Ratpack, a comprehensive sets of modules allowing you to elegantly add new features, has a registry that can be easily extended using a DI framework like Guice or Spring. The Ratpack Registry is a store of objects available from the context of a handler, and it can be used to avoid opening multiple Cluster connections. By default, the registry is empty, but it can be populated with object instances, which are retrieved by type. In this example, a CouchbaseCluster instance is added to the registry for use in asynchronous operations. The Guice module allows for more complex DI configurations and can be activated by adding the appropriate dependency to the build.gradle file. The RxJava module creates a bridge between Rx Observables and Ratpack Promises, allowing users to easily convert between the two. With these features, developers can create cleaner and more efficient code, making it easier to build lightweight applications or more complex REST APIs.