Fanout Pattern in System Design

Fanout is the pattern you use when one event needs to create work for many recipients: feed entries, notifications, counters, search updates, ranking candidates, or analytics events.

Consider a user posting "Hello world" on a social app. Storing the post is one write. The hard part is deciding how and when that post becomes visible to followers, notification channels, and downstream systems.

In interviews, the important question is not "how do we send it to everyone?" The important question is where do we pay the distribution cost: at write time, at read time, or with a hybrid model.

The fanout pattern is central to timelines, activity feeds, notification systems, subscription updates, and content distribution systems. This chapter uses the social feed as the running example, since it exercises every hard part of fanout: amplification, freshness, ranking, and cleanup. The same trade-offs carry over to notifications, counters, and other fan-out workloads.

Where This Pattern Applies