Pod in Kubernetes Explained

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This article is an assignment for CKA lessons. It explains how to set up a local Docker registry using kind, create Kubernetes pods using both imperative and declarative methods, and leverage dry runs to validate configurations. Additionally, it offers troubleshooting tips, useful kubectl commands, and guidance on fixing YAML errors in Kubernetes manifests.

Two Ways to Create a pod

There are two methods to create a pod: the imperative way and the declarative way.

Imperative Way (Using Commands or API Calls)

Run the following command to create a pod:

kubectl run nginx-pod --image=localhost:5001/nginx:alpine-slim

To checl the pod’s status, run:

$ kubectl get pods
NAME        READY   STATUS    RESTARTS   AGE
nginx-pod   1/1     Running   0          8m40s

To view the pod’s logs, use the following commands:

kubectl logs nginx-pod

Declarative way (By Creating Manifest Files)

First, check the pod API version in cluster:

kubectl explain pod

This will return v1.

Now, create a YAML file named pod.yaml with the following content:

apiversion: v1
kind: Pod
metadata:
  name: nginx-pod
  labels:
    env: demo
    type: frontend
spec:
  containers:
  - name: nginx-container
    image: nginx
    ports:
    - containerPort: 80

To create the pod, first delete the one created in the previous steps:

kubectl delete pod nginx-pod
or
kubectl delete -f pod.yaml

Then create the pod using the YAML file:

kubectl create -f pod.yaml
or
kubectl apply -f pod.yaml

To get more details about the pod:

kubectl describe pod nginx-pod

To edit the pod:

kubectl edit pod nginx-pod

To access the pod’s shell:

kubectl exec -it nginx-pod -- sh

What is a “dry run”

In Kubernetes, a dry run refers to a way to test a configuration or operation without actually making changes to the cluster. It’s useful for validating manifests and ensuring that the requested operation would succeed without impacting any resources.

Key Aspects of Dry Run:

1. Purpose:

• Validate the resource definition (e.g., syntax, required fields).
• Check whether an operation would succeed without creating, updating, or deleting any resources.

2. Usage:

• The--dry-run flag is used with kubectl commands.
• You can use--dry-run=client or --dry-run=server.

3. Types:

• Client-side (--dry-run=client):
• Validation happens locally on the client without contacting the API server.
• Good for checking syntax and local configurations.
• Server-side (--dry-run=server):
• Validation happens on the API server but does not persist the changes.
• Useful for ensuring server-side admission controllers and resource quota validations are applied correctly.

4. Example Commands:

• Validate creating a pod:

kubectl apply -f pod.yaml --dry-run=server

• Check a delete operation without actually deleting:

kubectl delete pod my-pod --dry-run=client

5. Benefits:

• Prevent unintended changes or errors in production.
• Test configurations before applying them to a live cluster.
• Ensure compliance with resource constraints and admission policies.

“dry run” examples

To perform a “dry run”:

kubectl run nginx --image=nginx --dry-run=client

To generate YAML output:

kubectl run nginx --image=nginx --dry-run=client -o yaml

To save the YAML output to a file:

kubectl run nginx --image=nginx --dry-run=client -o yaml > pod-new.yaml

To save the JSON output to a file:

kubectl run nginx --image=nginx --dry-run=client -o json > pod-new.json

More commands

To display the lables of a pod:

kubectl get pods nginx-pod --show-labels

To display detailed information about the pod:

kubectl get pods -o wide

Task: Identify the Error in the following YAML File

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: test
  name: redis
spec:
  containers:
  - image: rediss
    name: redis

Answer: The image value is incorrect. The correct value is redis (with only one s).

Reference

• Pod in Kubernetes Explained