In this chapter, we have revisited a few well-known design patterns by GoF—including Observer, Publish-Subscribe, and Iterator to build the basis of reactive programming. We have written a few implementations to review both the strong and weak sides of the instruments we already have for asynchronous programming. We have also leveraged Spring Framework support for Server-Sent Events, WebSockets, and also played with Event-Bus provided by Spring. Also, we have used Spring Boot and start.spring.io for fast application bootstrapping. Even though our examples were pretty simple, they demonstrated the potential issues that arise from immature approaches that are used for asynchronous data processing.

We also looked at reactive programming's history to highlight architectural problems, which reactive programming was invented to fight against. In this context...