System designMediumsd-g219

Subject Leader electionLevel Mid–Senior~40 minCommon in Reliability & on-call · Distributed systems interviewsIndustries Technology, Software development

Question

Design a highly-available singleton service: a global rate-limit/quota aggregator that must have exactly one active instance computing the authoritative cross-region usage total (so quotas aren't double-counted), but must fail over to a standby within seconds if the active dies. It also can't be a bottleneck — thousands of services report usage to it. How do you elect the one active instance, fail over fast, and not lose counts during the switch?

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.