Expose services through port bindings

A Twelve-Factor App is expected to be self-contained or standalone. Traditionally, applications are deployed into a server--a web server or an application server, such as Apache Tomcat or JBoss, respectively. A Twelve-Factor App ideally does not relay on an external web server. A HTTP listener, such as Tomcat, Jetty, and more, has to be embedded in the service or application itself.

Port binding is one of the fundamental requirements for microservices to be autonomous and self-contained. Microservice embeds the service listeners as a part of the service itself.