System designHardsd-g573

Subject System designLevel Mid–Senior~40 minCommon in Networking & APIs · Distributed systems interviewsIndustries Software development, Technology

Question

Design an API rate-limiter service fronting a public API gateway handling ~80K req/sec across a fleet of ~40 gateway instances. Limits are per-API-key and tiered (e.g. 100 req/min free, 10K req/min enterprise) plus a global per-key burst limit. Requirements: enforce limits accurately across all gateway instances (not per-instance), add <2ms to each request, fail-open if the limiter backend is down (don't take the API offline), and return correct rate-limit headers (remaining, reset). Smooth bursting is preferred over hard cliffs.

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.

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