Upstream and Downstream

When reading Reactor’s reference documentation or the javadoc of the API, you’ll find two terms that confused me a little bit the first time I found them: upstream and downstream.

For example, in the javadoc of Mono.fromRunnable(Runnable), in the section Type Parameters, they define T as:

T - The generic type of the upstream, which is preserved by this operator

In simple words, the upstream is the stream that comes before the current operator.

On the other hand, downstream is the stream that comes after the current operator.

For example, consider this operator chain:

Flux
  .map(...)
  .flatMap(...)
  .map(...)
  .subscribe()

In this case, taking the flatMap operator as reference, the source Flux and the first map operator would be the upstream.

Whereas the second map operator, as well as the subscribe operator, would be the downstream.

Some operators will be affected by what happens upstream or downstream, so always review the documentation to look for references to these terms.