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04_main_kubectl_commands.md

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Kubectl commands

Get status of different components

To get status of a Node

kubectl get nodes

To get status of a pod

kubectl get pod

To get status of a services

kubectl get services

create a pod/deployment

Usually pod is the smallest unit in a kubernetes cluster but in practice you do not create a pod, you create a deployment to manage the pod configuration. You can say that a deployment is basically an abstraction over a pod. To create a deployment from a docker image we can do

kubectl create deployment nginx-depl-kubectl --image=nginx

This will give the output

NAME                                  READY   STATUS              RESTARTS   AGE
nginx-depl-kubectl-5bbcdc84cc-g2lp7   0/1     ContainerCreating   0          5s

Now there is another entity in kubernetes, which are replicasets. So the pod that we created above is part of a replca set whose Id is 5bbcdc84cc and the id of the pod is g2lp7

and when we type

kubectl get replicasets

we get

NAME                            DESIRED   CURRENT   READY   AGE
nginx-depl-kubectl-5bbcdc84cc   1         1         1       3m59s

In this deployment we specified only 1 replica of the pod.

layers of abstraction

So this is the layer of abstraction
Deployment -> ReplicaSets -> Pods -> Containers
And here everything below the deployments is usually managed by kubernetes.

change the pod/deployment

To edit a Pod or replicaset we will have to edit the deployment

kubectl edit deployment nginx-depl-kubectl

After running the above command we will be shown a yaml confiuration file, and if we edit and save it, a new replicaset will be created and the old pod in the old replicaset will be deleted.

kubectl get pod

will give

NAME                                  READY   STATUS    RESTARTS   AGE
nginx-depl-kubectl-5c9878c688-85tdr   1/1     Running   0          3m19s

and

kubectl get replicaset

NAME                            DESIRED   CURRENT   READY   AGE
nginx-depl-kubectl-5bbcdc84cc   0         0         0       24m
nginx-depl-kubectl-5c9878c688   1         1         1       3m52s

debugging pods

To know the changes that are happening inside a pod you can do

kubectl describe pod mongo-depl-kubectl

And events will be like

Events:
Type    Reason     Age   From               Message
----    ------     ----  ----               -------
Normal  Scheduled  100s  default-scheduler  Successfully assigned default/mongo-depl-kubectl-54678f769c-fx59n to minikube
Normal  Pulling    91s   kubelet            Pulling image "mongo"

To see the logs of a container we can use

kubectl logs mongo-depl-kubectl-54678f769c-fx59n

Now to enter a container, via a terminal shell, we can use the following command

kubectl exec -it mongo-depl-kubectl-54678f769c-fx59n -- bin/bash

This can be very usefull while project setup and debugging

delete pod/deployment

To delete a deployment you can use, deleting the deployment means deleting the entire replicset and thus all the pods in it.

kubectl delete deployment mongo-depl-kubectl

CRUD by applying configuration file

It is a tideous process to remember commands for deploying same configurations, so we can define configurations in a configuration file and use that using the following command

kubectl apply -f ./configs/nginx-config.yaml

nginx-config.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-depl
spec:
  selector: 
    matchLabels:
      app: nginx-depl
  template:
    metadata:
      labels:
        app: nginx-depl
    spec:
      containers:
      - name: nginx-depl
        image: nginx
        resources:
          limits:
            memory: "128Mi"
            cpu: "500m"
        ports:
        - containerPort: 80

Now after running

kubectl get pods

NAME                          READY   STATUS              RESTARTS   AGE
nginx-depl-86b8689f6f-j4jgv   0/1     ContainerCreating   0          8s

Now if we want to make any changes to the deployment we can simply change the configuration file and run the apply command again, for example if we want to create 2 replicas we can simply add a replica fiends and run apply command and we'll see

NAME                          READY   STATUS              RESTARTS   AGE
nginx-depl-86b8689f6f-6n9p9   0/1     ContainerCreating   0          6s
nginx-depl-86b8689f6f-j4jgv   1/1     Running             0          3m45s

Conf

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-depl
spec:
  replicas: 2
  selector: 
    matchLabels:
      app: nginx-depl
  template:
    metadata:
      labels:
        app: nginx-depl
    spec:
      containers:
      - name: nginx-depl
        image: nginx
        resources:
          limits:
            memory: "128Mi"
            cpu: "500m"
        ports:
        - containerPort: 80