The previous chapter explained why it is important to build reactive systems and how reactive programming helps to do this. In this section, we will look at some toolsets that have already been present in Spring Framework for some time. We will also learn the important basic concepts of reactive programming by exploring the RxJava library, which is the first and most well-known reactive library in the Java world.

In this chapter, we will cover the following topics:

• Observer pattern
• Publish-Subscribe implementation provided by Spring
• Server-sent events
• RxJava history and base concepts
• Marble diagrams
• Business cases implemented by applying reactive programming
• The current landscape of reactive libraries