Most of you have probably heard about JBoss Application Server; WildFly is its successor. It is an open source implementation of the Java EE specification, and, more importantly in the context of this book, it is the base of Swarm project.
WildFly has an extensible architecture, which enables building differently sized distributions on top of its high-performance core, which, as we will learn in the next chapter, is utilized by Swarm to a great extent.