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A timeout does not tell a client whether a server completed a request. If the client retries a mutation such as creating an order, the server may perform it twice unless the operation has a defined way to recognize retries. An idempotency key can provide that recognition—but only when the service stores or coordinates the key, associates it with the right request, and defines how repeats behave. It is a retry-safety mechanism, not an exactly-once guarantee.
What is an idempotency key?
An idempotency key is a unique identifier a caller supplies for one logical operation. The caller keeps using that same key when retrying the operation after an ambiguous outcome, such as a lost response. The service uses the key to identify a repeat and follow its documented duplicate-request behavior, rather than blindly performing the mutation again.
The key alone does nothing. The service needs to coordinate the key with the operation and retain enough outcome information to handle a later retry consistently. AWS describes idempotent request tokens as a way to avoid duplicate records or side effects and return a prior response; see its Well-Architected guidance.
How are idempotency keys different from HTTP method idempotency?
HTTP method idempotency is a property of the intended effect of repeated identical requests. RFC 9110, Section 9.2.2, says: “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.” PUT, DELETE, and safe methods are idempotent by definition; see RFC 9110.
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An idempotency key is instead an application-level mechanism for recognizing a logical operation across requests. An API can define a POST operation as safe to retry with a key, but the client must rely on that API’s contract. Do not infer retry safety merely from the presence of a header or from a method name.
How do I safely retry a POST request?
- Confirm the API contract. Establish that the specific POST operation supports idempotent retries, how to supply the key, and what responses to expect for completed or in-progress duplicates. HTTP warns against automatically retrying a non-idempotent request unless the client can establish that its semantics are safe.
- Create one key for the logical operation. Generate a high-entropy unique value, such as a UUID, when the user or system initiates the operation. Keep it for every transport retry; do not generate a new key for each attempt.
- Retry the same request identity. Send the same key and the same intended payload. Do not reuse that key for a different operation or payload. The IETF HTTPAPI Idempotency-Key document is an Internet-Draft, not an RFC; it recommends unique keys and says not to reuse a key with a different payload. Follow the provider’s current contract for exact header syntax and request matching.
- Use bounded backoff with jitter. Space out retries and add randomness so clients do not all retry at once. Stripe discusses exponential backoff and random jitter in its idempotency article. Set a retry limit or deadline appropriate to the operation rather than retrying indefinitely.
- Handle ambiguity as ambiguity. A timeout or lost connection is not proof of failure. Retry with the original key according to the API’s policy, then interpret the response using that policy. If the API provides a way to retrieve operation status, use its documented process.
What should an API implement?
A robust contract covers more than the key’s format. The service must decide which caller or tenant the key belongs to, how it identifies the request associated with it, how it handles simultaneous arrivals, which outcomes it retains, and how long it retains them. The AWS Builders’ Library discussion of making retries safe and the IETF draft inform these design considerations; they do not prescribe one database mechanism for every service.
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- Scope keys appropriately. Associate a key with the relevant caller, account, or tenant so unrelated callers do not accidentally collide.
- Match requests explicitly. Decide whether to compare a request fingerprint or reject a payload mismatch. Document what happens when a client sends the same key with changed parameters.
- Coordinate concurrent requests. A completed duplicate and a simultaneous in-flight duplicate are different cases. The service needs coordination that prevents both requests from independently applying the mutation before either records the outcome.
- Retain a useful outcome. Specify which successes and failures are replayed, and what a repeat receives while the original request is still running. Persist enough information to answer retries consistently.
- Define expiration and recovery. Publish the retention period or expiration policy when applicable, and explain what happens if a retry arrives after the record expires. A retry after expiry may no longer be recognized as a duplicate.
What happens if I send the same idempotency key twice?
There is no universal response. A service might return the saved result for a completed operation, reject a key reused with a different payload, or return a distinct response while the first request is still in progress. Its contract should also say which failures are retained and replayed. The IETF document is still an Internet-Draft and recommends that resource owners publish their requirements, including expiration policy when applicable.
Do not assume two APIs behave alike because both use a header called an idempotency key. Check the specific API documentation for key scope, syntax, payload-mismatch rules, duplicate and concurrent responses, retained outcomes, expiration, and retry guidance.
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How long should idempotency keys be stored?
There is no single retention duration established by HTTP or by the IETF draft. The API owner should choose and publish an expiration policy that fits the operation and the period during which callers may retry. The client must not assume a key will be recognized indefinitely: after its record expires, the service may treat a retry as a new operation. For a named provider, use that provider’s current API contract rather than general guidance.
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What are the common implementation mistakes?
- Generating a fresh key for every network attempt, which makes retries look like new operations.
- Reusing one key for a different logical operation or changed payload.
- Recording a key only after performing the side effect, leaving a race in which concurrent requests can both act.
- Returning an arbitrary response for an in-progress duplicate without documenting what the caller should do next.
- Failing to state expiry behavior, or letting clients assume a key remains valid forever.
- Retrying aggressively without a bound, backoff, and jitter, which can worsen an overloaded service.
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