5.9 News Feed Fan-Out

Three hundred million people open a social app each day, and each of them expects a ranked list of recent posts from the accounts they follow in under 200 milliseconds. A post is written once but read by many followers, so the main design question is when to do the work of building feeds: when the post is created, or when each follower reads. Copying a post into the feeds of all followers is called fan-out, and the choice of when to fan out defines the design.

Requirements and Arithmetic

This lesson works with one set of assumptions throughout