A worker commits an order update to an external database and crashes before acknowledging the message. On retry, which protection prevents a duplicate business update?

Choose the single best answer.

A.Increase message retention without changing business update logic
B.Assume broker duplicate detection suppresses every consumer retry forever
C.Application idempotency or an appropriate transactional design around the side effect
D.Assume acknowledgement guarantees exactly-once effects before it occurs

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✓ Correct answer

Application idempotency or an appropriate transactional design around the side effect

Redelivery after a crash can repeat the external side effect. Application idempotency or coordinated transaction design addresses that window. Acknowledgement is incomplete, broker deduplication is not a universal side-effect guarantee, and longer retention does not prevent duplicate updates. Source guide: PDF page(s) 66, 69.

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