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Kubernetes: 1. Pod

CREATE NEW POD

Every kubernetes pod has 4 root level properties. They are

  1. apiVersion
  2. kind
  3. metadata
  4. spec

For a pod these are as below.
metadata -> Data about the Pod, note that we cannot have anything we want here. Only allowed values have to be used.
metadata -> labels. This can be any key:value pairs, think of this as custom properties. 
spec -> containers -> "- name". This is list, because we can have multiple containers running in a pod. "-" Indicates its a list

pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
    location: IN

spec:
  containers:
  - name: nginx-container
    image: nginx
  - name: backend-db
    image: redis

The easy way to create a pod is using kubectl run
kubectl run <pod-name> --image=<image-name>


DELETE A POD
Get the list of pods running and then delete the pod
kubectl get pods

kubectl delete pod <pod-name

kubectl delete -f <pod-definition-file> -f 
-> Filename, directory, or URL to a file containing the resource to delete.


UPDATE A POD
There are two ways to update a pod
  1. kubectl edit pod <pod-name> -> You will observe that full definition of pod as created by kubernetes will be shown
    With this the changes are automatically applied to the pod and the pod is restarted
  2. Directly update the YAML file and use kubectl apply command
    With this the changes are updated only after running the kubectl apply command

Note that Kubernetes does not allow many properties of the pod to be changed once created. Below specifications are NOT ALLOWED to be changed.
  • spec.containers[*].image
  • spec.initContainers[*].image
  • spec.activeDeadlineSeconds
  • spec.tolerations

But if you need to make certain changes, then there are 2 options
  1. Use kubectl edit pod
  2. Extract pod output in YAML format and make changes
  • With kubectl edit pod, kubernetes gives an error that the changes cannot be made
  • But at the same time kubernetes makes a copy of the changes under /tmp directory. It gives the full path for the definition file
  • Simply delete the existing pod and recreate the new pod using the updated definition file
  • With second option, get the pod output in yaml format by using "-o yaml" option
  • Update the yaml file with new changes
  • Delete the old pod and create the new pod with the updated yaml file
  • As we can see both the options are bit painful, that is why it is better to create a deployment file
  • Kubernetes allow changes to the deployment definition
  • This is because kubernetes deletes all the pod templates defined in deployment and re-creates the pod everytime there is a change to deployment definition

kubectl edit pod <pod-name>

kubectl apply -f <pod-definition-yaml-file-name>

kubectl get pod webapp -o yaml > my-new-pod.yaml

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