Communicate Between Containers in the Same Pod Using a Shared Volume [page]deterministic

This page shows how to use a Volume to communicate between two Containers running in the same Pod. See also how to allow processes to communicate by [sharing process namespace](/docs/tasks/configure-pod-container/share-process-namespace/) between containers.

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## Creating a Pod that runs two Containers

In this exercise, you create a Pod that runs two Containers. The two containers share a Volume that they can use to communicate. Here is the configuration file for the Pod:

In the configuration file, you can see that the Pod has a Volume named `shared-data`.

The first container listed in the configuration file runs an nginx server. The mount path for the shared Volume is `/usr/share/nginx/html`. The second container is based on the debian image, and has a mount path of `/pod-data`. The second container runs the following command and then terminates.

echo Hello from the debian container > /pod-data/index.html

Notice that the second container writes the `index.html` file in the root directory of the nginx server.

Create the Pod and the two Containers:

kubectl apply -f https://k8s.io/examples/pods/two-container-pod.yaml

View information about the Pod and the Containers:

kubectl get pod two-containers --output=yaml

Here is a portion of the output:

apiVersion: v1 kind: Pod metadata: ... name: two-containers namespace: default ... spec: ... containerStatuses:

• containerID: docker://c1d8abd1 ... image: debian ... lastState: terminated: ... name: debian-container ...

• containerID: docker://96c1ff2c5bb ... image: nginx ... name: nginx-container ... state: running: ...

You can see that the debian Container has terminated, and the nginx Container is still running.

Get a shell to nginx Container:

kubectl exec -it two-containers -c nginx-container -- /bin/bash

In your shell, verify that nginx is running:

root@two-containers:/# apt-get update root@two-containers:/# apt-get install curl procps root@two-containers:/# ps aux

The output is similar to this:

USER PID ... STAT START TIME COMMAND root 1 ... Ss 21:12 0:00 nginx: master process nginx -g daemon off;

Recall that the debian Container created the `index.html` file in the nginx root directory. Use `curl` to send a GET request to the nginx server:

``` root@two-containers:/# curl localhost ```

The output shows that nginx serves a web page written by the debian container:

``` Hello from the debian container ```

## Discussion

The primary reason that Pods can have multiple containers is to support helper applications that assist a primary application. Typical examples of helper applications are data pullers, data pushers, and proxies. Helper and primary applications often need to communicate with each other. Typically this is done through a shared filesystem, as shown in this exercise, or through the loopback network interface, localhost. An example of this pattern is a web server along with a helper program that polls a Git repository for new updates.

The Volume in this exercise provides a way for Containers to communicate during the life of the Pod. If the Pod is deleted and recreated, any data stored in the shared Volume is lost.

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* Learn more about [patterns for composite containers](/blog/2015/06/the-distributed-system-toolkit-patterns/).

* Learn about [composite containers for modular architecture](https://www.slideshare.net/Docker/slideshare-burns).

* See [Configuring a Pod to Use a Volume for Storage](/docs/tasks/configure-pod-container/configure-volume-storage/).

* See [Configure a Pod to share process namespace between containers in a Pod](/docs/tasks/configure-pod-container/share-process-namespace/)

* See [Volume](/docs/reference/generated/kubernetes-api//#volume-v1-core).

* See [Pod](/docs …(trimmed)