Reactive programming started as a Reactive Streams model initially implemented in the .NET Framework but popularized by Pivotal and Netflix. This programming paradigm is supported by a specification used by many developers to extend and implement libraries that can solve Reactive-related problems. JavaScript, Python, and Java are some of the languages that have already shown their support by including this specification in their platforms. Based on the Reactive Stream JVM specification, Java 1.8 and above can now support Reactive programming. Java 1.9, especially, has a dedicated Flow API (java.util.concurrent.Flow) which consists of all the Reactive Streams API written within the context of Java language specification.

This chapter will introduce Reactive programming concepts and will provide recipes on how this paradigm started using the popular observer design pattern.