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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: {}
<|endoftext|>
# source: k8s_examples/_archived/javaweb-tomcat-sidecar/README.md type: docs
## 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](https://kubernetes.io/docs/concepts/workloads/pods/init-containers/) 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:
![](workflow.png?raw=true "Workflow")
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 wi...
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:
```console
$ 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_](https://kubernetes.io/docs/user-guide/pods.md) 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](javaweb.yaml) for Java Web pod:
NOTE: you should define `war` init-container **first** as it is the "provider".
<!-- BEGIN MUNGE: javaweb.yaml -->
```yaml
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: {}
```
<!-- END MUNGE: EXAMPLE -->
The only magic here is the `resouer/sample:v1` image:
```
FROM busybox:latest
ADD sample.war sample.war
CMD "sh" "mv.sh"