In this chapter, we covered many topics. We briefly outlined the history of Reactor to ascertain the motivation behind yet another reactive library, Project Reactor. We also looked at the most important milestones of this library—milestones that were needed to build such a versatile and powerful tool. Furthermore, we looked at an overview of the main problems with the RxJava 1.x implementation, as well as problems with early Reactor versions. By looking at what has been changed in Project Reactor after the Reactive Streams specification, we highlighted why reactive programming—so efficient and straightforward—requires such a challenging implementation.

We also described the Mono and Flux reactive types, as well as the different ways to create, transform, and consume Reactive Streams. We looked inside a running stream and controlled backpressure with the pull-push model via the Subscription object. We also described how operator fusion improves the performance of Reactive Streams. In summary, the Project Reactor library provides a powerful toolset for reactive programming and asynchronous and IO-intensive applications.

In the following chapters, we are going to cover how the Spring Framework has been improved in order to leverage the power of reactive programming in general and Project Reactor in particular. We will focus on building efficient applications with Spring 5 WebFlux and Reactive Spring Data.