Classic idempotency problem and I knew the concept but fumbled the specifics under pressure.
Start by clarifying the context: is this a client retry, a network duplicate, or a malicious replay? Then propose idempotency as the core principle, using idempotency keys for write operations and conditional requests for reads. Discuss trade-offs between client-generated keys, server-side deduplication windows, and exactly-once semantics in distributed systems.
Pro tip: Mention that true exactly-once delivery is impossible in distributed systems; instead, aim for effectively-once processing by making operations idempotent and using deduplication with a unique key and a reasonable time window. This shows you understand the theoretical limits and practical solutions.
Ask whether the duplicate is from a client retry, network issue, or malicious replay, and whether the operation is read or write. Determine the required consistency and latency trade-offs.
For write operations, have the client generate a unique idempotency key (e.g., UUID) and send it with the request. The server stores the key and the result, returning the same response for duplicate keys.
Use a distributed cache or database with TTL to store idempotency keys and their associated responses. Ensure atomic check-and-set to avoid race conditions.
For reads, use conditional requests (ETags, If-None-Match) or versioning to avoid redundant processing. For writes, consider optimistic concurrency control with version numbers.
Address key expiration, storage overhead, failure scenarios (e.g., key stored but response lost), and how to handle non-idempotent operations like payments.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.