How to use a Volume to communicate between two Containers running in the same Kubernetes-Pod

Question

How to establish communication between containers in the same Kubernetes-Pod Using a Shared Volume?

Answer 1

Create a Pod that runs two Containers

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:

pods/two-container-pod.yaml 

apiVersion: v1
kind: Pod
metadata:
  name: two-containers
spec:

  restartPolicy: Never

  volumes:
  - name: shared-data
    emptyDir: {}

  containers:

  - name: nginx-container
    image: nginx
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html

  - name: debian-container
    image: debian
    volumeMounts:
    - name: shared-data
      mountPath: /pod-data
    command: ["/bin/sh"]
    args: ["-c", "echo Hello from the debian container > /pod-data/index.html"]

• 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

The Volume in this example 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.