Fan-Out

What it is

Fan-out means one input becomes many outputs.

An event comes in once and triggers multiple consumers or paths.

When it matters

This appears when the same fact needs to feed different things:

• notifications
• indexing
• analytics
• cache updates

Common mistake

The classic mistake is seeing fan-out only as a scaling pattern.

It also multiplies:

• failure points
• cost
• observability needs
• ordering complexity

Short example

A user publishes a post.

That event may trigger at the same time:

• feed updates
• notifications
• search indexing
• counters

Why it helps

Fan-out helps split responsibilities so one flow does not need to do everything.

But the more outputs you create, the more clearly you need to define who may fail, lag, or retry.

Fan-out decouples work, but it also spreads complexity around.