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| Communicating Between Containers Running in the Same Pod |
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This page shows how to use a Volume to communicate between two Containers running in the same Pod.
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{% capture prerequisites %}
{% include task-tutorial-prereqs.md %}
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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:
{% include code.html language="yaml" file="two-container-pod.yaml" ghlink="/docs/tasks/configure-pod-container/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 create -f http://k8s.io/docs/tasks/configure-pod-container/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:/# 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:/# apt-get update
root@two-containers:/# apt-get install curl
root@two-containers:/# curl localhost
The output shows that nginx serves a web page written by the debian container:
Hello from the debian container
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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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{% capture whatsnext %}
-
Learn more about patterns for composite containers.
-
Learn about composite containers for modular architecture.
-
See Volume.
-
See Pod.
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{% include templates/task.md %}