What is an idempotency key and why does it prevent double-charging or duplicate inserts?
An idempotency key is a unique identifier (usually a client-generated UUID plus timestamp) attached to every mutation. If the network fails after a successful server write but before the client receives the ACK, retrying with the same key tells the server 'I've already done this' and it returns the cached result instead of repeating the action.
Answered in
Optimistic UI: The Illusion of InstantApply mutations locally and reconcile in the background, collapsing perceived latency from 150 ms to instant. Rollback, idempotency keys, offline queues.
Read the full analysisOther questions this article answers
- What is optimistic UI execution and why is perceived latency different from actual latency?
- How does rollback work when the server rejects an optimistic mutation?
- How do you handle ordering when mutations are queued offline and replayed later?
- When is optimistic UI the wrong call and what should you use instead?
More system design questions
- Why does an index's internal data structure matter if it all ends up 'faster than a scan'?
- Why is disk I/O the thing index structures are actually optimizing for?
- Why can't a hash index handle range queries?
- What makes a bitmap index different from a B-tree, and when is it better?
- Why do B-trees stay balanced automatically as data is inserted?
- Why does a database need an index at all — why can't it just scan the table?
- When is a hash index better than a B-tree index?
- What is a composite index and why does column order matter?
Every answer on Crashtech is written by the editor of the article it comes from — never auto-summarised. Browse all answers or the System Design beat.