News feed

How this question breaks down

1. 1Fan-outWrite path29 minWhen someone posts, copy a small reference to the post into every active follower's precomputed feed, so reading a feed is one lookup instead of a query across hundreds of accounts.
2. 2Feed readsRead path25 minServe a page of the feed in under 200 ms by reading the precomputed list, merging in large accounts, dropping what the reader shouldn't see, filling in post bodies from cache, and handing back a cursor for the next page.
3. 3RankingRead path24 minServe the For you tab by gathering ~900 candidates, narrowing them with a cheap model and a costly one, applying variety and safety rules, and caching the ranked list for the session, inside a 400 ms budget with a newest-first fallback.

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