Social feed ranking
Design the ranking service for a social feed with 400M daily active users where each session must rank ~500 candidate posts down to a scrolling feed in under 200ms p95. Ranking must balance multiple objectives (predicted like, comment, share, watch-time, and a 'don't show me this' negative signal) into a single ordering, respect integrity demotions, and adapt within a session as the user scrolls. Cover the model architecture, how you combine objectives, the serving path, and how you measure whether a ranking change is actually good.
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.
Clarify6:00 left
Estimate6:00 planned
Design18:30 planned
Deep dive14:30 planned
Failure10:00 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.