News feed

Show each user a feed of recent posts from the people they follow. The central decision is when to do the work: at write time (fan-out on write, cheap reads) or at read time (fan-out on read, cheap writes).

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Duration 45:00

Requirements checklist

Publish a post (text, media references).Read a feed of recent posts from followed accounts, newest first (ranking is out of scope here), paginated.Follow and unfollow.Feed reads p99 under 200 ms; reads outnumber posts ~100:1.A new post appears in followers' feeds within a few seconds (eventual consistency is fine).Posting must stay fast and available even when fan-out lags.

Clarifying questions

Chronological or ranked feed?
Follower count distribution — are there accounts with millions of followers?
How fresh must a feed be?
Media in posts, and is hydration in scope?

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requirements

Suggested time: about 6 minutes. Distinguishes availability of posting from freshness of feeds.

Notes for requirements