Demo a sidecar mode within pod for java users
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harry
Harry Zhang
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<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- BEGIN STRIP_FOR_RELEASE -->
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
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If you are using a released version of Kubernetes, you should
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refer to the docs that go with that version.
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<strong>
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The latest 1.0.x release of this document can be found
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[here](http://releases.k8s.io/release-1.0/examples/javaweb-tomcat-sidecar/README.md).
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Documentation for other releases can be found at
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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--
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<!-- END STRIP_FOR_RELEASE -->
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<!-- END MUNGE: UNVERSIONED_WARNING -->
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## Java Web Application with Tomcat and Sidercar Container
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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 a sidecar container as `war` file provider.
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### Prerequisites
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https://github.com/kubernetes/kubernetes/blob/master/docs/user-guide/prereqs.md
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### Overview
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This sidecar mode brings a new workflow for Java users:
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As you can see, user can create a `sample:v2` container as sidecar to "provide" war file to Tomcat by copying it to the shared `emptyDir` volume. And Pod will make sure the two containers compose a "atomic" scheduling unit, which is perfect for this case. Thus, your application version management will be totally seperated from web server management.
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For example, if you gonna change the configurations of your Tomcat:
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```
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docker exec -it <tomcat_container_id> /bin/bash
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# do your change, and then commit it to a new image
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docker commit <tomcat_container_id> mytomcat:7.0-dev
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```
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Done! The new Tomcat image **will not** mess up with your `sample.war` file.
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You can also upgrade your app to new version seperately, without creating a new "Tomcat plus app" image (and this image will become huge as unionfs will keep all the old war files in its layers).
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**Why don't put my `sample.war` in a host dir and mount it to tomcat container?**
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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.
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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 seriously, it's a overkill.
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### How To
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In Kubernetes a [_Pod_](../../docs/user-guide/pods.md) is the smallest deployable unit that can be created, scheduled, and managed. Its a collocated group of containers that share an IP and storage volume.
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Here is the config [javaweb.yaml](javaweb.yaml) for Java Web pod:
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NOTE: you should define `war` contaienr **first** as it is the "provider".
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<!-- BEGIN MUNGE: javaweb.yaml -->
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```
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apiVersion: v1
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kind: Pod
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metadata:
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name: javaweb
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spec:
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containers:
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- image: resouer/sample:v1
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name: war
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volumeMounts:
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- mountPath: /app
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name: app-volume
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- image: resouer/mytomcat:7.0
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name: tomcat
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command: ["sh","-c","/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
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volumeMounts:
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- mountPath: /root/apache-tomcat-7.0.42-v2/webapps
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name: app-volume
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ports:
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- containerPort: 8080
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hostPort: 8001
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volumes:
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- name: app-volume
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```
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<!-- END MUNGE: EXAMPLE -->
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The only magic here is the `resouer/sample:v1` image:
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```
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FROM busybox:latest
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ADD sample.war sample.war
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CMD "sh" "mv.sh"
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```
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And the content inf `mv.sh` is:
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```sh
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cp /sample.war /app
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tail -f /etc/hosts
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```
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#### Explaination
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1. 'war' container only contains the `war` file of your app
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2. 'war' container's CMD try to copy `sample.war` to the `emptyDir` volume path
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3. The last line of `tailf -f` is just used to hold the container, as RC does not support one-off task
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4. 'tomcat' container will load the `sample.war` from volume path
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What's more, if you don't want to add a build in `mv.sh` script in the `war` container, you can use Pod lifecycle handler to do the copy work, here's a example [javaweb-2.yaml](javaweb-2.yaml):
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<!-- BEGIN MUNGE: javaweb-2.yaml -->
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```
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apiVersion: v1
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kind: Pod
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metadata:
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name: javaweb-2
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spec:
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containers:
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- image: resouer/sample:v2
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name: war
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lifecycle:
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postStart:
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exec:
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command:
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- "cp"
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- "/sample.war"
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- "/app"
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volumeMounts:
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- mountPath: /app
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name: app-volume
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- image: resouer/mytomcat:7.0
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name: tomcat
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command: ["sh","-c","/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
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volumeMounts:
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- mountPath: /root/apache-tomcat-7.0.42-v2/webapps
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name: app-volume
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ports:
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- containerPort: 8080
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hostPort: 8001
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volumes:
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- name: app-volume
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```
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<!-- END MUNGE: EXAMPLE -->
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And the `resouer/sample:v2` Dockerfile is quite simple:
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```
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FROM busybox:latest
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ADD sample.war sample.war
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CMD "tail" "-f" "/etc/hosts"
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```
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#### Explaination
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1. 'war' container only contains the `war` file of your app
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2. 'war' container's CMD use `tail` to hold the container, nothing else
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3. The `postStart` lifecycle handler will do `cp` after the `war` container is started
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4. Again 'tomcat' container will load the `sample.war` from volume path
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Done! Now your `war` container contains nothing except `sample.war`, clean enough.
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### Test It Out
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Create the Java web pod:
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```sh
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kubectl create -f examples/javaweb-tomcat-sidecar/javaweb-2.yaml
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```
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Check status of the pod:
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```sh
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kubectl get -w po
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NAME READY STATUS RESTARTS AGE
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javaweb-2 2/2 Running 0 7s
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```
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Wait for the status to `2/2` and `Running`. Then you can visit "Hello, World" on `http://localhost:8001/sample/index.html`
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You can also test `javaweb.yaml` in the same way.
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### Delete Resources
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All resources created in this application can be deleted:
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```sh
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kubectl delete -f examples/javaweb-tomcat-sidecar/javaweb-2.yaml
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```
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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@ -0,0 +1,29 @@
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apiVersion: v1
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kind: Pod
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metadata:
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name: javaweb-2
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spec:
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containers:
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- image: resouer/sample:v2
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name: war
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lifecycle:
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postStart:
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exec:
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command:
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- "cp"
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- "/sample.war"
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- "/app"
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volumeMounts:
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- mountPath: /app
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name: app-volume
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- image: resouer/mytomcat:7.0
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name: tomcat
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command: ["sh","-c","/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
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volumeMounts:
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- mountPath: /root/apache-tomcat-7.0.42-v2/webapps
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name: app-volume
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ports:
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- containerPort: 8080
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hostPort: 8001
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volumes:
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- name: app-volume
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@ -0,0 +1,22 @@
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apiVersion: v1
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kind: Pod
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metadata:
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name: javaweb
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spec:
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containers:
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- image: resouer/sample:v1
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name: war
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volumeMounts:
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- mountPath: /app
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name: app-volume
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- image: resouer/mytomcat:7.0
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name: tomcat
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command: ["sh","-c","/root/apache-tomcat-7.0.42-v2/bin/start.sh"]
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volumeMounts:
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- mountPath: /root/apache-tomcat-7.0.42-v2/webapps
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name: app-volume
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ports:
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- containerPort: 8080
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hostPort: 8001
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volumes:
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- name: app-volume
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