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Imperative vs Declaritive approaches

There's generally two ways to use kubectl when managing your kubernetes objects (pods, services,...etc), they are the Declaritive and Imperative approaches.

The Declaritive approach is about using the kubectl apply command when creating objects. That's the approach we took in our hello-world demo. The declarative approach therefore depends on us first creating the yaml files before we can create the objects.

The imperative approach involves using any of the verb based commands, here are some them:

  • kubectl run
  • kubectl create
  • kubectl expose
  • kubectl delete
  • kubectl edit

For example to imperitavely create our pod we would use the kubectl run command:

kubectl run pod-httpd --image=httpd --labels="app=apache_webserver" --restart=Never

You can also feed yaml files into some of the imperative commands:

$ kubectl create -f configs/
pod/pod-httpd created

$ kubectl delete -f configs/
pod "pod-httpd" deleted

In the documentation it says that the Declaritive technique is the recommended approach when working in production. However in reality it depends on the the task you want to perform. For example you should take the declarative approach when creating pods, so that your pod's yaml files are documentend and version controlled in Git. On the otherhand, we use the imperative approach to delete objects.

One very handy feature with imperative commands is that you can use them to generate example yaml files. You can then use these as boilerplate templates to create your own yaml files:

kubectl run pod-httpd --image=httpd --restart=Never --dry-run -o yaml > pod.yaml

kubectl create service nodeport svc-nodeport-httpd --node-port=31000 --tcp=3050:80 > service.yaml

CKA Exam tip: Your not allowed to copy+paste more than a couple of lines at a time during the exam. You can write them by hand, but that's time consuming and prone to errors, which will use up even more precious seconds. Which is why it's best to use the imperative commands to create the yaml files for you. We'll include a complete list of these commands in the appendix.

References

https://ansilh.com/03-pods/03-create-pod/