A distributed system is not just isolated instances of your applications running unaware of the existing world. They constantly communicate with each other and have to be properly integrated together to provide the most value.
Much was already said on the topic of integration, so in this section, we'll try to showcase just a handful of patterns for effective integration of both entirely new systems, as well as new parts of the system that needs to coexist with other existing parts, often legacy ones.
To not make this chapter be a whole book on its own, let's start this section with a recommendation of an existing one. If you're interested in integration patterns, especially focused on messaging, then Gregor Hohpe and Bobby Woolf's Enterprise Integration Patterns book is a must-read for you.
Let's take a brief look at two patterns covered by this book.