• Kubernetes doesn’t run containers directly on the nodes. Every container is encapsulated by a pod.
• Smallest unit of computing Kubernetes.
• A pod is a single instance of an application. If another instance of the application needs to be deployed, another pod is deployed with the containerized application running inside pod.
• Pods are epheremeral resources, meaning that Pods can be terminated at any point and then restarted on another node within our Kubernetes cluster.
• Creates a running environment over the container so that we only interact with the Kubernetes layer. This allows us to replace the container technology like Docker.
• Each pod gets an internal IP address for communicating with each other (virtual network created by K8).
• If a pod is restarted (maybe after the application running on it crashed), its IP address may change.

Simple Config for a Pod

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apiVersion: v1
kind: Pod
metadata:
  labels:
    name: webserver
spec:
  containers:
	  - name: httpd
	    image: httpd:latest

• apiVersion Which version of the Kubernetes API we’re using to create this object. You can read more about API versioning in Kubernetes here.

• kind This defines what kind of Kubernetes object we want to create.

• metadata This is data that helps us uniquely identify the object that we want to create. Here we can provide a name for our app, as well as apply labels to our object.

• spec This defines the state that we want or our object. The format that we use for spec. For our Pod file, we have provided information about the containers that we want to host on our Pod.

To see what else we can define in our Pod YAML file, this documentation from Kubernetes will help you.

• With kubectl, we can create a Pod using our YAML definition using below command:

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kubectl apply -f pod_defination.yaml

We can list all of our Pods using below command:

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kubectl get pods

Restart Policy

The default behavior of K8s is to restart a pod if it terminates. This is desirable for long running containers like web applications or databases. But, this is not desirable for short-lived containers such as a container to process an image or run analytics.

restartPolicy allows us to specify when K8s should restart the pod.

• Always - restart the pod if it goes down (default)
• Never - never restart the pod
• OnFailure - restart the pod only if the container inside failed (returned non zero exit code after execution)

Config of restartPolicy usage

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apiVersion: v1
kind: Pod
metadata:
  labels:
    name: process
spec:
  containers:
	  - name: analytics
	    image: analytics
	restartPolicy: Never