4: Understanding Kubernetes: Deep Dive into kubectl create vs kubectl apply, Labels, and Selectors

Kubernetes (K8s) provides several commands to manage resources effectively. This blog explores concepts like kubectl create vs kubectl apply, YAML configurations, labels, and selectors with examples.

---

kubectl create vs kubectl apply

• kubectl create: This command is used to create new resources in Kubernetes. However, it only verifies the resource's name during creation. If the resource already exists, it throws an error.
  Example:

    kubectl create -f deployment.yaml
  

  • If deployment.yaml defines a resource already present in the cluster, you'll get an error like:

      Error from server (AlreadyExists): deployments.apps "example-deployment" already exists
    

• kubectl apply: Unlike create, the apply command allows you to modify resources declaratively. It compares the Live Object Configuration (LOC), stored in etcd, with the Last Applied Configuration (LAC).

  • When you use apply, Kubernetes updates the resource and retains a history of the last applied configuration for reference.
    Example:

      kubectl apply -f deployment.yaml
    

How Kubernetes Works with YAML Files

1. You define resources in a YAML file.

2. When you create or apply the YAML, Kubernetes converts it into a Live Object Configuration (LOC) stored in etcd.

3. For kubectl create, there is no Last Applied Configuration (LAC) stored, as the command doesn't track changes.

4. For kubectl apply, both LOC and LAC are maintained, enabling smooth updates.

---

Viewing Pod Details

To understand the state of a pod:

• View complete YAML configuration:

    kubectl get po qr-app -o yaml
  

  This displays the YAML file representing the pod qr-app.

• View labels of pods:

    kubectl get pods --show-labels
  

  Labels provide metadata about applications, helping identify and organize resources.

• Filter pods by labels:

    kubectl get po -l "app=dev"
  

  This retrieves all pods where the label app=dev is present.

• Delete pods by labels:

    kubectl delete po -l "app=dev"
  

  This deletes all pods with the label app=dev.

  

---

Kubernetes Labels: A Best Practice

Labels are key-value pairs attached to Kubernetes objects, providing meaningful metadata. Kubernetes recommends adding at least the following 5 labels for clarity:

1. name: Unique name for the application, e.g., abc.com/name=dotnet

2. managed-by: Who or what manages this resource, e.g., abc.com/managed-by=hassan

3. version: Application version, e.g., v1.0

4. component: Application component, e.g., abc.com/component=frontend

5. part-of: The larger application this component belongs to, e.g., abc.com/part-of=projectX

---

Label Selectors

Label selectors allow you to filter Kubernetes objects based on specific criteria. Examples:

• Basic selector: Find pods labeled with app=nginx:

    kubectl get po -l "app=nginx" --show-labels
  

• Exclusion selector: Exclude pods labeled with client=abc:

    kubectl get po -l "app=nginx,client!=abc" --show-labels
  

For more details on labels and selectors, visit the Kubernetes Documentation on Labels and Selectors.

---

Modifying Labels

Labels can be modified imperatively or declaratively:

• Imperative approach:

    kubectl label pod nginx managed-by=hassan
  

  • Note: This method updates the labels immediately but doesn't affect the Last Applied Configuration (LAC).

• Declarative approach: Update the YAML file and reapply it using kubectl apply. This method also updates the LAC, ensuring consistency.

---

Conclusion

Understanding commands like kubectl create and kubectl apply is essential for resource management in Kubernetes. Labels and selectors enhance your ability to organize and manage resources effectively. Adopting best practices such as recommended labels and leveraging LAC for tracking changes will improve your workflow.

Ready to explore more? Check out the official Kubernetes documentation for in-depth guidance.