Part 3. Patterns

We have spent a fair amount of time so far discussing the what and why of being Reactive. Now it is time to focus on the how. In part 3, we will present patterns of development that will help you implement Reactive applications. We will begin by discussing how to test to ensure that your application is Reactive so you can build forward with confidence that you are meeting the Reactive contract, from the smallest components to an entire cross–data center deployment. Then we will delve into specific patterns for building Reactive systems across all dimensions of Reactive concepts.

In this part, you will learn about the following:

• Testing Reactive systems, with a specific emphasis on asynchronous testing
• Layering internal and external fault tolerance into your application
• Managing the resources used by your Reactive application
• Managing the flow of messages and data within and between your applications
• Managing state and persist data in Reactive systems

We will present the individual patterns by first introducing their essence in one short paragraph (for easy reference when you revisit them), followed by information about where the pattern emerged, and then details of an example where the pattern is applied to a concrete problem. Each pattern is then summarized with the concerns it addresses, its quintessential features, and its scope of applicability.

Part 2 discussed the building blocks on which a Reactive system can be built. It may be worthwhile to frequently recall that background while reading forward—in our experience, it is usually the second approach to a topic that brings the “eureka!” moments.