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An idempotent request has the same intended effect on a server whether it is applied once or multiple times. That makes idempotence important when a client times out or loses its connection before receiving a response: the client may not know whether the server completed the first attempt, but repeating an idempotent operation should not change the intended outcome.
What does idempotent mean in an API?
Idempotence describes the requested effect on the server, not whether the server receives or records a request only once. The IETF defines an HTTP method as idempotent when multiple identical requests have the same intended server effect as one request. A server can still log every attempt or record each one in revision history.
The response to a repeat does not have to match the first response. The important guarantee is the intended effect, not identical status codes or response bodies. See RFC 9110, Section 9.2.2.
Which HTTP methods are idempotent?
RFC 9110 classifies PUT, DELETE, and all safe methods as idempotent. Safe methods are read-oriented by definition. POST is not defined as idempotent by the HTTP standard, but an individual POST operation can still be designed to have idempotent behavior or use an API-specific retry mechanism.
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| Method or category | Idempotent by HTTP semantics? | Practical meaning |
|---|---|---|
| Safe methods | Yes | Defined as read-oriented; repeated requests have the same intended effect. |
| PUT | Yes | Repeated identical requests are intended to leave the resource in the same state. |
| DELETE | Yes | Repeating the request has the same intended effect, even if a later response differs. |
| POST | Not by method definition | An API may make a particular operation idempotent or provide a separate mechanism such as an idempotency key. |
These are method semantics, not a guarantee that every API implementation behaves correctly. The standard defines the expected semantics; an API’s documented behavior determines what its specific operation actually guarantees.
Why does idempotence matter when retrying requests?
A client can send a request, have the server apply it, and then lose the connection before the response arrives. A timeout or connection failure alone does not establish whether the operation ran. If the request is idempotent, the client can repeat it without changing the intended outcome.
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For a non-idempotent request, RFC 9110 says a client should not automatically retry unless it knows the operation is idempotent despite the method or can detect that the first attempt was never applied. A retry without either basis could perform the operation more than once.
Can you retry a POST request after a timeout?
Not automatically based on POST alone. Check whether the API documents the operation as idempotent or supports an idempotency key. If it supports keys, retry the same logical operation with the same key and the same parameters; creating a new key for each attempt does not identify the attempts as duplicates.
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Whether a POST is safe to retry depends on that API’s contract. The HTTP method itself does not provide a universal deduplication guarantee.
How do idempotency keys prevent duplicate operations?
An idempotency key is an application-level token that lets an API recognize retries as attempts at one logical operation. It is not a universal HTTP feature, and APIs can differ in how they handle keys, parameter mismatches, repeated responses, retention, and concurrent requests. Read the provider’s documentation rather than assuming that a header with this name behaves the same everywhere.
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Stripe’s documented behavior
Stripe says it saves the first result for a key and returns the same status and response body for later requests using that key, including when the result is a 500 error. It compares parameters and rejects requests that reuse a key with different parameters. Stripe documents a maximum key length of 255 characters and says it may remove keys after they are at least 24 hours old; if a pruned key is reused, Stripe treats the request as new. These limits and behaviors apply to Stripe’s API, not to idempotency keys generally. See Stripe’s idempotent requests documentation.
What to verify in an API’s key contract
- How the API identifies the logical operation and scopes a key.
- Whether it compares request parameters and what happens if they differ.
- Whether a retry returns the original result or produces another response.
- How long the key is retained and what happens after expiration or removal.
- How the API handles simultaneous requests using the same key.
What should API designers do to make mutations idempotent?
A robust design must bind the token to the operation and its result reliably. If a server records a key but fails before applying the mutation, or applies the mutation but fails before recording the key, a retry can produce the wrong outcome. AWS guidance emphasizes handling the token and operation with atomic, consistent, isolated, and durable behavior.
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- Define how a request represents one logical operation and how its token is scoped.
- Specify what happens when the same token arrives with different parameters.
- Coordinate recording the token, applying the mutation, and saving the result so retries cannot split those actions into inconsistent states.
- Document key retention, repeated-response behavior, and handling of concurrent retries.
For implementation guidance, see the AWS Well-Architected Framework guidance on idempotent mutating operations and the AWS Builders’ Library discussion of making retries safe with idempotent APIs.
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