main.go

package main

import (
	"io/ioutil"
	"log"
	"os"
	"strings"

	apps_v1 "k8s.io/api/apps/v1"
	"k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/cache"
	"k8s.io/client-go/tools/clientcmd"
)

func main() {
	kubeconfig := os.Getenv("KUBECONFIG")
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		panic(err.Error())
	}

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

	factory := informers.NewSharedInformerFactoryWithOptions(clientset, 0, informers.WithNamespace(namespace()))
	informer := factory.Apps().V1().Deployments().Informer()
	stopper := make(chan struct{})
	defer close(stopper)

	informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		UpdateFunc: func(old interface{}, new interface{}) {
			var deployment = new.(*apps_v1.Deployment).DeepCopy()
			var oldDeployment = old.(*apps_v1.Deployment).DeepCopy()

			var nrOfReplicas = *deployment.Spec.Replicas
			var oldNrOfReplicas = *oldDeployment.Spec.Replicas

			if nrOfReplicas != oldNrOfReplicas {
				log.Printf("Deployment: %s, Replicas: %d", deployment.GetName(), nrOfReplicas)
				// do something here
			}
		},
	})

	informer.Run(stopper)
}

// get current namespace
func namespace() string {
	// This way assumes you've set the POD_NAMESPACE environment variable using the downward API.
	// This check has to be done first for backwards compatibility with the way InClusterConfig was originally set up
	if ns, ok := os.LookupEnv("POD_NAMESPACE"); ok {
		return ns
	}

	// Fall back to the namespace associated with the service account token, if available
	if data, err := ioutil.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/namespace"); err == nil {
		if ns := strings.TrimSpace(string(data)); len(ns) > 0 {
			return ns
		}
	}

	return "default"
}