Multi-WAR tomcat vs Docker containers

Question

I'm wondering if a Docker solution is faster and more memory efficient than my current Tomcat deployment. I will explain both solutions.

The current: I have a Tomcat server with about 20 WAR's deployed. The WAR's are Spring Boot applications. It takes up a lot of memory and boottime and money too.

The docker alternative: The alternative I'm thinking about is a docker host with 20 docker containers, one for each app. It seems Spring recommends using JAR's on JDK images.

Now, does Docker, or containerization in general, improve memory and speed?

One improvement I am expecting is that applications can start in parallel. This will hopefully speed up boot-time (assuming multi-core hardware). Am I right here?

Secondly I'm wondering which approach will handle memory most efficient. What happens when I have multiple WAR's, sharing the exact same dependency? Will Tomcat reuse dependency memory for that? And will Docker?

Answer 1

Memory (and thus likely CPU) efficiency can be debated and probably needs to be measured. Let me give some insight.

Let's assume you create 20 containers, one for each of the war's you want to run. At that time you have 20 different JVMs in memory. Depending whether they come from the same container image or from different ones, the OS recognizes they are the same, and the codebase could be shared. So this depends on whether you bake your wars into the container images or have one image only and mount the wars at runtime.

What about permgen space, heap or other memory regions? I doubt the OS can share much between the processes here. And the JVMs cannot share on their level since the docker container isolation would not allow them to talk to each other. So shared memory on JVM level is lost.

With that, every JVM would start up and run the JIT for hotspot code locations, and no synergy between the applications can be used. With a bigger codebase in memory, also the CPU would have to jump more between processes, invalidating the cache more often.

All in all I believe dockerizing your setup is an improvement in application isolation. You can more easily install/uninstall your stuff, and one application running havoc cannot impact the others. But performance-wise, you should notice lower execution times and higher memory usage. To what extent might only be benchmarked.