THE SHORT ANSWER

Make repeated event delivery safe without accidentally discarding legitimate updates.

Expect repeated delivery

A provider may retry when it cannot confirm receipt. Your application can also time out after completing work. Design the handler so repeating the same event does not duplicate the business action. This is especially important for payments, stock changes and messages, where duplicate work has a visible consequence.

Choose a stable identity

Use the provider’s event identifier where appropriate and understand its scope. Store processing state with the operation it protects. A naive check followed by a separate write can still race under concurrent requests. The right persistence approach depends on the system, but the business requirement is one consistent outcome for the same event.

Decision flow: Receive, then Verify, then Deduplicate, then Apply
Receive Verify Deduplicate Apply. This diagram summarises the approach; it is not a substitute for checking your project’s specific requirements.

Verify before processing

Validate authenticity using the provider’s documented mechanism and avoid trusting arbitrary incoming payloads. Record relevant metadata without retaining secrets. Reject malformed requests and handle unknown event types deliberately. A public endpoint should not treat every correctly shaped request as an authorised instruction.

Illustrative api photography accompanying the guide
Illustrative stock photography · Backend & cloud

Handle order and recovery

Events may arrive late or out of sequence. Compare the current record state before applying a change. Give failed handlers a retry process and monitor work that remains unresolved. Test concurrent duplicate events, not only a repeated request sent after the first one has already finished.

PUT IT INTO PRACTICE

Your next working session.

  • Identify repeated events
  • Verify the sender
  • Test concurrent delivery

Bring one real, anonymised example to the discussion. Work through the choices above with the person responsible for the outcome, record the unresolved questions and agree who will investigate them.

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Elite Apps editorial · Practical planning guidance. Examples and diagrams are illustrative; implementation depends on your requirements. Technical and supplier guidance can change.