At this stage, you've gotten a good taste of how to whip up a file-handling controller, and hitch it to a service that reads and writes files to disk. But the question that often arises is why do I need to do this reactively?

With imperative programming, the process of taking inputs, building intermediate collections and other steps often leaves us with lots of intermediate states--some of it potentially blocking in bad places.

Using the functional style as we've explored so far moves away from the risk of inefficiently building up this state, and switches to building a stream of data instead. And Reactor's operations let us have one stream feed another in lots of different ways. We can merge streams, filter streams, and transform streams.

When we engage in reactive programming, the level of abstraction moves up a level. We find ourselves...