Fanout Pattern in System Design
Fanout (sometimes called fan-out on write or fan-out on read) is the pattern you reach for when one event needs to reach many recipients—fast, reliably, and at scale.
Picture a single user posting “Hello world” on a social app. That’s one write. But the moment it happens, thousands (or millions) of people might need to see it in their feeds, get a push notification, update counters, refresh recommendations, and trigger analytics.
The system’s real work isn’t storing the post, it’s expanding that one input into many outputs without melting down.
The fanout pattern solves this problem. It's a fundamental technique for distributing data from one source to many destinations, and it powers some of the most demanding systems in tech: Twitter's timeline, Facebook's news feed, LinkedIn's activity feed, and notification systems at scale.