Distributed

Idempotency Key

A client-generated unique key stored with a uniqueness constraint so a retry replays the stored response instead of re-executing.

Why interviewers ask about it

Insert first and catch the unique violation - never read-then-write, which races. Commit the key and the effect in one transaction.

At-least-once delivery combined with idempotent consumers that deduplicate, committing the dedup record with the side effect.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.