Software Design

Idempotency Keys in Payment APIs

UML Sequence Diagrams · made in Skemio

Sequence diagram showing Client, API Gateway, Payment Service, and Redis coordinating an idempotency key lock and payment response cache.
Sequence diagram showing Client, API Gateway, Payment Service, and Redis coordinating an idempotency key lock and payment response cache.
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What it shows

  • Atomic key lookups prevent double charges during automated client retries
  • Mutex locks on pending requests reject duplicate calls with HTTP 409
  • Completed transaction responses are cached to return identical payloads

The takeaway

Always combine unique idempotency keys with distributed locks to guarantee safe at-most-once billing.

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