Lesson 13: Kubernetes Operators and CRDs

In this lesson, we will explore Kubernetes Operators and Custom Resource Definitions (CRDs) to extend the functionality of Kubernetes. Operators are a method of packaging, deploying, and managing a Kubernetes application. CRDs allow you to define your own custom resources, which can be managed just like built-in Kubernetes resources.

What are Custom Resource Definitions (CRDs)?

Custom Resource Definitions (CRDs) enable you to extend Kubernetes capabilities by defining your own resource types. A CRD allows you to create, update, and delete custom resources in your Kubernetes cluster.

Example of a CRD

Let's create a simple CRD for a resource called Database. Here’s how you can define it:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: databases.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
  scope: Namespaced
  names:
    plural: databases
    singular: database
    kind: Database

To apply this CRD, save it to a file named database-crd.yaml and run:

kubectl apply -f database-crd.yaml

What are Kubernetes Operators?

Kubernetes Operators are a way to automate the management of complex applications on Kubernetes. They use CRDs to manage the state of applications and can handle tasks like deployment, scaling, and backups.

Creating an Operator

To create an Operator, you typically use a framework like Operator SDK. Below is a basic outline of how to create an Operator:

1. Install Operator SDK: Follow the Operator SDK installation guide to get started.
2. Create a new Operator: bash operator-sdk init --domain example.com --repo github.com/example/database-operator
3. Create an API: bash operator-sdk create api --group db --version v1 --kind Database
4. Implement the Controller: Modify the generated controller to manage your Database resources.

Example Operator Controller Code

Here’s a simplified example of what your controller logic might look like:

package controllers

import (
    "context"
    "sigs.k8s.io/controller-runtime/pkg/controller"
    "sigs.k8s.io/controller-runtime/pkg/controller/controllerutil"
    "sigs.k8s.io/controller-runtime/pkg/reconcile"
)

// DatabaseReconciler reconciles a Database object
type DatabaseReconciler struct {
    // add your own fields here
}

// Reconcile reads that state of the cluster for a Database object and makes changes based on the state
func (r *DatabaseReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    // Logic to manage your Database resource
}

Best Practices and Common Mistakes

Best Practice: Always version your CRDs. This helps manage changes and ensures backward compatibility.

Common Mistake: Forgetting to set the proper permissions for your Operator. Ensure your Operator has the necessary RBAC permissions to manage its resources.

Summary Table

By utilizing CRDs and Operators, you can effectively manage complex applications on Kubernetes, allowing for automation and enhanced functionality.

Exercise 1: Create a Custom Resource Definition

1. Define a CRD for a Cache resource similar to the Database CRD example above.
2. Apply your CRD using kubectl.

Exercise 2: Build a Simple Operator

1. Use the Operator SDK to create a new Operator for managing Cache resources.
2. Implement basic reconciliation logic to log when a Cache resource is created.

Exercise 3: Test Your Operator

1. Deploy your Operator in the cluster.
2. Create a Cache resource and observe the logs to ensure your Operator is functioning correctly.

• Custom Resource Definitions (CRDs) allow you to extend Kubernetes with new resource types.
• Kubernetes Operators automate the management of complex applications using CRDs.
• Always version your CRDs for better management.
• Ensure proper RBAC permissions for your Operators to function correctly.
• Operators use controllers to manage the state of custom resources.