A quick guide on How to extend kubectl with plugins?

• kubectl
• What is a plugin?
• Why write a plugin?
  • Restart a Pod
• How to write a plugin?
• Demo
• Notes
• Show Your Support

kubectl

kubectl is a command-line interface (CLI) used to interact with the Kubernetes cluster. With the help of kubectl you can perform the following operations on the Kubernetes cluster:

• Deploy & Manage containerize application.
• Connect to multiple Kubernetes clusters.
• Get Kubernetes cluster information.
• Many more...

Visit here for more detail on kubectl.

What is a plugin?

• kubectl plugin is a standalone executable file used to enhance the functionality of kubectl with new subcommands.
• With plugins, you can add new features to kubectl as per your requirement.

Why write a plugin?

To build a complex use case using existing kubectl commands. Let us understand this with the following use case:

Restart a Pod

To restart a Pod you can use the following methods:

Method 1:

• Retrieve the Pod name using the following command:

kubectl get pods

• Delete the Pod using the following command:

kubectl delete pod <POD_NAME>

Method 2:

• Retrieve the Deployment name using the following command:

kubectl get deployment

• Scale down the replica to zero using the following command:

kubectl scale --replicas=0 deploy/<DEPLOYMENT_NAME>

• Scale up the replica to one using the following command:

kubectl scale --replicas=1 deploy/<DEPLOYMENT_NAME>

Note: Both the methods are valid only If Pod is created through Deployment.

In both the methods to restart a Pod You have to execute multiple kubectl commands. But can you achieve the same using a single command? The answer is YES. Through plugins, you can wrap the above kubectl commands into a single command. For example, You can have a command like kubectl restart pod nginx where nginx is the name of the Deployment.

How to write a plugin?

Let us understand the whole process step by step by writing a plugin for the Restart a Pod use case.

1. Create a file called kubectl-restart-pod.sh by using the following command:

touch kubectl-restart-pod.sh

1. Add the following content into the kubectl-restart-pod.sh file:

#!/bin/bash

# Logging script execution start
echo "-------------------------"

DEPLOYMENT=$1
NAMESPACE=default

if [ ! -z $2 ]; then
 if [ $2 == "-n" ]; then
    NAMESPACE=$3
  fi
fi


# Logging name of the Deployment
echo "Deployment: $DEPLOYMENT"

# Logging name of the Namespace
echo "Namespace: $NAMESPACE"

# Retrieving Deployment
DEPLOYMENT_DETAILS={}

{
 DEPLOYMENT_DETAILS=$(kubectl get deploy $DEPLOYMENT -o json -n $NAMESPACE)
} || {
 echo "-------------------------"
 exit 1
}

# Retrieving the current replicas
CURRENT_REPLICAS=$(echo $DEPLOYMENT_DETAILS | jq .spec.replicas)

# Retrieving the labels
LABELS=$(echo $DEPLOYMENT_DETAILS | jq .spec.selector.matchLabels)
LABEL=$(echo $LABELS | jq -r 'to_entries|map("\(.key)=\(.value|tostring)")|.[0]')

# Scaling down the replicas
echo "Scaling down the replicas to 0"
kubectl scale --replicas=0 deploy/$DEPLOYMENT -n $NAMESPACE

while true; do

    PODS_COUNT=$(kubectl get po -l $LABEL -n $NAMESPACE -o json | jq '.items | length')

    if [ $PODS_COUNT == 0 ]; then
        break
    else
        printf "."
        sleep 5
    fi

done

# Scaling up the replicas
echo ""
echo "Scaling up replicas to $CURRENT_REPLICAS"
kubectl scale --replicas=$CURRENT_REPLICAS deploy/$DEPLOYMENT -n $NAMESPACE

while true; do

    INDEX=0
    RUNNING_COUNT=0
    PODS=$(kubectl get po -l $LABEL -n $NAMESPACE -o json | jq .items)

    while [ $INDEX -lt $CURRENT_REPLICAS ]; do
        PHASE=$(echo $PODS | jq -r --arg index $INDEX '.[$index | tonumber].status.phase')

        if [ $PHASE == "Running" ]; then
            RUNNING_COUNT=$((RUNNING_COUNT + 1))
        fi

        INDEX=$((INDEX + 1))
    done

    if [ $RUNNING_COUNT == $CURRENT_REPLICAS ]; then
        break
    else
        printf "."
        sleep 5
    fi

done

# Logging script execution end
echo ""
echo "-------------------------"

Refer to source code here.

1. Make kubectl-restart-pod.sh file executable by using the following command:

chmod +x kubectl-restart-pod.sh

1. Move kubectl-restart-pod.sh file to /usr/local/bin directory by using following command:

mv kubectl-restart-pod.sh /usr/local/bin/kubectl-restart-pod

Demo

• Install jq on the workstation machine, where you can access the Kubernetes cluster. Visit here for steps to install jq.

• List all the available plugins by using the following command:
  

kubectl plugin list

kubectl-restart-pod plugin should list in the output.

• Create namespace demo by using the following command:

kubectl create ns demo

• Deploy nginx application on Kubernetes cluster by using the following command:

kubectl create deploy nginx --image=nginx:latest -n demo

• Check the status of nginx Pod by using the following command:

kubectl get po -n demo

nginx Pod should be in the Running state.

• Restart the nginx Pod by using the following command:

kubectl restart pod nginx -n demo

Notes

• As a prerequisite, you should have kubectl CLI installed on your workstation machine, where you can access the Kubernetes cluster. Visit here for steps to install kubectl CLI.
• Name of the plugin standalone executable file should start with kubectl- for example kubectl-restart-pod.sh
• You cannot override the existing kubectl command using plugins for example you cannot create a plugin kubectl-delete-pod. If you create such plugin then it will be ignored when you run kubectl delete pod command.

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