Java Web Application with Tomcat and Init Container

The following document describes the deployment of a Java Web application using Tomcat. Instead of packaging war file inside the Tomcat image or mount the war as a volume, we use an init-container as war file provider.

Prerequisites

https://github.com/kubernetes/kubernetes/blob/master/docs/user-guide/prereqs.md

Overview

This sidecar mode brings a new workflow for Java users:

As you can see, user can create a sample:v2 container as an initContainers object to "provide" war file to Tomcat by copying it to the shared emptyDir volume. And Pod will make sure the two containers compose an "atomic" scheduling unit, which is perfect for this case. Thus, your application version management will be totally separated from web server management.

By using an init-container the tomcat container is assured of the war file existing before start up as the pod will not start normal containers until all init-containers have completed successfully.

For example, if you are going to change the configurations of your Tomcat:

$ docker exec -it <tomcat_container_id> /bin/bash
# make some change, and then commit it to a new image
$ docker commit <tomcat_container_id> mytomcat:7.0-dev

Done! The new Tomcat image will not mess up with your sample.war file. You can re-use your tomcat image with lots of different war container images for lots of different apps without having to build lots of different images.

Also this means that rolling out a new Tomcat to patch security or whatever else, doesn't require rebuilding N different images.

Why not put my sample.war in a host dir and mount it to tomcat container?

You have to manage the volumes in this case, for example, when you restart or scale the pod on another node, your contents is not ready on that host.

Generally, we have to set up a distributed file system (NFS at least) volume to solve this (if we do not have GCE PD volume). But this is generally unnecessary.

How To Set this Up

In Kubernetes a Pod is the smallest deployable unit that can be created, scheduled, and managed. It's a collocated group of containers that share an IP and storage volume.

Here is the config javaweb.yaml for Java Web pod:

NOTE: you should define war init-container first as it is the "provider".

apiVersion: v1
kind: Pod
metadata:
  name: javaweb
spec:
  initContainers:
    - image: resouer/sample:v1
      name: war
      volumeMounts:
        - mountPath: /app
          name: app-volume
  containers:
    - image: resouer/mytomcat:7.0
      name: tomcat
      command: ["sh", "-c", "/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
      volumeMounts:
        - mountPath: /root/apache-tomcat-7.0.42-v2/webapps
          name: app-volume
      ports:
        - containerPort: 8080
          hostPort: 8001
  volumes:
    - name: app-volume
      emptyDir: {}

The only magic here is the resouer/sample:v1 image:

FROM busybox:latest
ADD sample.war sample.war
CMD "sh" "mv.sh"

And the contents of mv.sh is:

cp /sample.war /app
tail -f /dev/null

Explanation

1. 'war' container only contains the war file of your app
2. 'war' container's CMD tries to copy sample.war to the emptyDir volume path
3. The last line of tail -f is just used to hold the container, as Replication Controller does not support one-off task
4. 'tomcat' container will load the sample.war from volume path

What's more, if you don't want to enclose a build-in mv.sh script in the war container, you can use Pod's command to do the copy work, here's a example javaweb-2.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: javaweb-2
spec:
  initContainers:
  - image: resouer/sample:v2
    name: war
      command:
        - "cp"
        - "/sample.war"
        - "/app"
    volumeMounts:
    - mountPath: /app
      name: app-volume
  containers:
  - image: resouer/mytomcat:7.0
    name: tomcat
    command: ["sh","-c","/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
    volumeMounts:
    - mountPath: /root/apache-tomcat-7.0.42-v2/webapps
      name: app-volume
    ports:
    - containerPort: 8080
      hostPort: 8001
  volumes:
  - name: app-volume
    emptyDir: {}

And the resouer/sample:v2 Dockerfile is quite simple:

FROM busybox:latest
ADD sample.war sample.war
CMD "tail" "-f" "/dev/null"

Explanation

1. 'war' container only contains the war file of your app
2. 'war' container's CMD uses tail -f to hold the container, nothing more
3. The command will do cp after the war container is started
4. Again 'tomcat' container will load the sample.war from volume path

Done! Now your war container contains nothing except sample.war, clean enough.

Test It Out

Create the Java web pod:

$ kubectl create -f examples/javaweb-tomcat-sidecar/javaweb-2.yaml

Check status of the pod:

$ kubectl get -w po
NAME        READY     STATUS    RESTARTS   AGE
javaweb-2   2/2       Running   0         7s

Wait for the status to 0/1 and Running. Then you can visit "Hello, World" page on http://localhost:8001/sample/index.html

You can also test javaweb.yaml in the same way.

Delete Resources

All resources created in this application can be deleted:

$ kubectl delete -f examples/javaweb-tomcat-sidecar/javaweb-2.yaml