System designMediumsd-g590

Subject Real time personalizationLevel Mid–Senior~45 minCommon in Distributed systems interviewsIndustries Technology

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.