Twitter home timeline

Serve home timelines at hundreds of thousands of reads per second. Twitter's answer was fan-out on write into in-memory timelines, with celebrities merged in at read time — a hybrid that bounds both read cost and write amplification.

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Duration 45:00

Requirements checklist

Post a tweet.Read the home timeline: recent tweets from followed accounts, newest first.Read a user timeline (one author's tweets).Home timeline p99 under ~200 ms; reads dominate writes by ~60:1.A tweet reaches followers' timelines within seconds; ~5 s is acceptable, minutes for the very largest accounts is not.Posting never blocks on fan-out.

Clarifying questions

How skewed is the follower distribution?
How fresh must the home timeline be?
Ranked or chronological?
How far back must the timeline paginate?

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requirements

Suggested time: about 6 minutes. Explicit staleness budget and celebrity behaviour.

Notes for requirements