Idempotent Consumers

Summary

Designing message consumers so processing the same message twice — which queues can occasionally guarantee only 'at-least-once' delivery of — produces the same result as processing it once.

• 1Design a consumer that uses a unique message ID to detect and skip duplicates.
• 2Learn why 'at-least-once' delivery is far more common than 'exactly-once' in real systems.
• 3Identify a real operation, e.g. charging a card, where non-idempotent processing would be dangerous.