System Design Simulator
News Feed
Design a social or product feed with follow graphs and ranking.
Al Beltran Engineering Lab
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How the challenge works
- Celebrity fan-out on write explodes. Hybrid pull for huge follow graphs is the usual escape hatch.
- Precompute a short cached timeline. Rank asynchronously.
Key concepts
Fan-out on write
Works when most users have a modest follower count.
Cache as the product
The home API should read a prepared list, not run a graph query live.
Common mistakes
- JOIN follows × posts on every homepage load.
- Blocking publish until ranking finishes.
Recommended architecture
- API + cache timeline, database for graph and posts, queue/workers for fan-out and ranking, CDN for media.
Interview tips
- Ask what 'seen' means and whether ranking is personal or chronological.
Related challenges
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