Real-time personalization shelf
Design a real-time personalization service for a streaming-media app that re-ranks the home shelves based on what the user did in the last few minutes (played, skipped, searched), updating within 2 seconds of an action for 30M concurrent users. The personalized response must return in under 100ms p95. Cover how you capture and process in-session signals, the serving path, and how you reconcile fast session signals with the slower batch user profile.
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.