Unlike other chapters and applications that we have seen so far in this book, this chapter introduced a new type of application development in a distributed environment. The term microservice has existed since 2011. It has emerged as an enhancement of previous architectures. 

With the introduction of Spring Cloud, developers can provide an implementation of various common patterns in a distributed environment. Starting with Spring Boot, creating a microservice application just takes a few configurations.

At the beginning of this chapter, we have explored what a microservice is and how it differs from the monolithic architecture, followed by various principles and criteria that need to be adhered to if you want to develop a microservice system. We then explored various Spring Cloud components and other Netflix OSS components in brief.

We have also learned how to create a microservice application by building a real-life example—an online bookstore application. We started with the application's architecture and the database design. We looked at creating microservices as Spring Boot applications with the required configurations.

We then saw practical examples of how to build various Netflix OSS and Spring Cloud components, such as a Eureka discovery server, a Zuul proxy server, a Feign client, Ribbon, and Spring Cloud Config. These components are the building blocks for developing a distributed application. We also saw various options and approaches to building a microservice frontend. Finally, we secured the application with Spring Cloud Security.

Java was the only de facto programming language for building Spring-based applications for quite a long time. However, the team at Pivotal (the company behind Spring) has started to support other functional programming languages, such as Scala. Starting with version 5, Spring announced support for Kotlin, a JVM-based language mainly used for Android applications. In the next chapter, we will dive into an altogether new way of building Spring applications with Kotlin.