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When a user publishes a post, that post may need to appear in the feeds of everyone who follows them. A single action can therefore create work for thousands, or even millions, of other users. The challenge is distributing that update efficiently without overwhelming the system.

This lesson covers what fanout is, the two main ways to deliver one piece of content to many users, and how systems handle the hard cases, like a celebrity with 100 million followers.

It also covers how to approach fanout in a system design interview, since it's a key part of several common interview problems, like designing a news feed.