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An idempotent request has the same intended effect on the server whether it is applied once or repeatedly. That makes retries safer when a client times out or loses its connection before receiving a response—but it does not mean the server processed the request only once, or that every repeat gets the same response.
What does idempotent mean in an API?
RFC 9110 defines an HTTP method as idempotent when multiple identical requests have the same intended server effect as one request. The standard focuses on the requested effect, not every side effect of handling the request: a server can still log each attempt or add entries to a revision history. The response to a repeat can also differ from the first response.
For example, setting a resource to a specified state can be idempotent: applying the same update again leaves the resource in that state. By contrast, an operation that creates a new charge or adds another item each time may have a different effect on every application. The key question is what the operation does to server state, not whether two responses look alike.
RFC 9110, Section 9.2.2 defines the standard’s semantics and retry guidance.
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Which HTTP methods are idempotent?
RFC 9110 classifies PUT, DELETE, and all safe methods as idempotent. Safe methods are read-oriented by definition. These are method semantics; an API still needs to honor the semantics of the method it exposes.
| Method category | Idempotent by HTTP definition? | What that means for retries |
|---|---|---|
| Safe methods, such as GET | Yes | Repeating the request has the same intended effect. Safe methods are read-oriented. |
| PUT | Yes | Repeating an identical request has the same intended effect as applying it once. |
| DELETE | Yes | Repeating an identical request has the same intended effect as applying it once; the response may differ. |
| POST | No, not by method definition | Do not assume automatic retries are safe from the method alone. An individual POST operation may nevertheless be designed to be idempotent or protected by an API-specific mechanism. |
These classifications describe intended effects, not guaranteed identical response bodies or an exactly-once delivery mechanism.
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Why does idempotence matter when a request times out?
A connection failure can happen after the server has applied a request but before the client receives the response. From the missing response alone, the client cannot tell whether the operation ran. If the operation is idempotent, repeating it is designed to leave the intended outcome unchanged whether or not the first attempt succeeded.
RFC 9110 advises clients not to automatically retry a non-idempotent request unless they know its semantics are idempotent despite the method or can detect that the original request was never applied. A timeout is not, by itself, proof that the server did nothing.
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Not automatically based on POST alone. First check whether the API documents the operation as safe to repeat, provides an idempotency-key mechanism, or gives you another reliable way to determine that the first attempt was not applied. Without such a basis, a retry could perform the operation twice.
When the API supports idempotency keys
- Generate or obtain a key for the logical operation, not for each network attempt.
- Send that same key and the same operation parameters on every retry.
- Follow the API’s documented rules for key scope, parameter matching, retention, and the response returned for repeats.
Do not generate a new key for each retry: a new token may be treated as a new operation rather than a repeat. An idempotency key only provides protection when the particular API implements and documents its behavior.
How do idempotency keys prevent duplicate operations?
An idempotency key is an application-level token that lets an API recognize repeated attempts for one logical operation. It is not a universal HTTP feature, and providers can implement different rules.
Stripe documents one example: subsequent requests with the same key return the same result, including 500 errors, and the API checks whether parameters match. Stripe says keys may be removed after they are at least 24 hours old; if a pruned key is reused, Stripe treats the request as new. Stripe also documents a maximum key length of 255 characters. These limits and behaviors are specific to Stripe’s API reference, not general rules for idempotency keys. See Stripe’s idempotent requests documentation for its contract.
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| Behavior to check | Why it matters |
|---|---|
| Key scope | Establishes which operations or resources the API considers when recognizing a repeat. |
| Parameter matching | Determines what happens if the same key is sent with different operation details. |
| Response replay | Explains whether a repeat returns a saved result or is handled another way. |
| Retention | Sets how long the API recognizes a key before it may be treated as new. |
| Concurrent requests | Clarifies how simultaneous attempts with the same key are coordinated. |
What should API designers implement?
Reliable idempotency requires more than storing a token after an operation. The API needs a clear contract for recognizing one logical request and coordinating the token with the mutation and its result.
- Define how the API identifies a logical operation and the scope of its token.
- Specify what happens when the same token arrives with different parameters.
- Document the response behavior for repeats, token retention, and concurrent requests.
- Coordinate recording the token and performing the mutation so failures cannot leave the operation applied without a corresponding deduplication record, or vice versa.
AWS guidance emphasizes handling the token and associated operation with atomic, consistent, isolated, and durable behavior. See AWS Well-Architected guidance on making mutating operations idempotent and AWS Builders’ Library guidance on making retries safe with idempotent APIs.
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