Twitter

How this question breaks down

1. 1PostingWrite path25 minAccept a 280-character post exactly once, store it durably under a time-ordered ID, and hand it to fan-out, search and notifications without making the author wait for any of them.
2. 2CelebritiesWrite path22 minWhy pushing one tweet into 48 million timelines breaks fan-out, and how a hybrid design pulls big accounts' tweets at read time instead.
3. 3TimelineRead path31 minServe a page of someone's home timeline in tens of milliseconds by reading a precomputed list of IDs, merging big accounts, hydrating tweets in bulk and filtering what the reader must not see.
4. 4CountsRead path26 minRecord each like exactly once as an edge, show the viewer their own like instantly, and keep like, retweet, reply and view counts fresh within seconds without a hot row per viral tweet.

Work through the parts in order and you’ve covered the full design. Each part is one topic.