Per-user rate limiter
Design a simple per-user API rate limiter for a small backend — say, allow each user 100 requests per minute, returning HTTP 429 when they exceed it. The backend runs as a couple of app instances behind a load balancer, so a user's requests can land on different instances. Cover the algorithm, where the counters live, and what happens when the limit is hit.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify3:30 left
Estimate3:30 planned
Design10:00 planned
Deep dive8:00 planned
Failure5:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.