Beyond the Twelve-Factor App

CHAPTER 11
Port Binding
Factor 7 states that cloud-native applications export services via port
binding.
Avoiding Container-Determined Ports
Web applications, especially those already running within an enterprise,
are often executed within some kind of server container. The
Java world is full of containers like Tomcat, JBoss, Liberty, and Web‐
Sphere. Other web applications might run inside other containers,
like Microsoft Internet Information Server (IIS).
In a noncloud environment, web applications are deployed to these
containers, and the container is then responsible for assigning ports
for applications when they start up.
One extremely common pattern in an enterprise that manages its
own web servers is to host a number of applications in the same
container, separating applications by port number (or URL hierarchy)
and then using DNS to provide a user-friendly facade around
that server. For example, you might have a (virtual or physical) host
called appserver, and a number of apps that have been assigned
ports 8080 through 8090. Rather than making users remember port
numbers, DNS is used to associate a host name like app1 with app
server:8080, app2 with appserver:8081, and so on.
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