Activity feed with power users
Design an activity feed for a mid-size social/collaboration app with ~2M users where users follow each other and a home feed shows recent activity from everyone you follow. Distribution is skewed: most users follow ~150 accounts, but some 'power' accounts have 100K+ followers. Requirements: home-feed loads in <150ms p95 (read-heavy), new activity appears within seconds, feeds are reverse-chronological with light ranking, and the system shouldn't fall over when a power account posts (the fan-out spike). Roughly 20:1 read:write on activity.
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.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 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.