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Problem Understanding
Restate the problem in your own words.
Design Twitter (Timeline)
Design a Twitter/X-class microblogging system: short posts (with optional media), a follow graph, and a home timeline that surfaces posts from the accounts a user follows. The system is read-heavy with a long-tailed distribution — most users have 100s of follows, a few celebrities have 100M. The decisive trade-offs are fan-out on write vs. fan-out on read, how to handle celebrity accounts without melting the writers, and keeping cold-cache home-timeline reads under 200 ms while the firehose ingests 6k+ posts/sec at peak.
- Twitter / XThe canonical microblog. ~500M MAU; ~6k posts/sec at peak; a heavy fan-out + Redis-backed timeline.
- MastodonOpen-source, federated alternative — same UX, different scaling profile because each instance owns its own users.
- Bluesky / AT ProtocolDecentralised microblogging on a content-addressable graph; same fan-out problem with a federated twist.
- Threads (Meta)Built on top of Instagram’s social graph; demonstrates how a microblog can ride an existing follow graph.
Your task: read the problem above, then write what the system is, who uses it, the rough scale, and the headline UX expectation — in your own words. Submit for AI review when you're ready.
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