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Save a durable record of the payment operation and its stable idempotency key before calling the payment provider. If the provider charges the customer but your server crashes before saving the result, a retry with the same key can let a provider that supports idempotency return the original outcome rather than create another charge. A timeout is an unknown outcome—not proof that the payment failed.
Why saving the key after charging creates a duplicate-payment risk
A charge at a payment provider and a write to your own database are separate operations. A local database transaction cannot make both systems commit atomically. AWS describes this broader dual-write problem for database writes paired with external events; the same separation matters when your application writes a payment record and calls a processor. AWS’s transactional outbox guidance explains the general pattern.
- Your application receives a request to create a payment.
- It calls the provider before durably recording the logical payment operation and its stable idempotency key.
- The provider processes the charge.
- The process crashes, the connection drops, or the application fails before its local completion write.
- On recovery, the application lacks durable evidence of the first attempt. If it cannot recover the original key and sends a fresh request or new key, the provider may process another operation.
The uncertainty is fundamental: if a connection fails before a response arrives, the caller may not know whether the remote side effect happened. In Stripe’s 2017 engineering article, Brandur Leach describes failures before a call, during fulfillment, and after a successful operation whose response never reaches the client. Stripe’s explanation of idempotency and distributed state covers that failure model.
What to persist before the provider call
Create a local payment-intent or operation record before making the external request. Give it a stable identity and store the idempotency key that will identify this logical payment across retries. This ordering is an architectural recommendation for handling ambiguous failures; it is not a universal, provider-specific transaction recipe.
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- One logical payment, one key: retries and replays for the same operation must use the original key, not generate a replacement.
- Keep the request consistent: Stripe compares parameters when a key is reused and returns an error if they differ. Preserve the original request data needed to retry accurately. Stripe’s current idempotency reference documents this behavior.
- Keep the operation recoverable: leave it pending or otherwise unresolved until your application can record or reconcile the provider’s outcome.
AWS’s Durable Execution guidance makes the replay issue explicit: generate the key once inside a durable step and pass that same key to every attempt. Generating a new key during replay defeats deduplication. AWS Durable Execution design best practices describe this stable-key principle.
What if the charge succeeds but the server crashes before saving the result?
Treat the payment as unresolved, not failed. The provider may have completed the charge even though your application never received or persisted the response. Retrying with the same key is safer than issuing a new logical operation, where the provider supports idempotency.
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For Stripe, subsequent requests using the same key return the original result, including a 500 response. Stripe also says it saves the first request’s resulting status code and body whether that request succeeds or fails. This behavior means an idempotent retry is not necessarily a fresh attempt that will eventually succeed; the original outcome may itself be an error. Check the operation’s state and use the provider’s documented retrieval or event mechanisms to resolve it. The appropriate lookup or reconciliation procedure depends on the integration and is not specified here.
Can you safely retry a payment after a timeout?
A timeout or lost connection does not establish whether the provider processed the request. If the endpoint supports idempotency, retry the same logical operation with the same key and the same parameters. Do not interpret a missing response as permission to start over with a new key.
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Stripe recommends exponential backoff with random jitter to avoid synchronized retries adding load. The retry schedule should be bounded and paired with a recovery path for operations that remain unresolved; retries alone do not replace durable local state. Stripe’s engineering article discusses backoff and jitter.
Do idempotency keys prevent duplicate charges forever?
No. Stripe says it may remove keys once they are at least 24 hours old. Reusing a key after it has been pruned creates a new request, so the original key cannot serve as permanent protection for delayed recovery. Keep durable local operation records and consult the current provider documentation when designing recovery beyond the provider’s retention period. Stripe’s current API reference describes key retention.
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What to check when designing payment retries
Stripe’s documented behavior should not be assumed to apply to another processor or every endpoint. Before relying on idempotency, check the documentation for the specific provider and operation:
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- Does the charge or payment-creation endpoint support idempotency?
- What is the key’s scope, and how are concurrent requests with the same key handled?
- How long are keys retained, and when may they be pruned?
- Are requests with mismatched parameters rejected?
- Which outcomes are cached, including server errors?
- What documented retrieval, event, or reconciliation mechanisms can resolve an unknown outcome?
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