Highly available rate-limit aggregator
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.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 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.