The document discusses building reactive microservices using Vert.x. It begins with definitions of microservices and reactive systems. It then covers reactive programming and the reactive manifesto. A large portion of the document is dedicated to explaining Vert.x, including that it is a toolkit for building reactive applications on the JVM in a non-blocking way. It covers Vert.x concepts like verticles, the event bus, and service discovery. It provides examples of how to implement various Vert.x patterns and components like regular verticles, worker verticles, the event bus, circuit breakers, and service discovery.