Java – Why Does Java Require a Servlet Container for Simple RPC Service?

Treat an EAR like a pseudo-virtual machine

An EAR is simply a collection of WAR files that share common configuration and libraries, usually from JARs. This enables a collection of inter-dependent services to be managed more easily within an application container. Thus you can think of an EAR as a simple form of virtual machine once it is deployed into its container.

In the same way that one process on a virtual machine cannot affect another, the same is true for an EAR. All the WARs are isolated to protect their internal state.

Scaling authentication

In general web applications need to be stateless to scale well. Having a lot of information in the session is an anti-pattern that prevents this. This leads to a conflict between the stateless nature of HTTP and the need to maintain a speedy, customised user experience. Authentication is a classic use case, and is prevalent in chatty APIs that require a lot of authenticated requests to deliver end-user functionality.

Single Sign On and Single Sign Off need careful signalling to work correctly (consider partial Sign Off), particularly when horizontal scaling is in place. Simply sharing session state is almost never a good solution and a better approach is to use a single point of reference for the user information that all nodes in the cluster have access to.

Concentrating on speedy cached access to relevant information will yield far more scalable results than some complex, fragile session sharing solution.

The recommendations I've seen is to have all-in-one container: Docker Misconceptions:

Misconception: You should have only one process per Docker container!

It's important to understand that it is far simpler to manage Docker if you view it as role-based virtual machine rather than as deployable single-purpose processes. For example, you'd build an 'app' container that is very similar to an 'app' VM you'd create along with the init, cron, ssh, etc processes within it. Don't try to capture every process in its own container with a separate container for ssh, cron, app, web server, etc.

One way to think about it is to ask yourself if you'd ever need one piece running without the others. OK, maybe you'd want the DB running without the app server, but how often?