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Background

I have a Spring Boot Application that I am deploying in a Docker container.

The application must be running 24/7. If it crashes, it must restart.

I have the option of configuring the container to restart on crash, and of using Kubernetes, but these are relatively slow operations.

I was under the impression that Tomcat, like IIS on Windows is able to restart applications that fail. Reloading an application within Tomcat should be quicker than reloading the entire container.

So, I tried deploying the application as an exploded war in Tomcat.

But it turns out, that all Tomcat does for me is to start the application and provide manual restart through its manager application.

I learned that Tomcat supports "reloading" if specified files are changed, but this is intended for development environments and it is recommended not to use it in production.

My Questions:

  1. Is it possible to configure my application under Tomcat so that if it crashes, Tomcat will restart it for me? How?
  2. If not, what advantages, if any, do I have when using Tomcat to host my application inside the container rather than simply running java -jar in the container?
David Sackstein
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  • In Kubernetes it's not just that the cluster manager will restart the container, but you also typically have multiple copies of it running behind a load balancer (and all of this infrastructure is provided by the cluster) so if one fails and restarts, the other two can keep handling traffic. Your use case might not justify a multi-node cluster setup, but the pattern of running multiple containers behind a load balancer could still be reused. – David Maze Jan 15 '20 at 03:03

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One option you could look into, is running systemd within the docker container and then installing spring boot as a service. The auto restart of spring boot would then be handled by systemd. You do however have to jump through some hoops to get systemd to work in docker and be careful if you're using Kubernetes and OpenShift (see my first link below for details). Some links that might be useful...

https://developers.redhat.com/blog/2019/04/24/how-to-run-systemd-in-a-container/

https://singlebrook.com/2017/10/23/auto-restart-crashed-service-systemd/

httPants
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  • So there is no advantage to running the app under Tomcat. Correct? – David Sackstein Jan 15 '20 at 09:52
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    No because when the application crashes, that means the JVM running the application crashed, which would be the tomcat container hosting the application. Running an executable spring boot jar uses an embedded version of tomcat (by default). The only benefit I can see to redeploying an application in a running tomcat server is if the application has memory leaks then redeploying the app would free up memory, but for a crash, then tomcat is what would have actually crashed and you'd need something to restart that tomcat server. – httPants Jan 16 '20 at 21:51
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    I would preference the restarting of docker containers rather than restarting the app within a container. If high availability is a priority, then running multiple container instances behind a load balancer and autoscaler would provide that. You can use springboot actuator to get a health endpoint which you then configure in the load balancer so it knows when your application has crashed or become unhealthy. The autoscaler would then startup new instances of the container to replace the unhealthy ones. Requests are routed to already running instances while the container starts up. – httPants Jan 16 '20 at 22:07