Feeds & Timelines

Assembling a fresh, ranked stream for every reader: fan-out on write versus on read, the celebrity hot-key, feed caches kept as ID lists, and the hydration step that turns those IDs back into posts.

Explainers

• Design a News Feed — Open the app and see a fresh, ranked stream of posts from everyone you follow. The whole design is one fork — do you push each post into every follower’s feed as it’s written, pull them all together at read time, or split the difference? Built from zero: a commit-first envelope on 10M users, the push/pull/hybrid deliberation with cost meters computed live, the celebrity hot-key that melts the write path, the feed cache kept as bare <post-id, user-id> lists and why not full posts, the hydration step, an honest paragraph on the ranking hand-off, and the failure sweep.