Lab 2: Init Containers

Init containers are specialized containers that run before or after app containers in a Pod. They can contain utilities or setup scripts not present in an app image.

Init containers are exactly like regular containers, except:

• Init containers always run to completion.

• Each init container must complete successfully before the next one starts.

• If you specify multiple init containers for a Pod, Kubelet runs each init container sequentially.

• If a Pod’s init container fails, Kubernetes repeatedly restarts the Pod until the init container succeeds.

• Init containers support all the fields and features of app containers, including resource limits, volumes, and security settings, however they do not support lifecycle, livenessProbe, readinessProbe, or startupProbe for obvious reasons.

  

Init Containers Applied

1. Run a standard nginx container

   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: nginx-demo
     namespace: default
     labels:
       app: nginx-demo
   spec:
     selector:
       matchLabels:
         app: nginx-demo
     replicas: 1
     template:
       metadata:
         labels:
           app: nginx-demo
       spec:
         containers:
         - name: nginx
           image: nginx
           imagePullPolicy: IfNotPresent 
           ports:
           - containerPort: 80
   

   ``

   kubectl apply -f ./training/initContainers/container.yaml
   

   ``

2. Expose it with a service

    apiVersion: v1
    kind: Service
    metadata:
      name: nginx-demo-service
    spec:
      selector:
        app: nginx-demo
      ports:
        - protocol: TCP
          port: 8000
          targetPort: 80
      type: NodePort
   

   ``

   kubectl apply -f ./training/initContainers/service.yaml
   

   ``

3. Open the deployment in Firefox

   minikube service nginx-demo-service
   

   ``

   You should see the default nginx welcome page.

TODO

4. Update the deployment with the init container

   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: nginx-demo-init
     namespace: default
     labels:
       app: nginx-demo
   spec:
     selector:
       matchLabels:
         app: nginx-demo
     replicas: 1
     template:
       metadata:
         labels:
           app: nginx-demo
       spec:
         containers:
         - name: nginx
           image: nginx
           imagePullPolicy: IfNotPresent 
           ports:
           - containerPort: 80
             
          --> Volume mapped into the nginx pod
           volumeMounts:
           - name: workdir
             mountPath: /usr/share/nginx/html
                
         --> This container is run during pod initialization
         initContainers:
   
         --> Sleep for 2 Seconds
         - name: sleep-1
           image: busybox
           imagePullPolicy: IfNotPresent
           command: ['sh', '-c', 'echo sleep-1 start; sleep 2;echo sleep-1 complete;']
            
         --> Sleep again for 2 Seconds
         - name: sleep-2
           image: busybox
           imagePullPolicy: IfNotPresent
           command: ['sh', '-c', 'echo sleep-2 start; sleep 2;echo sleep-2 complete;']
         - name: install
           image: busybox
              
         --> Download index.html and store into work-dir
           command:
           - wget
           - "-O"
           - "/work-dir/index.html"
           - http://kubernetes.io
           volumeMounts:
           - name: workdir
             mountPath: "/work-dir"
         dnsPolicy: Default
         volumes:
         - name: workdir
           emptyDir: {}
   

   ``

   To initialize nginx with some content:

   • We create a local volume (work-dir)
   • This volume is mapped into the nginx container path (/usr/share/nginx/html) where nginx serves static HTML content from
   • An init container (install) that downloads the index.html file from the official Kubernetes webpage and stores it into the volume

   kubectl apply -f ./training/initContainers/init-container.yaml
   

   `

5. Refresh the page in Firefox

   You should see the Kubernetes page that has been downloaded by the init container.

Init Containers Examined

Now let’s have a look at what has happened in the background.

1. Start k9s and select the nginx-demo-init Pod and hit enter

   

2. Select the install Container and hit enter

   

3. You can see that the shell command has downloaded the html code into the work-dir volume.

   

4. Hit ESC to go back and select the nginx Container and hit enter

   

5. Nginx is serving the index.html page from the work-dir volume.

   

6. If you go back with ESC you can also have a look at how the two other containers have slept ;-)

   

7. Quit k9s by hitting Ctrl-C several times

Congratulations!!! This concludes the Lab on Init Containers