Design a Social News Feed

Design Twitter's home timeline or Facebook's news feed. The core problem is fanout — how do you push one post to millions of followers fast enough that the feed feels real-time?

What you will learn

• Explain the fanout-on-write vs fanout-on-read tradeoff and when each breaks down
• Design a hybrid fanout strategy that handles celebrity accounts without degrading feed freshness
• Model the feed storage layer and choose between precomputed timelines and query-time assembly
• Handle ranking, pagination, and feed freshness at scale

The news feed — Twitter's home timeline, Facebook's feed, Instagram's scroll — is one of the most studied distributed systems problems. The challenge is deceptively simple: when a user opens the app, show them a ranked, paginated list of recent posts from accounts they follow. The complexity lies in the scale. A single celebrity post triggers a wave of writes or reads that can cascade across the entire system.

Functional requirements:

• Users see a feed of posts from accounts they follow, sorted by recency (or relevance)
• New posts appear in followers' feeds within seconds
• Infinite scroll with pagination — load more as user scrolls
• Users can create posts (text, images, links)
• Like and comment counts visible inline (eventual consistency acceptable)

Non-functional requirements:

• Feed load time < 200ms p99
• Post fan-out latency < 5 seconds (follower sees post within 5s of publish)
• Read-heavy: 100:1 read/write ratio
• High availability — feed downtime is user-visible

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