How can you ensure idempotent processing in a serverless architecture?

• A. Rely on at-least-once semantics without dedup
• B. Implement idempotency keys with deduplication (e.g., DynamoDB) and ensure retries are safe
• C. Use static random delays
• D. Disable retries entirely

Answer: (B)

In a serverless setup, you often encounter at-least-once delivery, so the goal is to recognize duplicate requests and make reprocessing harmless. Implementing idempotency keys and deduplication with a durable store lets you do just that. When a request arrives, you use a unique idempotency key to check a persistent store (for example, DynamoDB). If that key has already been processed, you return the previously produced result or a known safe response instead of re-running the operation. If it’s new, you perform the work and atomically record the outcome alongside the key. Designing the operation to be safe on retries—using conditional writes, upserts, or transactional updates—ensures that even if the same request is retried, it won’t produce duplicate side effects or inconsistent state.

This approach beats relying on at-least-once semantics alone, which can still lead to duplicates without any dedup logic. It also avoids relying on static delays or disabling retries, which don’t provide a reliable mechanism to prevent duplicate processing or to guarantee that a retried invocation won’t cause unintended changes.