📝 System Design Interview Classics · Data Flow
Social News Feed (Twitter-style)
How posts reach followers' feeds: fan-out on write for normal users, fan-out on read for celebrities, and a ranked feed built from a cache.
Drawing diagram…
What this diagram shows
- Normal users' posts are pushed to followers' feed caches
- Celebrity posts are merged in at read time to avoid millions of writes
- Feeds are ranked before they are shown
Prompt used
Data flow for a social feed: a post is saved in the post store and sent to a fan-out service. For users with fewer than 10,000 followers the fan-out service writes the post ID into each follower's feed cache in Redis. For celebrities it does nothing at write time. When a user opens the app, the feed service reads their cached feed, merges recent posts from celebrities they follow, ranks them, and hydrates post details from the post store.
Mermaid code
flowchart LR U[User] -->|New post| PS[Post Service] PS -->|Post| PDB[(Post Store)] PS -->|Post ID and author| FO[Fan-out Service] G[(Follower Graph)] -->|Followers| FO FO -->|Post ID per follower| FC[(Feed Cache - Redis)] R[Reader] -->|Open feed| FS[Feed Service] FC -->|Cached post IDs| FS CEL[(Celebrity Posts)] -->|Recent posts| FS PDB -->|Post details| FS FS -->|Candidates| RK[Ranking Model] RK -->|Ranked feed| R
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