Building Custom Kubernetes Controllers

Kubernetes operators are a powerful tool for extending the functionality of Kubernetes clusters. They are built on top of custom controllers and Custom Resource Definitions (CRDs), allowing for the automation of complex tasks specific to certain applications or platforms. This blog will delve into the technical aspects of building custom Kubernetes controllers, which are the core components of Kubernetes operators.

Understanding Kubernetes Controllers

Kubernetes controllers are components that observe the state of resources within a Kubernetes cluster and take actions to maintain the desired state. They are essential for managing the lifecycle of applications and ensuring that the cluster remains in a consistent state. Controllers can be generic, managing common resources like Deployments and Services, or they can be custom, managing specific resources defined by CRDs.

Basic Components of a Controller

A Kubernetes controller typically consists of the following components:

1. Informer: This is responsible for watching the Kubernetes API for changes to specific resources. Informers provide a way to listen to events such as creation, update, and deletion of resources.

2. Work Queue: This is a queue that holds the objects that need to be processed. When an event is received from the Informer, the object is added to the work queue.

3. Processor: This is the main logic of the controller that processes the objects in the work queue. It retrieves the object from the queue, performs the necessary actions, and then updates the object in the Kubernetes API.

4. Reconciler: This is the function that performs the actual reconciliation logic. It compares the current state of the object with the desired state and makes the necessary changes.

Building a Custom Controller

To build a custom controller, you need to follow these steps:

Step 1: Set Up the Development Environment

First, you need to set up your development environment. This includes installing the necessary tools such as operator-sdk and kubectl.

# Install operator-sdk
curl -sL https://github.com/operator-framework/operator-sdk/releases/download/v1.11.0/operator-sdk-v1.11.0-x86_64-linux-gnu.tar.gz | tar xvf - -C /usr/local/bin/

# Verify the installation
operator-sdk version

Step 2: Create the Operator Project

Next, create a new operator project using the operator-sdk tool.

# Create a new operator project
mkdir -p $GOPATH/src/operators && cd $GOPATH/src/operators
operator-sdk init --domain=example.com --repo=github.com/example/operator

Step 3: Define the Custom Resource

Define the custom resource that your controller will manage. This involves creating a CRD and the corresponding Go structs.

# Create a new API and controller
operator-sdk create api --version=v1alpha1 --kind=Traveller

This command will create the necessary files for the CRD and the controller.

Step 4: Implement the Controller Logic

Implement the controller logic in the controller.go file. This involves defining the reconciler function that performs the actual reconciliation logic.

package controllers

import (
    "context"
    "fmt"
    "log"

    "k8s.io/apimachinery/pkg/runtime"
    ctrl "sigs.k8s.io/controller-runtime"
    "sigs.k8s.io/controller-runtime/pkg/client"
    "sigs.k8s.io/controller-runtime/pkg/controller/controllerutil"

    "github.com/example/operator/api/v1alpha1"
)

func (r *TravellerReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    log.Println("Reconciling Traveller")

    // Fetch the Traveller instance
    traveller := &v1alpha1.Traveller{}
    err := r.Get(ctx, req.NamespacedName, traveller)
    if err != nil {
        log.Println(err)
        return ctrl.Result{}, err
    }

    // Perform the reconciliation logic
    if traveller.Status.Phase == "" {
        traveller.Status.Phase = "Running"
        err = r.Status().Update(ctx, traveller)
        if err != nil {
            log.Println(err)
            return ctrl.Result{}, err
        }
    }

    return ctrl.Result{}, nil
}

Step 5: Run the Controller

Finally, run the controller using the operator-sdk tool.

# Run the controller
operator-sdk run --local

This will start the controller in your local environment.

Testing the Controller

To test the controller, you can create a sample Traveller resource and observe how the controller reconciles it.

apiVersion: example.com/v1alpha1
kind: Traveller
metadata:
  name: example-traveller
spec:
  phase: Running

Apply this YAML file to your Kubernetes cluster:

kubectl apply -f example-traveller.yaml

Observe the logs of the controller to see the reconciliation process:

kubectl logs -f <controller-pod-name>

Conclusion

Building custom Kubernetes controllers is a powerful way to extend the functionality of Kubernetes clusters. By following the steps outlined in this blog, you can create a custom controller that manages specific resources defined by CRDs. This approach is particularly useful in Platform Engineering, where custom automation logic is required to manage complex applications.