VII. Port binding
Export services via port binding
Web apps are sometimes executed inside a webserver container. For example, PHP apps might run as a module inside Apache HTTPD, or Java apps might run inside Tomcat.
The twelve-factor app is completely self-contained and does not rely on runtime injection of a webserver into the execution environment to create a web-facing service. The web app exports HTTP as a service by binding to a port, and listening to requests coming in on that port.
In a local development environment, the developer visits a service URL like http://localhost:5000/ to access the service exported by their app. In deployment, a routing layer handles routing requests from a public-facing hostname to the port-bound web processes.
This is typically implemented by using dependency declaration to add a webserver library to the app, such as Tornado for Python, Thin for Ruby, or Jetty for Java and other JVM-based languages. This happens entirely in user space, that is, within the app's code. The contract with the execution environment is binding to a port to serve requests.
HTTP is not the only service that can be exported by port binding. Nearly any kind of server software can be run via a process binding to a port and awaiting incoming requests. Examples include ejabberd (speaking XMPP), and Redis (speaking the Redis protocol).
Note also that the port-binding approach means that one app can become the backing service for another app, by providing the URL to the backing app as a resource handle in the config for the consuming app.