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Why saving the key after the charge is risky
A payment processor and your application database are separate systems. A local database transaction cannot make a processor charge and your own record commit atomically; one can succeed while the other does not. AWS describes this general dual-write problem for a database write paired with an external event, a pattern analogous to payments: AWS transactional outbox guidance.
Consider the unsafe sequence: your server sends a charge request, the processor completes it, and then the server crashes before writing the operation’s key or completed status. When the request is retried, your application has no durable local record to show that the first attempt may have succeeded. If it sends a fresh request or generates a fresh key, the processor may treat it as a second payment.
This is a distributed-systems ambiguity, not proof that the charge failed. Brandur Leach’s Stripe engineering article describes failures before a request reaches the server, interruptions while it is being handled, and successful operations whose responses never reach the client: Stripe: Idempotency and retries.
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Should I save the idempotency key before or after charging?
Save it before making the external charge request. Create a durable local payment-intent or operation record first, with a stable identity and enough state to resume or reconcile the attempt. Then send the same idempotency key to the processor for every retry of that logical payment.
- Create the operation: Record the intended payment in your database and assign its stable idempotency key before calling the processor.
- Submit the request: Send the payment request with that key and the intended parameters.
- Record the outcome: When you receive a result, persist the provider response and update the local operation state.
- Recover ambiguity: If the request times out or the process fails before saving a result, keep the operation unresolved and recover it using the same key and provider-supported retrieval or event mechanisms.
This ordering is an architectural response to the documented lost-response failure mode; it is not a universal, provider-specific transaction recipe. Your database and the processor still do not share one atomic commit.
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What happens if the charge succeeds but my server crashes?
The processor may have completed the payment even though your application never received or saved its response. Treat that operation as unknown, not failed. Keep its durable local record pending until you can establish the outcome. Do not replace its key with a new one simply because the original request timed out.
For Stripe, requests using the same idempotency key return the first request’s saved result, including a saved 500 response. Stripe also compares parameters for a reused key and returns an error if they differ. Preserve the original request parameters when retrying; do not reuse one key for a materially different payment. See the Stripe idempotent requests reference.
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Can I safely retry after a timeout?
Where the provider supports idempotency for the specific endpoint, retry the same logical operation with the same key and original parameters. A timeout only tells you that your caller did not get a timely response; it does not establish whether the remote side effect occurred.
Stripe’s engineering guidance recommends exponential backoff with random jitter, which spaces retries and helps avoid synchronized retry load: Stripe: Idempotency and retries. For unresolved operations, use the provider’s documented retrieval, event, or reconciliation mechanisms; the exact procedure depends on the integration.
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Keep the key stable across application replays, too. AWS Durable Execution guidance recommends generating it once inside a durable step and passing it to every attempt; generating a new key during replay defeats deduplication: AWS Lambda durable execution idempotency.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does an idempotency key prevent duplicate charges forever?
No. Idempotency behavior and key retention are provider-specific. Stripe says keys can be removed once they are at least 24 hours old. Reusing a key after it has been pruned creates a new request, so that key alone cannot protect a delayed recovery indefinitely. Keep durable local operation records and consult the provider’s current documentation when defining a longer-term reconciliation policy: Stripe idempotent requests reference.
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What to check when choosing a provider or designing retries
Do not assume one processor’s exact idempotency rules apply to another, or to every endpoint at the same processor. Verify these behaviors for the payment operation you use:
- Whether that specific endpoint supports idempotency keys.
- How keys are scoped and how concurrent requests using the same key behave.
- How long keys are retained and whether they are pruned.
- Whether reusing a key with different parameters is rejected.
- Which outcomes are saved and returned on retry, including server errors.
- Which documented retrieval, event, or reconciliation options can resolve an unknown outcome.
Stripe’s current API reference establishes the Stripe-specific behaviors described above. Its engineering article explains the failure model; AWS’s durable-execution and transactional-outbox guidance supplies general patterns, not guarantees for every payment provider.
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