Creating Kubernetes Objects: Imperative vs Declarative Approaches

In Kubernetes, there are two primary ways to create and manage objects: the imperative way and the declarative way. Each approach has its use cases and benefits. In this blog post, we’ll explore both methods and work through some practical examples.

Imperative vs Declarative: An Overview

1. Imperative Approach:

   • Uses commands or API calls to directly create or modify Kubernetes objects
   • Ideal for quick, one-off tasks or when learning Kubernetes
   • Example: kubectl create pod nginx --image=nginx

2. Declarative Approach:

   • Uses YAML or JSON manifest files to describe the desired state of objects
   • Better for version control and maintaining complex configurations
   • Example: kubectl apply -f nginx-pod.yaml

Now, let’s dive into some practical tasks to illustrate these approaches.

Task 1: Creating a Pod Imperatively

Let’s create a pod using the imperative command with nginx as the image:

kubectl run nginx-pod --image=nginx --port=80

This command creates a pod named “nginx-pod” using the nginx image and exposing port 80.

To verify the pod creation:

kubectl get pods

You should see your nginx-pod listed in the output.

Task 2: From Imperative to Declarative

Now, let’s convert our imperatively created pod into a YAML file:

1. Generate the YAML from the existing pod:

kubectl get pod nginx-pod -o yaml > nginx-pod.yaml

2. Edit the YAML file to update the pod name:

sed -i 's/name: nginx-pod/name: nginx-new/' nginx-pod.yaml

3. Create a new pod using this YAML file:

kubectl apply -f nginx-pod.yaml

Verify the new pod creation:

kubectl get pods

You should now see both nginx-pod and nginx-new in the list of pods.

Task 3: Troubleshooting a Pod YAML

Let’s apply the following YAML and fix any errors:

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

Applying this YAML:

kubectl apply -f redis-pod.yaml

You might encounter an error like this:

Error: ImagePullBackOff: Back-off pulling image "rediss"

This error suggests that Kubernetes couldn’t pull the specified image. Let’s troubleshoot:

1. Check the pod status:

kubectl describe pod redis

In the events section, you’ll likely see an error indicating that the image “rediss” couldn’t be found.

2. The issue is a typo in the image name. Let’s correct it:

sed -i 's/image: rediss/image: redis/' redis-pod.yaml

3. Apply the corrected YAML:

kubectl apply -f redis-pod.yaml

4. Verify the pod is now running:

kubectl get pods

You should now see the redis pod in a “Running” state.

Conclusion

We’ve explored both imperative and declarative approaches to creating Kubernetes objects. While the imperative method is quick for simple tasks, the declarative approach with YAML files offers better version control and is more suitable for complex configurations.

Remember, when troubleshooting Kubernetes issues:

1. Always check the pod status using kubectl get pods
2. Use kubectl describe pod <pod-name> for detailed information
3. Look for common issues like typos in image names or misconfigured resources

By mastering both approaches and understanding how to troubleshoot common issues, you’ll be well-equipped to manage Kubernetes objects effectively.

References

1. Kubernetes Official Documentation: Imperative Commands
2. Kubernetes Official Documentation: Declarative Management
3. Kubernetes Official Documentation: Troubleshooting Applications

kubectl apply -f nginx-pod.yaml