152 lines
4.2 KiB
Markdown
152 lines
4.2 KiB
Markdown
---
|
|
title: Communicate Between Containers in the Same Pod Using a Shared Volume
|
|
content_template: templates/task
|
|
weight: 110
|
|
---
|
|
|
|
{{% capture overview %}}
|
|
|
|
This page shows how to use a Volume to communicate between two Containers running
|
|
in the same Pod.
|
|
|
|
{{% /capture %}}
|
|
|
|
|
|
{{% capture prerequisites %}}
|
|
|
|
{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
|
|
|
|
{{% /capture %}}
|
|
|
|
|
|
{{% capture steps %}}
|
|
|
|
## 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 create -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
|
|
|
|
{{% /capture %}}
|
|
|
|
|
|
{{% capture discussion %}}
|
|
|
|
## 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.
|
|
|
|
{{% /capture %}}
|
|
|
|
|
|
{{% capture whatsnext %}}
|
|
|
|
* Learn more about
|
|
[patterns for composite containers](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns).
|
|
|
|
* Learn about
|
|
[composite containers for modular architecture](http://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 [Volume](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#volume-v1-core).
|
|
|
|
* See [Pod](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#pod-v1-core).
|
|
|
|
{{% /capture %}}
|
|
|
|
|
|
|