How to mimic '--volumes-from' in Kubernetes

Question

I'm looking for a pattern that allows to share volumes between two containers running on the same pod in Kubernetes.

My use case is: I have a Ruby on Rails application running inside a docker container. The docker image contains static assets in /app/<app-name>/public directory, and I need to access those assets from the nginx container running alongside in the same pod.

In 'vanilla' docker I would have used --volumes-from flag to share this directory:

docker run --name app -v /app/<app-dir>/public <app-image> docker run --volumes-from app nginx 

After reading this doc: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/volumes.md I tried this (only relevant entries presented):

spec:   containers:     - image: <app-image>       name: <app-name>       volumeMounts:         - mountPath: /app/<app-name>/public           name: assets     - image: nginx       name: nginx       volumeMounts:         - mountPath: /var/www/html           name: assets           readOnly: true     volumes:       - name: assets         hostPath:           path: /tmp/assets 

But:

• Even though /tmp/assets on the node exists, it's empty
• /app/<app-name>/public inside the app container is also empty

As a workaround I'm gonna try to populate the shared directory when the application container is up (simply cp /app/<app-name>/public/* to shared directory), but I really dislike this idea.

Question: how to mimic --volumes-from in Kubernetes, or if there is no direct counterpart, how can I share files from one container to other running in the same pod ?

apiVersion: v1beta3

Client Version: version.Info{Major:"0", Minor:"17", GitVersion:"v0.17.0", GitCommit:"82f8bdac06ddfacf493a9ed0fedc85f5ea62ebd5", GitTreeState:"clean"} Server Version: version.Info{Major:"0", Minor:"17", GitVersion:"v0.17.0", GitCommit:"82f8bdac06ddfacf493a9ed0fedc85f5ea62ebd5", GitTreeState:"clean"} 

Answer 1

[update-2016-8] In latest Kubernetes release, you can use a very nice feature named init-container to replace the postStart part in my answer below, which will make sure the container order.

apiVersion: v1 kind: Pod metadata:   name: javaweb-2 spec:   initContainers:   - name: war     image: resouer/sample:v2     command: ["cp", "/sample.war", "/app"]     volumeMounts:     - mountPath: /app       name: app-volume   containers:   - name: tomcat     image: resouer/mytomcat:7.0     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: {} 

NOTE: initContainer is still a beta feature so the work version of this yaml is actually like: http://kubernetes.io/docs/user-guide/production-pods/#handling-initialization, please notice the pod.beta.kubernetes.io/init-containers part.

---original answer begin---

Actually, you can. You need to use container life cycle handler to control what files/dirs you want to share with other containers. Like:

--- apiVersion: v1 kind: Pod metadata:     name: server spec:     restartPolicy: OnFailure     containers:     - image: resouer/sample:v2       name: war       lifecycle:         postStart:           exec:             command:               - "cp"               - "/sample.war"               - "/app"       volumeMounts:       - mountPath: /app         name: hostv1      - name: peer       image: busybox       command: ["tail", "-f", "/dev/null"]       volumeMounts:       - name: hostv2         mountPath: /app/sample.war     volumes:     - name: hostv1       hostPath:           path: /tmp     - name: hostv2       hostPath:           path: /tmp/sample.war 

Please check my gist for more details:

https://gist.github.com/resouer/378bcdaef1d9601ed6aa

And of course you can use emptyDir. Thus, war container can share its /sample.war to peer container without mess peer's /app directory.

If we can tolerate /app been overridden, it will be much simpler:

--- apiVersion: v1 kind: Pod metadata:   name: javaweb-2 spec:   restartPolicy: OnFailure   containers:   - image: resouer/sample:v2     name: war     lifecycle:       postStart:         exec:           command:             - "cp"             - "/sample.war"             - "/app"     volumeMounts:     - mountPath: /app       name: app-volume   - 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: {}