Kubernetes Operators

While Custom Resource Definitions (CRDs) extend the Kubernetes API with new resource types, Operators add application-specific logic to manage these resources. Operators are the next level of Kubernetes extensibility, allowing you to automate complex, stateful applications by encoding domain knowledge into Kubernetes-native controllers.

What are Kubernetes Operators?

An Operator is a method of packaging, deploying, and managing a Kubernetes application. It extends Kubernetes by adding application-specific operational knowledge.

The Operator Pattern

The operator pattern consists of:

1. Custom Resource Definition (CRD): Defines the schema for your custom resource
2. Custom Controller: Watches for changes to custom resources and takes action to reconcile the actual state with the desired state
3. Domain-Specific Knowledge: Encodes operational knowledge about a specific application or service

At its core, an operator implements the reconciliation loop pattern used throughout Kubernetes, continuously working to ensure the actual state of the system matches the desired state defined in your custom resources.

Why Use Operators?

Operators provide several key benefits:

Examples of Common Operators

Many popular applications have operators available:

How Operators Work

Operators implement a controller that watches for changes to custom resources and takes action to reconcile the actual state with the desired state.

The Reconciliation Loop

The heart of an operator is its reconciliation loop:

1. Watch: Monitor for changes to custom resources
2. Compare: Compare the current state with the desired state
3. Act: Take action to align the current state with the desired state
4. Update: Update the status of the custom resource to reflect the current state
5. Repeat: Continue watching for changes

Example: Simple Operator Implementation

Here’s a high-level view of what an operator might do for our Website example:

// Simplified pseudo-code for a Website operator
func (c *Controller) reconcileWebsite(website Website) error {
    // Check if deployment exists
    deployment, err := getDeployment(website.Name)
    if err != nil {
        // Create a new deployment
        deployment = createDeploymentSpec(website)
        return createDeployment(deployment)
    }

    // Update deployment if specs have changed
    if deploymentNeedsUpdate(deployment, website) {
        updateDeployment(deployment, website)
    }

    // Check if service exists
    service, err := getService(website.Name)
    if err != nil {
        // Create a new service
        service = createServiceSpec(website)
        return createService(service)
    }

    // Update service if needed
    if serviceNeedsUpdate(service, website) {
        updateService(service, website)
    }

    // Handle ingress for the domain
    ingress, err := getIngress(website.Name)
    if err != nil {
        // Create a new ingress
        ingress = createIngressSpec(website)
        return createIngress(ingress)
    }

    return nil
}

This operator would watch for Website resources and ensure the corresponding Deployments, Services, and Ingresses are created and maintained.

Building Operators

Several frameworks and toolkits are available for building Kubernetes operators, each with different approaches and strengths:

Operator Development Toolkits

Language Choices

The choice of programming language affects how you’ll implement your operator:

• Go: The most common choice, with the strongest ecosystem support and direct access to Kubernetes client libraries
• Python: Easier learning curve but may have performance limitations for complex operators
• Ansible: Good for operators that primarily perform configuration management tasks
• Helm: Suitable for operators that primarily deploy applications defined in Helm charts

Operator Maturity Model

Operators can range from simple to highly sophisticated. The Operator Capability Levels define this progression:

Operator Lifecycle Manager (OLM)

The Operator Lifecycle Manager (OLM) provides a framework for installing, managing, and upgrading operators in a Kubernetes cluster. It simplifies operator management by handling dependencies and updates through a catalog-based system, similar to a package manager for operators.

OLM Components and Resources

OLM organizes operators into catalogs, allowing administrators to control which operators are available to users while ensuring proper versioning and dependency resolution.

Operators in AKS

Azure Kubernetes Service fully supports operators and provides several benefits when using them:

AKS-Specific Operator Considerations

When deploying operators in AKS, consider these important factors:

Azure-Specific Operators

Several operators are designed specifically for Azure:

Example: Using Azure Service Operator

The Azure Service Operator lets you provision Azure resources directly from Kubernetes:

apiVersion: azure.microsoft.com/v1alpha1
kind: PostgreSQLServer
metadata:
  name: my-db-server
spec:
  location: westus2
  resourceGroup: my-resource-group
  serverVersion: "11"
  sku:
    name: GP_Gen5_2
    tier: GeneralPurpose
    family: Gen5
    size: "2"
  administratorLogin: myusername
  administratorLoginPassword:
    name: my-secret
    key: password

Best Practices for Operators

Designing, implementing, and operating Kubernetes operators requires careful consideration of several factors to ensure reliability, maintainability, and security.

Design Best Practices

When designing operators, follow these principles to create robust, maintainable solutions:

Implementation and Operational Considerations

The implementation and operational aspects of operators are equally important:

Common Operator Use Cases in Azure

Further Reading and Resources

• Kubernetes Operators Documentation
• Operator SDK Documentation