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Design a distributed news feed

Twitter/Instagram-style timeline: fan-out, ranking, and hot users — a staple at FAANG-style system design.

~28 minDesign interview
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Interview script (outline)
  1. Clarify: Twitter-style public vs Facebook friends; ranking vs chronological; global scale.
  2. Estimate: DAU, posts/day, follows per user, feed load QPS, celebrity fan-out.
  3. Fan-out on write vs read — when to hybrid.
  4. Storage: posts, graph, feed cache; hot users.
  5. Ranking: candidate gen + ML + mixer; freshness vs engagement.
  6. Reliability: queue lag, idempotent fan-out, stale feed degradation.
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