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A timed-out request leaves an awkward question: did the server apply the change, or did the response simply get lost? Retrying a mutation such as creating an order can duplicate its effect unless the API provides a safe retry contract. An idempotency key helps a service recognize repeated attempts at the same logical operation—but only when the service implements matching, coordination, and outcome handling for that key.
What is an idempotency key?
An idempotency key is a caller-supplied identifier attached to a request so a service can recognize that a later attempt refers to an operation it has already seen. For example, a client might create a unique key when a user submits an order, then send that same key if it must retry because the connection drops before a response arrives.
The key is not an exactly-once guarantee by itself. The server needs a defined way to associate the key with the caller and request, coordinate simultaneous attempts, and retain enough of the outcome to answer later repeats consistently. AWS describes idempotency tokens as a way to avoid duplicate records or side effects and return a prior response: AWS Well-Architected guidance.
How is a key different from HTTP method idempotency?
HTTP defines idempotency by the intended effect of repeating an identical request: “A request method is considered "idempotent" if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request.” That is the wording of RFC 9110, Section 9.2.2. Safe methods, PUT, and DELETE are idempotent by definition; POST is not automatically so.
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A service can design a particular operation to be idempotent regardless of its HTTP method, but a client needs the API contract or another reliable signal to know that. An idempotency key is one common way to let a service recognize retries of an operation that would otherwise create an additional effect. The method’s semantics and the service’s key-handling behavior are related, but they are not interchangeable.
How do I safely retry a POST request?
First establish that the endpoint supports idempotency keys and follow its documented syntax and behavior. If it does, create one key for the logical operation—not for each network attempt—and retain it while retrying that operation. A new key on every retry makes each attempt look like a new operation.
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- Define the operation boundary. Decide what counts as the same logical change, such as placing one order or creating one payment instruction.
- Create a unique, high-entropy key. Keep it with the operation and reuse it for its retries. The IETF HTTPAPI Idempotency-Key document recommends UUIDs or similar random identifiers, says keys must be unique for requests, and says not to reuse a key with a different payload. It is an Internet-Draft, not an RFC; the API provider’s contract takes precedence.
- Send the same operation identity on each attempt. Keep the key and the relevant request data consistent. Do not attach the key to an unrelated operation or use it to change the payload.
- Retry only under the endpoint’s documented rules. If the outcome is uncertain, the service’s key contract should tell you whether to retry and what a repeat can return. Without a contract that establishes safety, do not assume that retrying a non-idempotent request is harmless; RFC 9110 cautions against automatic retries in that case.
- Use bounded backoff with jitter. Increase the delay between attempts, add randomness so clients do not all retry together, and stop according to a retry limit or deadline appropriate to the operation. Stripe discusses exponential backoff and random jitter in its idempotency article.
What happens if I send the same idempotency key twice?
There is no universal response. A completed duplicate might receive the original result, an error, or another response defined by that API. A simultaneous duplicate arriving while the first request is still in progress is a separate case: the service may wait, return an in-progress response, or reject it. The key’s meaning depends on the API’s published contract.
For an implementation, specify these behaviors rather than leaving them implicit:
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- Scope: whether a key is unique per account, tenant, endpoint, or some other boundary.
- Request matching: whether the service compares a request fingerprint, rejects a payload mismatch, or applies another documented rule.
- Concurrency: how simultaneous requests with the same key are coordinated so both cannot perform the mutation before either records the operation.
- Outcome handling: which successful or failed outcomes are retained and what result a later retry receives.
- In-progress behavior: what the client sees while the original attempt has not reached a final outcome.
The AWS Builders’ Library article on making retries safe with idempotent APIs and the IETF draft inform these design considerations; neither makes every provider’s behavior identical.
How long should idempotency keys be stored?
There is no single retention period that fits every operation. Choose a period that covers the retries your clients may reasonably make, then document what happens after it expires. If a client retries after the server has discarded the key record, the service may no longer recognize the attempt as a repeat.
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The IETF draft says resource owners should publish idempotency requirements, including an expiration policy when applicable. That is guidance to specify the policy, not a universal duration. Retention also affects how long a service must keep the outcome or other information needed to respond consistently.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should an API contract document?
The phrase “idempotency key” does not imply one shared contract across APIs. An API provider should make the practical differences explicit:
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- Where the key goes, its syntax, and its scope.
- How the service handles the same key with a changed payload.
- What a completed duplicate returns and what happens for an in-flight duplicate.
- Which outcomes are recorded and replayed.
- How long keys remain valid and what can happen after expiry.
- Which failures are safe to retry, along with recommended pacing or limits.
For clients, the operational rule is simple: treat the provider’s current documentation as authoritative. Stripe’s engineering discussion and AWS’s guidance are useful examples, not universal guarantees for other APIs.
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