Suraj Deshmukh
2b2fb7d84b
The doc asks users to use `ps` command inside the nginx container, when ps is not installed inside the container. So this commit adds that command to install ps.
150 lines
4.2 KiB
Markdown
150 lines
4.2 KiB
Markdown
---
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title: Communicate Between Containers in the Same Pod Using a Shared Volume
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---
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{% capture overview %}
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This page shows how to use a Volume to communicate between two Containers running
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in the same Pod.
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{% endcapture %}
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{% capture prerequisites %}
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{% include task-tutorial-prereqs.md %}
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{% endcapture %}
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{% capture steps %}
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## Creating a Pod that runs two Containers
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In this exercise, you create a Pod that runs two Containers. The two containers
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share a Volume that they can use to communicate. Here is the configuration file
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for the Pod:
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{% include code.html language="yaml" file="two-container-pod.yaml" ghlink="/docs/tasks/access-application-cluster/two-container-pod.yaml" %}
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In the configuration file, you can see that the Pod has a Volume named
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`shared-data`.
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The first container listed in the configuration file runs an nginx server. The
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mount path for the shared Volume is `/usr/share/nginx/html`.
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The second container is based on the debian image, and has a mount path of
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`/pod-data`. The second container runs the following command and then terminates.
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echo Hello from the debian container > /pod-data/index.html
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Notice that the second container writes the `index.html` file in the root
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directory of the nginx server.
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Create the Pod and the two Containers:
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kubectl create -f https://k8s.io/docs/tasks/access-application-cluster/two-container-pod.yaml
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View information about the Pod and the Containers:
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kubectl get pod two-containers --output=yaml
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Here is a portion of the output:
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apiVersion: v1
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kind: Pod
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metadata:
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...
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name: two-containers
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namespace: default
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...
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spec:
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...
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containerStatuses:
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- containerID: docker://c1d8abd1 ...
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image: debian
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...
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lastState:
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terminated:
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...
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name: debian-container
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...
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- containerID: docker://96c1ff2c5bb ...
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image: nginx
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...
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name: nginx-container
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...
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state:
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running:
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...
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You can see that the debian Container has terminated, and the nginx Container
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is still running.
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Get a shell to nginx Container:
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kubectl exec -it two-containers -c nginx-container -- /bin/bash
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In your shell, verify that nginx is running:
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root@two-containers:/# apt-get update
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root@two-containers:/# apt-get install curl procps
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root@two-containers:/# ps aux
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The output is similar to this:
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USER PID ... STAT START TIME COMMAND
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root 1 ... Ss 21:12 0:00 nginx: master process nginx -g daemon off;
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Recall that the debian Container created the `index.html` file in the nginx root
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directory. Use `curl` to send a GET request to the nginx server:
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root@two-containers:/# curl localhost
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The output shows that nginx serves a web page written by the debian container:
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Hello from the debian container
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{% endcapture %}
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{% capture discussion %}
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## Discussion
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The primary reason that Pods can have multiple containers is to support
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helper applications that assist a primary application. Typical examples of
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helper applications are data pullers, data pushers, and proxies.
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Helper and primary applications often need to communicate with each other.
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Typically this is done through a shared filesystem, as shown in this exercise,
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or through the loopback network interface, localhost. An example of this pattern is a
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web server along with a helper program that polls a Git repository for new updates.
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The Volume in this exercise provides a way for Containers to communicate during
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the life of the Pod. If the Pod is deleted and recreated, any data stored in
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the shared Volume is lost.
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{% endcapture %}
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{% capture whatsnext %}
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* Learn more about
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[patterns for composite containers](http://blog.kubernetes.io/2015/06/the-distributed-system-toolkit-patterns.html).
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* Learn about
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[composite containers for modular architecture](http://www.slideshare.net/Docker/slideshare-burns).
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* See
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[Configuring a Pod to Use a Volume for Storage](/docs/tasks/configure-pod-container/configure-volume-storage/).
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* See [Volume](/docs/api-reference/{{page.version}}/#volume-v1-core).
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* See [Pod](/docs/api-reference/{{page.version}}/#pod-v1-core).
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{% endcapture %}
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{% include templates/task.md %}
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