Pod

• Kubernetes doesn’t run containers directly on the nodes. Every container is encapsulated by a pod.

• Smallest unit of 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 it.

• 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

⛔

Sometimes we need to have a helper container for the application container. In that case, we can run both containers inside the same pod. This way both containers share the same storage and network and can reference each other as localhost

Without using Pods, making a setup like this would be difficult as we need to manage attaching the helper containers to the application containers and kill them if the application container goes down.

Although, most use cases of pods revolve around single containers, it provides flexibility to add a helper container in the future as the application evolves.

Config file for a Pod

apiVersion: v1
kind: Pod
metadata:
  labels:
    name: frontend
spec:
  containers:
	  - name: httpd
	    image: httpd:2.4-alpine

Restart Policy

• 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)

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

Multi Container Pod

• Containers inside a pod share the same:
• Lifecycle - created and destroyed together
• Network - can refer each other as localhost
• Storage - can access the same storage volumes

• All the containers inside a pod must keep running for the pod to remain in running state. If any of them fails, the pod restarts.

• Sharing volumes in multi-container pods

apiVersion: v1
kind: Pod
metadata:
	name: web-pod
spec:
	volumes:
		- name: shared-vol
			emptyDir: {}
	containers:
		- name: webapp
			image: webapp
			volumeMounts:
		    - name: shared-vol
		      mountPath: /files
		- name: log-agent
			image: log-agent
			volumeMounts:
		    - name: shared-vol
		      mountPath: /logs

Init Containers

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
spec:
  containers:
  - name: myapp-container
    image: busybox:1.28
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-myservice
    image: busybox:1.28
    command: ['sh', '-c', 'until nslookup myservice; do echo waiting for myservice; sleep 2; done;']
  - name: init-mydb
    image: busybox:1.28
    command: ['sh', '-c', 'until nslookup mydb; do echo waiting for mydb; sleep 2; done;']

Sidecar Containers