In this chapter, you learned about the Reactive pattern and its principles. It is not a new innovation in programming--it is a very old concept, but it very fits in very well with the demands of modern applications.

Reactive programming has four principles: responsiveness, resilience, elasticity, and message-driven architecture. Responsiveness means a system must be responsive in all conditions: odd conditions and even conditions.

The Spring 5 Framework provides support for the reactive programming model by using the Reactor framework and reactive stream. Spring has introduced new a reactive web module, that is, spring-web-reactive. It provides the reactive programming approach to a web application by either using Spring MVC's annotations, such as @Controller, @RestController, and @RequestMapping, or by using the functional programming approach using the Java 8 Lambda expression.

In this chapter, we created a web application by using the spring web reactive modules. The code for this application is available on GitHub. In the next chapter, you will learn about implementation of concurrency patterns.