System designMediumsd-g581

Subject Recommendation systemLevel Mid–Senior~45 minCommon in ML systems interviewsIndustries Technology

Question

Design a recommendation system for an e-commerce homepage serving 80M monthly active users and a catalog of 50M items. The 'recommended for you' shelf must return within a 150ms p99 budget at ~30k requests/sec, blend recency of browsing behavior with long-term preferences, and exclude already-purchased or out-of-stock items. Walk through retrieval, scoring, the data model for user/item signals, and how you keep recommendations fresh as the catalog and user behavior change.

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.