A Basic Controller Sample Code

a-basic-controller-sample-code.go

package main

import (
	"flag"
	"fmt"
	"path/filepath"
	"time"

	"k8s.io/klog/v2"
    _ "k8s.io/client-go/plugin/pkg/client/auth"
	v1 "k8s.io/api/core/v1"
	"k8s.io/apimachinery/pkg/util/runtime"
	"k8s.io/apimachinery/pkg/util/wait"
	"k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/cache"
	"k8s.io/client-go/tools/clientcmd"
	"k8s.io/client-go/util/homedir"
	"k8s.io/client-go/util/workqueue"
)

// Controller demonstrates how to implement a controller with client-go.
type Controller struct {
	clientset   kubernetes.Interface
	podInformer cache.SharedIndexInformer
	queue       workqueue.RateLimitingInterface
}

// NewController creates a new Controller.
func NewController(clientset kubernetes.Interface) *Controller {
	informerFactory := informers.NewSharedInformerFactory(clientset, 0)
	podInformer := informerFactory.Core().V1().Pods().Informer()

	queue := workqueue.NewRateLimitingQueue(workqueue.DefaultControllerRateLimiter())

	// register event handlers to fill the queue with pod creations, updates and deletions
	podInformer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc: func(obj interface{}) {
			key, err := cache.MetaNamespaceKeyFunc(obj)
			if err == nil {
				queue.Add(key)
			}
		},
		UpdateFunc: func(old interface{}, new interface{}) {
			key, err := cache.MetaNamespaceKeyFunc(new)
			if err == nil {
				queue.Add(key)
			}
		},
		DeleteFunc: func(obj interface{}) {
			key, err := cache.DeletionHandlingMetaNamespaceKeyFunc(obj)
			if err == nil {
				queue.Add(key)
			}
		},
	})

	return &Controller{
		clientset:   clientset,
		podInformer: podInformer,
		queue:       queue,
	}
}

func (c *Controller) runWorker() {
	// hot loop until we're told to stop.  processNextWorkItem will
	// automatically wait until there's work available, so we don't worry
	// about secondary waits
	for c.processNextWorkItem() {
	}
}

// processNextWorkItem deals with one key off the queue.  It returns false
// when it's time to quit.
func (c *Controller) processNextWorkItem() bool {
	// Wait until there is a new item in the working queue
	key, quit := c.queue.Get()
	if quit {
		return false
	}
	// Tell the queue that we are done with processing this key. This unblocks the key for other workers
	// This allows safe parallel processing because two pods with the same key are never processed in
	// parallel.
	defer c.queue.Done(key)

	// Invoke the method containing the business logic
	err := c.syncToStdout(key.(string))
	// Handle the error if something went wrong during the execution of the business logic
	c.handleErr(err, key)
	return true
}

// syncToStdout is the business logic of the controller. In this controller it simply prints
// information about the pod to stdout. In case an error happened, it has to simply return the error.
// The retry logic should not be part of the business logic.
func (c *Controller) syncToStdout(key string) error {
	obj, exists, err := c.podInformer.GetIndexer().GetByKey(key)
	if err != nil {
		klog.Errorf("Fetching object with key %s from store failed with %v", key, err)
		return err
	}

	if !exists {
		// Below we will warm up our cache with a Pod, so that we will see a delete for one pod
		fmt.Printf("Pod %s does not exist anymore\n", key)
	} else {
		// Note that you also have to check the uid if you have a local controlled resource, which
		// is dependent on the actual instance, to detect that a Pod was recreated with the same name
		fmt.Printf("Sync/Add/Update for Pod %s\n", obj.(*v1.Pod).GetName())
	}
	return nil
}

// handleErr checks if an error happened and makes sure we will retry later.
func (c *Controller) handleErr(err error, key interface{}) {
	if err == nil {
		// Forget about the #AddRateLimited history of the key on every successful synchronization.
		// This ensures that future processing of updates for this key is not delayed because of
		// an outdated error history.
		c.queue.Forget(key)
		return
	}

	// This controller retries 3 times if something goes wrong. After that, it stops trying.
	if c.queue.NumRequeues(key) < 3 {
		klog.Infof("Error syncing pod %v: %v", key, err)

		// Re-enqueue the key rate limited. Based on the rate limiter on the
		// queue and the re-enqueue history, the key will be processed later again.
		c.queue.AddRateLimited(key)
		return
	}

	c.queue.Forget(key)
	// Report to an external entity that, even after several retries, we could not successfully process this key
	runtime.HandleError(err)
	klog.Infof("Dropping pod %q out of the queue: %v", key, err)
}

// Run begins watching and syncing.
func (c *Controller) Run(workers int, stopCh chan struct{}) {
	defer runtime.HandleCrash()

	// Let the workers stop when we are done
	defer c.queue.ShutDown()
	klog.Info("Starting Pod controller")

	go c.podInformer.Run(stopCh)

	// Wait for all involved caches to be synced, before processing items from the queue is started
	if !cache.WaitForCacheSync(stopCh, c.podInformer.HasSynced) {
		runtime.HandleError(fmt.Errorf("Timed out waiting for caches to sync"))
		return
	}

	// start up your workers.  Some controllers have multiple kinds of workers
	for i := 0; i < workers; i++ {
		// runWorker will loop until "something bad" happens.  The .Until will
		// then rekick the worker after one second
		go wait.Until(c.runWorker, time.Second, stopCh)
	}

	// wait until we're told to stop
	<-stopCh
	klog.Info("Stopping Pod controller")
}

func main() {
	var kubeconfig *string
	if home := homedir.HomeDir(); home != "" {
		kubeconfig = flag.String("kubeconfig", filepath.Join(home, ".kube", "config"), "(optional) absolute path to the kubeconfig file")
	} else {
		kubeconfig = flag.String("kubeconfig", "", "absolute path to the kubeconfig file")
	}
	flag.Parse()

	// Use the current context in kubeconfig
	config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
	if err != nil {
		panic(err)
	}

	// create the clientset
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		panic(err)
	}

	controller := NewController(clientset)

	// Now let's start the controller
	stop := make(chan struct{})
	defer close(stop)
	go controller.Run(1, stop)

	// Wait forever
	select {}
}