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Why a timeout does not mean the submission failed
A client can lose its connection after the server receives a request but before the response reaches the client. The server may not have received the request, may still be processing it, or may have completed it. A missing confirmation alone cannot distinguish those outcomes.
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This matters when repeating the operation could create a second effect: charging a payment twice, creating duplicate records, or launching the same job twice. HTTP guidance says a client should not automatically retry a non-idempotent request unless it can establish that the operation is safe to repeat or that the original was never applied. See RFC 9110, Section 9.2.2.
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When is retrying safe?
Safety depends on the operation’s intended effect and the endpoint’s documented behavior—not just on whether the request received a response.
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Idempotent operations
An operation is idempotent when repeating the same request has the same intended effect as performing it once. RFC 9110 identifies safe methods, PUT, and DELETE as idempotent. That does not mean every response will be identical, or that no other observable effect, such as logging, can occur. Nor does a method name prove that a particular application endpoint follows the expected contract: check its documentation.
Google Cloud’s HTTP guidance treats GET, PUT, and DELETE as idempotent, and POST and PATCH as neither safe nor idempotent. Treat that as a useful starting point, not a substitute for the endpoint’s own contract. A POST can be made safe to repeat through an explicit API guarantee; an endpoint’s actual behavior is what matters.
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Non-idempotent operations
If repeating an operation can produce an additional intended effect, a second attempt may duplicate it. RFC 9110’s advice is specific: a client should not automatically retry a non-idempotent request without a basis for knowing that its semantics are idempotent or that the original request was not applied.
Conditionally idempotent operations
Some APIs make retries safe only when the request includes a required precondition or parameter. For example, Google Cloud Storage documents conditional idempotency for certain operations. If the condition is absent or changes between attempts, the retry may no longer be safe. Follow the service’s retry strategy documentation; a response being classified as retryable does not, by itself, make repeating the operation safe.
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What to do after an uncertain submission
- Identify the operation. Determine what state it could change and whether performing the same logical operation twice would have the same intended effect. Do not treat the absence of a user-visible confirmation as proof that nothing happened.
- Check the endpoint contract. Look for documented idempotency behavior, supported keys, key scope and retention, parameter-matching rules, which outcomes are saved, status-query options, and any required preconditions.
- If supported, reuse the original idempotency key. Create one key for the logical operation and use that same key for every retry of that operation. A newly generated key can make a retry look like a new operation and defeat duplicate recognition.
- If there is no applicable guarantee, check authoritative state. Use the documented operation-status endpoint or reconcile against the system of record. Retry only when you have a reliable basis to conclude that the original was not applied, or the endpoint explicitly makes the repeated operation safe.
- Preserve any required conditions. For conditional idempotency, send the documented precondition or argument as required. Do not assume a retry remains safe if those inputs have changed.
What an idempotency key does—and what it does not
An idempotency key lets a provider recognize repeated requests as attempts at the same logical operation. It is useful only when the API supports it and the client follows that provider’s rules. The key does not create a universal HTTP guarantee, and its retention period and scope can affect whether a later request is recognized as a duplicate.
Stripe illustrates why details matter. Its documentation says that after endpoint execution begins, it saves the first status code and response body for a key, including a 500 response, and returns that saved result for a repeated key. Validation failures and certain concurrent conflicts are not saved. Stripe also says keys may be pruned once they are at least 24 hours old; after pruning, reuse may be treated as a new request. These are Stripe-specific rules, not defaults for other APIs. Check the provider’s current idempotency documentation before relying on them.
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In particular, reusing a key does not necessarily mean “try the work again.” Under Stripe’s documented behavior, a saved error can be replayed instead. The application must handle that result according to the provider’s contract rather than switching to a fresh key and risking a second operation.
How to design a client or workflow for uncertain outcomes
Keep one identity per logical operation
Generate the idempotency token once when the user or system initiates an operation, then persist it with the pending operation so a retry can reuse it. Do not derive a fresh token from the current time or generate a new one for each network attempt. AWS’s reliability guidance recommends consistent client tokens for this purpose; see REL04-BP04: Make mutating operations idempotent.
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Separate retry policy from retry safety
A timeout or a retryable status code answers whether another attempt may be worth making; it does not prove that repeating the operation cannot duplicate its effect. Google Cloud Storage explicitly advises considering idempotency when evaluating retries. Establish safety first, then apply the service’s retry policy.
Give users an honest recovery path
For a submission form, avoid responding to uncertainty with an immediate second submission under a new operation identity. Where available, show that the outcome is unknown and provide a status check or a way to reconcile the pending operation. This is a design implication of the HTTP retry guidance: without an idempotency guarantee, verify whether the original was applied before asking the system to perform it again.
Questions to ask when evaluating an API
- Does the endpoint document the operation as safe or idempotent, and does that guarantee cover the exact action you are taking?
- Can the client supply an idempotency key? What is its scope and retention period?
- Must retries match the original parameters, and which outcomes are stored and replayed?
- Is there an operation-status query or another authoritative way to determine whether the first request took effect?
- Does safety depend on a condition, such as an ETag or another precondition, that must be preserved?
These questions help distinguish an API that merely accepts retries from one that documents how retries avoid duplicate effects. Provider-specific rules can change, so verify the current contract when implementing a client.
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