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Idempotency is how an API can recognize that two requests are attempts to carry out the same operation, rather than two separate purchases. If a payment request times out, the client can retry it with the same idempotency key; a supporting service can then avoid repeating the charge and return the original result. This depends on the client reusing the key and the service implementing deduplication correctly—it is not a promise that every request is delivered only once.
Why one purchase can produce more than one request
Suppose you click Pay once. The payment service processes the request, but its response is delayed or lost. Your browser cannot tell whether the payment went through, so you click again or the software retries automatically. The two network attempts may represent one purchase intent.
Without a way to recognize that shared intent, the service could process both attempts and create two charges. With idempotency, the client attaches the same key to retries of the original operation. The service can associate the repeated request with the first operation and preserve or return its result. Stripe describes this as a way to retry requests without accidentally performing the same operation twice: Stripe’s idempotent requests documentation.
What an idempotency key does—and does not do
An idempotency key is an identifier for one logical operation, such as one attempt to create a particular payment. It lets a service distinguish a retry from a new request. The key does not make the network deliver a request exactly once, and it cannot by itself ensure that every later step in a distributed workflow happens only once.
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A service’s behavior depends on its own rules: which requests share a key scope, whether parameters must match, whether it replays the first response, how long it retains keys, and how it handles simultaneous requests. Downstream services and message consumers also need suitable duplicate handling. AWS recommends passing the token to downstream services and tracking duplicate message tokens where appropriate: AWS Well-Architected guidance on mutating operations.
How to use keys for safe retries
- Create one key for one intended operation. For example, generate a unique key when the customer submits a particular payment.
- Reuse that exact key for retries of that operation. If a timeout leaves the outcome uncertain, retry with the same key rather than creating a new one.
- Keep the operation’s parameters consistent. Providers may reject a reused key if the new request changes the original parameters.
- Use a new key for a genuinely new operation. A second purchase is not a retry of the first, even if the amount is identical.
- Check the provider’s rules and propagate the identifier when needed. If the operation triggers downstream work, those services or consumers need their own way to recognize duplicate processing.
Provider rules differ
Idempotency is a design pattern, not a universal contract. These examples show why an implementation must be checked against its provider’s documentation.
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| Behavior | Stripe | Amazon Pay |
|---|---|---|
| Key format and limit | UUID v4 or another random string with sufficient entropy; up to 255 characters, according to Stripe. | Up to 32 characters from a specified character set, according to Amazon Pay. |
| Changed parameters with the same key | Must match the original request; otherwise Stripe returns an error. | Returns a DuplicateIdempotencyKey error. |
| Retention | Keys may be pruned once they are at least 24 hours old; reusing a pruned key can start a new request. | Keys are stored indefinitely and scoped to a merchant account. |
| Result handling | After endpoint execution begins, Stripe saves the status and response body for the first request, including a 500 response. | A repeat with the same key returns the saved result. |
| Concurrent requests | A result is not saved when validation fails or a concurrent request conflicts before endpoint execution begins. | Not stated in the cited Amazon Pay documentation. |
These are provider-specific rules, not requirements that every API must follow. Stripe’s 24-hour pruning behavior also means a key should not be treated as a permanent record of a purchase. Amazon Pay’s indefinite retention and merchant-account scope are different choices, not a general standard.
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Idempotency beyond payments
The same problem appears whenever a retry can repeat a mutating action: creating a cloud resource, placing a job on a queue, or processing a message. AWS documents client tokens for EC2 and ECS API requests, and its Well-Architected guidance emphasizes duplicate handling for both mutating requests and messages: EC2 idempotency and ECS idempotency.
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For a workflow that crosses services, one protected request does not automatically protect the whole chain. Each service that can create a side effect must handle retries appropriately; forwarding an identifier helps only if the receiving service also recognizes and uses it.
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