotency means for an API update
Under RFC 9110, Section 9.2.2, a method is idempotent when multiple identical requests have the same intended effect on the server as one request. That definition is about the effect, not identical responses: a server may log each attempt or update incidental metadata while still applying the intended change only once.
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HTTP method semantics are a useful starting point, not proof that a particular endpoint is safe to repeat. PUT, DELETE, and safe methods are idempotent under HTTP semantics; POST and PATCH are not inherently idempotent in Google Cloud’s API style guidance. The implementation must honor the contract implied by its method.
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Choose state-setting semantics where they fit
Prefer expressing the desired state when that accurately represents the domain. Repeating “set quantity to 4” converges on the same quantity; repeating “add 1 to quantity” increments it again unless the API adds deduplication. This is a design choice, not a requirement imposed by HTTP. A PUT endpoint can be a natural fit for a state-setting update.
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For actions whose effect should happen once—such as creating a charge or triggering a job—an application-level idempotency key can identify the logical action even when it is sent using a method that is not inherently idempotent. Stripe’s API design guidance discusses using idempotency to make such operations predictable.
Define an idempotency-key contract
The client should generate one key for each logical operation and reuse it on every retry, along with the same operation parameters. Do not generate a fresh key for each network attempt: that makes each attempt look like a new action. Stripe recommends a V4 UUID or another random value with enough entropy to avoid collisions, and checks that a repeated key is used with matching parameters. AWS likewise recommends reusing the same token when repeating a request.
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Reject reuse of a key with different parameters rather than silently treating a different action as the original one. Also document the key’s scope—for example, whether it is unique per account or tenant and endpoint. There is no single scope mandated for every API; choose one that avoids both cross-user collisions and accidental deduplication of distinct operations. Avoid using a timestamp alone as the key: separate operations can share a time value, and AWS guidance flags timestamps as a poor choice.
Coordinate concurrent duplicate requests
Retries are not always sequential. A client may resend because it timed out while the original request is still running, so two copies with the same key can arrive together. The server needs coordinated in-progress and completed states so both requests cannot independently apply the mutation.
- Record or claim the operation key as in progress before applying the side effect.
- When another request with that key arrives during execution, do not apply the side effect again. Either return a documented in-progress or conflict response, or wait for the first request’s outcome.
- When execution finishes, transition the record to a completed outcome consistently with the mutation, so a later retry can replay it.
The precise transaction and storage strategy depends on the database and the operation’s consistency boundaries. Stripe documents that a conflict with a request already executing is not stored as a completed result and can be retried; other APIs should state their own concurrency behavior.
Store and replay a completed outcome
Once an operation reaches the point your API considers recordable, retain enough information to give a retry a stable logical outcome. A retry might receive the original status and body, or a signal that the operation has already happened; document which behavior clients can rely on.
Stripe’s implementation stores the first request’s resulting status code and body, including a 500 response, and returns that result for later requests using the same key. It does not save a result when validation fails before endpoint execution begins or when another request is still executing. These are Stripe-specific rules, not universal requirements. Your API should define which validation failures are outside execution, which outcomes are recorded, and what an in-progress duplicate receives.
Avoid promising “exactly once” as a blanket distributed-systems guarantee. AWS Well-Architected guidance distinguishes at-most-once and at-least-once behavior from the difficulty of achieving exactly-once effects. A more useful API contract is that requests with the same operation identity have one intended effect and receive a stable recorded outcome during the documented retention period.
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Set retry rules and key retention
When a response is lost, the client may not know whether the server committed the change. It should retry with the original key and payload while the server guarantees that the corresponding record still exists. Set a retention period that covers the client’s retry horizon, and specify what happens after expiry; otherwise, an old retry may be mistaken for a new operation.
Stripe says it may prune keys once they are at least 24 hours old; if a pruned key is reused, Stripe treats the request as new. That is Stripe’s policy, not a general HTTP rule or a duration to copy without considering your system.
For requests without an idempotency key, follow the method’s semantics. RFC 9110 advises clients not to automatically retry a non-idempotent method unless they can establish that the operation is idempotent in practice or that the original request was never applied. An uncertain connection failure alone does not establish that the server did nothing.
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Make the contract specific enough that a client can decide whether, when, and how to retry:
Quick Recap
- Which operations support idempotency keys and how keys are scoped.
- Whether parameters must match exactly and how mismatches are reported.
- What happens when the same key is received while an operation is in progress.
- Which completed outcomes are saved and what a replay returns.
- How long keys are retained, the client’s safe retry window, and what happens after expiry.
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