Java – Explains downstream and upstream in rxJava

I don’t quite understand what downstream and upstream mean in Rxjava.
What does that mean? Is it equivalent to subscribeOn and observableOn?

We can divide streams based on where the operator sees them.

         upstream          downstream
source <--------- operator -----------> consumer/further operators

So from top to operator, we call it upstream.

From the operator to the bottom, we call it downstream.

It is not equivalent to subscribeOn and observeOn. subscribeOn and observeOn are just operators. However, we can distinguish the behavior of these two operators by the concept of downstream and upstream.

subscribeOn is affecting both upstream and downstream of it. For example, subcsribeOn is on this code

just("Some String")
  .map(str -> str.length())
  .subsribeOn(Schedulers.computation()) // change thread
  .map(length -> 2 * length)
  .subscribe(number -> Log.d("", "Number " + number))

All streams (up and down) will be made to run on the compute thread.

On the other hand, observeOn only affects downstream. observeOn on this code

just("Some String")
  .map(str -> str.length())
  .observeOn(Schedulers.computation()) // change thread
  .map(length -> 2 * length)
  .subscribe(number -> Log.d("", "Number " + number))

Only let downstream run on the compute thread.

Hope the explanation helps you.