8.5 Design a News Feed

A post is written once but read by every follower, often several times. A feed is therefore a read-dominated system, and the central design question is where to spend the work: when a post is published, or when a feed is read. The answer used in practice is a hybrid. Most timelines are computed in advance, a few posts from very popular accounts are merged in at read time, and caches around profiles and posts keep each read cheap.

Requirements and Capacity

Functional. Users publish posts and follow or unfollow other accounts