System designMediumsd-g543

Subject Rate limiterLevel Entry–Mid~30 minCommon in Networking & APIs interviewsIndustries Software development, Technology

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.