Design a checkout API where clients may retry requests due to timeouts. How would you avoid duplicate orders or charges?

Design an idempotent checkout backend

Designs a checkout or order creation flow that survives client retries without duplicate orders, payments, or messages.

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I would require an idempotency key for checkout requests and store the key with the user, request hash, order id, status, and response outcome. If the client retries with the same key, the API returns the existing result instead of creating a second order. Order creation and payment intent state would be persisted with clear states like pending, payment_authorized, confirmed, failed, and cancelled. The client gets a stable response even when the first request timed out.

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Idempotency keys, transaction boundaries, payment side effects, order state, outbox, retries, and client contracts.

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Checkout is a classic backend design because correctness matters more than returning quickly. The design needs to survive retries, timeouts, partial failures, and external payment side effects.
I would require an idempotency key for checkout requests and store the key with the user, request hash, order id, status, and response outcome. If the client retries with the same key, the API returns the existing result instead of creating a second order. Order creation and payment intent state would be persisted with clear states like pending, payment_authorized, confirmed, failed, and cancelled. The client gets a stable response even when the first request timed out.
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I would separate internal state changes from external side effects. The database transaction can create the order and idempotency record, while payment confirmation or messages can use an outbox or a controlled worker so failures are retried safely. If the same key arrives with a different request body, I would reject it as a conflict. I would also design user-visible recovery: show pending checkout, allow retry by key, and reconcile with the payment provider when status is uncertain.

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Relying only on client-side button disabling.
No idempotency key or unique constraint.
Charging the card before durable order state exists.
No recovery path for uncertain payment status.

Design a checkout API where clients may retry requests due to timeouts. How would you avoid duplicate orders or charges?

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