Ratpack is a framework that allows developers to build web applications using a modular and reactive approach. It integrates well with Spring Boot, making it easy to replace the traditional MVC part of a Spring Boot application with Ratpack. The author of this blog post used Gradle as their build tool and added the necessary dependencies to integrate Ratpack with Spring Boot. They then created a custom configuration class that implements `RatpackServerCustomizer` and defines the handlers, bindings, and server configuration for their application. The authors also demonstrated how to convert synchronous services to asynchronous ones using RxJava and how to wrap blocking legacy code in promises. Overall, this post provides a comprehensive guide on how to integrate Ratpack with Spring Boot and migrate existing applications to a more reactive architecture.