Pod InitContainer

Pod InitContainer

Create 2 service, so we can emulate a clusterIP service.

---
kind: Service
apiVersion: v1
metadata:
  name: webservice
spec:
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
---
kind: Service
apiVersion: v1
metadata:
  name: mysql
spec:
  ports:
  - protocol: TCP
    port: 3306
    targetPort: 3306

Apply the file:

kubectl apply -f initcontainer-svc.yaml

service/webservice created
service/mysql created

This services will not have any pod attached, the clusterIP still be resolved.

kubectl get svc

NAME         TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
mysql        ClusterIP   10.106.152.72   <none>        3306/TCP   24s
webservice   ClusterIP   10.97.16.97     <none>        80/TCP     24s

Create a Pod with two InitContainers that wait until the service answer:

---
apiVersion: v1
kind: Pod
metadata:
  name: initcontainer-1
  labels:
    app: initcontainer-1
spec:
  containers:
  - name: container-1
    image: r.deso.tech/library/centos
    command: ['bash', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-container1
    image: r.deso.tech/library/netshoot
    command: ['sh', '-c', 'until nslookup webservice; do echo waiting for webservice application; sleep 2; done;']
  - name: init-container2
    image: r.deso.tech/library/netshoot
    command: ['sh', '-c', 'until nslookup mysql; do echo waiting for mysql database; sleep 2; done;']

Apply the yaml file:

kubectl apply -f initcontainer-1.yaml

Wait until the pod will start:

kubectl get pod

NAME                           READY   STATUS    RESTARTS   AGE
initcontainer-1                1/1     Running   0          9s

kubectl logs initcontainer-1

The app is running!

You should see “The app is running!”, because pass all the initcontainer without any problem.

Now delete the pod:

kubectl delete pod initcontainer-1 --force --grace-period=0

warning: Immediate deletion does not wait for confirmation that the running resource has been terminated. The resource may continue to run on the cluster indefinitely.
pod "initcontainer-1" force deleted

You are going to delete one of the services:

kubectl delete svc mysql

service "mysql" deleted

Now apply again the yaml file:

kubectl apply -f initcontainer-1.yaml

Immediately run:

kubectl get pod -w

NAME                           READY   STATUS     RESTARTS   AGE
initcontainer-1                0/1     Init:1/2   0          4s
initcontainer-1                0/1     Init:1/2   0          6s

You will see that the pod will stay indefinitely waiting that nslookup resolves it. After 1 minute the watching command will exit, but the pod still to wait the initContainer.

Now try to apply again the file of the service mysql. This operation will permit to resolve again the load balancer svc mysql and the initContainer will continue the step:

kubectl get pod

NAME                           READY   STATUS     RESTARTS   AGE
initcontainer-1                0/1     Init:1/2   0          4s
initcontainer-1                0/1     Init:1/2   0          6s

kubectl apply -f initcontainer-svc.yaml

service/webservice unchanged
service/mysql created

Check again the pod and you will see the Pod in running state.

kubectl get pod -w

NAME              READY   STATUS            RESTARTS   AGE
initcontainer-1   0/1     PodInitializing   0          4m26s
initcontainer-1   1/1     Running           0          4m27s

Delete the pod:

kubectl delete pod initcontainer-1 --force --grace-period=0

warning: Immediate deletion does not wait for confirmation that the running resource has been terminated. The resource may continue to run on the cluster indefinitely.
pod "initcontainer-1" force deleted

Delete the services:

kubectl delete svc mysql webservice

service "mysql" deleted
service "webservice" deleted