Facebook Feed System Design
Visual Problem Diagram
Scenario
Your friend posted ten minutes ago but your feed still looks like yesterday—fan-out lag or ranker timeout is the product failure. Hybrid fan-out, celebrity policy, and degraded ranking are what interviewers grade.
Design the home feed for a large social network: posts from followed accounts, ranked or chronological, paginated, fast to scroll. The interview targets unbounded work per swipe and celebrity fan-out—not ER diagrams alone.
You should support post creation, follow graph, feed read, reactions, ranking modes, and safety filters. Be ready to walk write path, read path, and degradation when fan-out or ranking back up.
Constraints
Create posts, follow graph, home feed pagination, reactions/comments, rank or recency modes, block/mute
< 300 ms feed p95, high availability, ranking may lag seconds, durable posts
1B+ MAU, 500M+ DAU, hundreds of millions posts/day, reads >> writes, 100M+ follower accounts