Explain Idempotent Consumer Pattern.
The idempotent consumer pattern ensures that processing the same message multiple times produces the same outcome as processing it once, preventing unintended side effects in distributed systems.
When to Use
This pattern is essential in event-driven systems with at-least-once delivery guarantees. It avoids problems like duplicate charges, repeated notifications, or double inventory deductions caused by retries or network failures.
Example
If a payment message is delivered twice, an idempotent consumer detects the duplicate and ignores it—ensuring the customer is only billed once.
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Why Is It Important
It increases reliability and correctness, ensuring duplicate events do not corrupt business logic. This is vital for financial transactions, order processing, and user experience.
Interview Tips
When asked, start by defining idempotency clearly, give a relatable example, and explain how to implement it (e.g., tracking processed message IDs or using unique transaction keys).
Trade-offs
The trade-off is extra complexity and storage overhead versus improved reliability. Systems may need to maintain logs or deduplication stores.
Pitfalls
Common mistakes include assuming duplicates won’t occur or failing to persist processed message IDs reliably, leading to silent duplicate processing.
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