This first chapter covers the principles of reactive programming and ReactiveX. It is composed of three parts. The first part explains what reactive programming is and how it compares to other concepts and paradigms that are often used in event-driven programming. The second part explains the foundations of ReactiveX and RxPY, its Python implementation. This exploration of RxPY is explained with a simple example that allows us to understand the basics of ReactiveX. Finally, the last part is dedicated to the documentation of the ReactiveX project and the documentation of your projects. By the end of this chapter, you will be able to write some ReactiveX code and understand existing code. 

The following topics will be covered in this chapter:

• What is reactive programming?
• An introduction to ReactiveX and RxPY
• A reactive echo application
• Marble diagrams
• Flow diagrams