You open the app and your feed is there in an instant, assembled from posts by everyone you follow, freshly ranked. Building that feed for one person is easy. Building it for hundreds of millions of people, where some accounts are followed by tens of millions and everyone expects an instant loa...
Source: [Dev.to](https://dev.to/roni_das_b1b76c5ee6583027/how-a-news-feed-works-fanout-on-write-vs-fanout-on-read-3lle)