website/content/en/docs/tasks/access-application-cluster/communicate-containers-same-pod-shared-volume.md

title	content_type	weight
Communicate Between Containers in the Same Pod Using a Shared Volume	task	110

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 between containers.

{{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}

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:

{{< codenew file="pods/two-container-pod.yaml" >}}

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.

{{% heading "whatsnext" %}}

• Learn more about patterns for composite containers.

• Learn about composite containers for modular architecture.

• See Configuring a Pod to Use a Volume for Storage.

• See Configure a Pod to share process namespace between containers in a Pod

• See [Volume](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#volume-v1-core).

• See [Pod](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#pod-v1-core).