In this chapter, we explored how to utilize Kotlin to build a Spring Boot REST application programming interface. In the process of doing so, we learned the basics of designing systems. We expressed the behavior of the messenger API system with a state diagram, and learned how to properly interpret the information represented in a state diagram. We went one step further by creating an E-R diagram to give a detailed diagrammatic representation of system entities and their relationships.

In addition, we learned how to set up PostgreSQL on a local machine and create a new PostgreSQL database. We also explored how to build a microservice with Spring Boot 2.0, connect the microservice to a database, and interact with data existing in a database with Spring Data.

Besides all this, we learned how to properly secure a RESTful Spring Boot web application with Spring Security and JSON Web Tokens (JWTs). We created custom Spring Security configurations to facilitate user authentication with the help of JWTs and created custom filters for the authentication process. Lastly, we learned how to deploy a Spring Boot application to AWS.

In the next chapter, you will explore Kotlin further in the Android domain by building the Android messenger application.