System designHardsd-g260

Subject Media systemsLevel Senior–Staff~50 minCommon in ML systems interviewsIndustries Technology

Question

Design the recommendation/serving system that builds a personalized home feed of videos for 1B users, choosing from a corpus of billions of items, with a strict <100ms p99 to assemble the ranked feed. Fresh content must surface quickly, watch behavior must feed back into recommendations within minutes, and the system can't re-score billions of items per request. Design the candidate-generation + ranking funnel, the feature/serving infrastructure, and the freshness path.

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.