155 lines
4.4 KiB
Markdown
155 lines
4.4 KiB
Markdown
---
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title: Communicate Between Containers in the Same Pod Using a Shared Volume
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redirect_from:
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- "/docs/user-guide/pods/multi-container/"
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- "/docs/user-guide/pods/multi-container.html"
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- "docs/tasks/configure-pod-container/communicate-containers-same-pod/"
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- "docs/tasks/configure-pod-container/communicate-containers-same-pod.html"
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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:/# 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:/# apt-get update
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root@two-containers:/# apt-get install curl
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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/v1.6/#volume-v1-core).
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* See [Pod](/docs/api-reference/v1.6/#pod-v1-core).
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{% endcapture %}
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{% include templates/task.md %}
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