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Step-by-step instructions for performing operations with Kubernetes.

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Configuring Pod Initialization

This page shows how to use an init Container to initialize a Pod before the application Container runs.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube.

Creating a Pod that has an init Container

In this exercise you create a Pod that has one application Container and one init Container. The init Container runs to completion before the application container starts.

Here is the configuration file for the Pod:

init-containers.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: init-demo
  annotations:
    pod.beta.kubernetes.io/init-containers: '[
        {
            "name": "install",
            "image": "busybox",
            "command": ["wget", "-O", "/work-dir/index.html", "http://kubernetes.io"],
            "volumeMounts": [
                {
                    "name": "workdir",
                    "mountPath": "/work-dir"
                }
            ]
        }
    ]'
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 80
    volumeMounts:
    - name: workdir
      mountPath: /usr/share/nginx/html
  dnsPolicy: Default
  volumes:
  - name: workdir
    emptyDir: {}

In the configuration file, you can see that the Pod has a Volume that the init Container and the application Container share. Notice that the init Container is a beta feature and is specified as an annotation.

The init Container mounts the shared Volume at /work-dir, and the application Container mounts the shared Volume at /usr/share/nginx/html. The init Container runs the following command and then terminates:

 wget -O /work-dir/index.html http://kubernetes.io

Notice that the init Container writes the index.html file in the root directory of the nginx server.

Create the Pod:

kubectl create -f http://k8s.io/docs/tasks/configure-pod-container/init-containers.yaml

Verify that the nginx Container is running:

kubectl get pod init-demo

The output shows that the nginx Container is running:

NAME      READY     STATUS    RESTARTS   AGE
nginx     1/1       Running   0          43m

Get a shell into nginx the Container running in the init-demo Pod:

kubectl exec -it init-demo -- /bin/bash

In your shell, send a GET request to the nginx server:

root@nginx:~# apt-get update
root@nginx:~# apt-get install curl
root@nginx:~# curl localhost

The output shows that nginx is serving the web page that was written by the init container:

<!Doctype html>
<html id="home">

<head>
...
"url": "http://kubernetes.io/"}</script>
</head>
<body>
  ...
  <p>Kubernetes is open source giving you the freedom to take advantage ...</p>
  ...

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