K8s Pods

Suppose you have a Kubernetes cluster set up on your machine , lets assume of nginx. You create a Pod using YAML and it looks like this :

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx:1.14.2
    ports:
    - containerPort: 80

You give the command to create a pod :

 kubectl apply -f pod.yaml

and it says the pod has been created .

To list the pods use the command :

kubectl get pods

And it will list the pods .

Everything is working Perfectly Fine.

Now if u run :

kubectl delete pod <pod-name>

It will delete the pod .

Now we know that Kubernetes is a container orchestration platform which orchestrates and manages the containers, Then why did it delete the pod ?

The answer is We have to create the right kind of resource ,which is called Deployment .

K8s Deployment

We will take the same nginx example to show what it's different from a pod.

Here's what a deployment looks like:

 apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

In YAML file we create a Deployment kind instead of Pod , we give a label and number of replicas.

The replicas define how much number of replicas we want and that much containers are always up and running . The replicasets are a kubernetes controller which implemets the autohealing process . The controller is something which ensures that the actual state specified in yaml manifest is same as the desired state .

Selector field defines how crated replicasets finds which pods to manage.matchLabels field is a map of {key,value} pairs .Which should satisfy some conditions in order to match the pairs .

Now,Create a deployment :

 kubectl apply -f deployment.yaml

When we create a deployment ,we are actually creating a pod which has a replicaset.

Now list the pods and the deployments you will see the nginx is a pod as well as a deployment.

Now ,delete the pod :

kubectl delete pod deployment.yaml

and simultaneously watch for the pods .

kubectl get pod -w

you will see that when the pod is getting deleted and simultaneously replicaset is creating another pod . And it ensures that the replicas defined in the yaml manifests are always up.

Diff between Pods and Deployments .

Pod	Deployment
1.Pod is not controlled by the Controller Manager.	1.Deployment is controlled by the controller Manager .
2.Pod can be deleted by the delete pod command .	2.Deployment cannot be deleted by the delete pod command .
3.Pod cannot be called as a Deployment.	3.Deployment is also a Pod and a Deployment.
4.Pods are managed by the replicasets of the controller Manager.	4.Deployment has a replica set which controls the pods .

Diagram

Here's a diagram to give more detailed overview of a Deployment:

In this way , Deployment Satisfies the condition of AutoHealing

Deployment also satisfies the condition of AutoScaling using Horizontal Pod AutoScaler(HPA).

There are many other ways That proves , Deployment is Always Better than creating a pod .

You can also Create a Pod for some Temporary Usage but its always better to use a Deployment.

You can Know more about Kubernetes Deployment on K8s Docs ..

Thanks For Reading ...

Credits to - Abhishek Veeramalla

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