watch-single-object.go

/*
Copyright 2015 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package main

import (
	"flag"
	"fmt"
	"os"
	"os/signal"
	"syscall"
	"time"

	"github.com/google/go-cmp/cmp"
	"github.com/google/go-cmp/cmp/cmpopts"
	corev1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/fields"
	"k8s.io/apimachinery/pkg/util/wait"
	discovery "k8s.io/apimachinery/pkg/version"
	kubeinformers "k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	cache "k8s.io/client-go/tools/cache"
	"k8s.io/client-go/tools/clientcmd"
	"k8s.io/ingress-nginx/internal/k8s"
	"k8s.io/klog"
)

func main() {
	klog.InitFlags(nil)
	flag.Set("logtostderr", "true")
	flag.CommandLine.Parse([]string{})

	kubeClient, err := createApiserverClient("http://127.0.0.1:8001", "/home/aledbf/.kube/config")
	if err != nil {
		handleFatalInitError(err)
	}

	// stop channel
	stopCh := make(chan struct{})

	// event channel (temporal)
	eventCh := make(chan string)

	// configmap to watch
	key := "default/test"

	klog.Infof("Watching configmap %v", key)

	w, err := watchConfigmap(key, eventCh, stopCh, kubeClient)
	if err != nil {
		handleFatalInitError(err)
	}

	go func(w *watcher, eventCh chan string) {
		for {
			select {
			case reason := <-eventCh:
				// for now just show a string with event and the configmap
				klog.Infof("[K8S data change] - reason: %v", reason)
				configmap, err := w.GetConfigMap()
				if err != nil {
					klog.Error(err)
					continue
				}
				klog.Infof("[K8S object] %v", configmap)
			case <-stopCh:
				break
			}
		}
	}(w, eventCh)

	signalChan := make(chan os.Signal, 1)
	signal.Notify(signalChan, syscall.SIGTERM)
	<-signalChan
	klog.Info("Received SIGTERM, shutting down")
	time.Sleep(10 * time.Second)
}

type ConfigmapWatcher interface {
	GetConfigMap() (*corev1.ConfigMap, error)
}

func NewConfigmapWatcher(key string, eventCh chan string, stopCh chan struct{}, kubeClient kubernetes.Interface) (ConfigmapWatcher, error) {
	w, err := watchConfigmap(key, eventCh, stopCh, kubeClient)
	if err != nil {
		return nil, err
	}

	return w, nil
}

type watcher struct {
	object *corev1.ConfigMap
}

func (w *watcher) GetConfigMap() (*corev1.ConfigMap, error) {
	return w.object, nil
}

func watchConfigmap(key string, eventCh chan string, stopCh chan struct{}, kubeClient kubernetes.Interface) (*watcher, error) {

	ns, name, err := k8s.ParseNameNS(key)
	if err != nil {
		return nil, err
	}

	kubeInformerFactory := kubeinformers.NewFilteredSharedInformerFactory(kubeClient, 0, ns,
		func(options *metav1.ListOptions) {
			options.FieldSelector = fields.OneTermEqualSelector("metadata.name", name).String()
		},
	)

	w := &watcher{}

	informer := kubeInformerFactory.Core().V1().ConfigMaps().Informer()

	var remove func(obj interface{})
	remove = func(obj interface{}) {
		switch obj := obj.(type) {
		case cache.DeletedFinalStateUnknown:
			remove(obj.Obj)
			break
		default:
			w.object = nil
		}

		eventCh <- "configmap removed"
	}

	informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc: func(obj interface{}) {
			w.object = obj.(*corev1.ConfigMap)
			eventCh <- "adding configmap"
		},
		UpdateFunc: func(old, cur interface{}) {
			if cmp.Equal(old, cur,
				cmpopts.IgnoreFields(metav1.ObjectMeta{}, "ResourceVersion")) {
				return
			}

			w.object = cur.(*corev1.ConfigMap)
			eventCh <- fmt.Sprintf("updating configmap to %v", w.object.Data)
		},
		DeleteFunc: remove,
	})

	// start the informer in a goroutine
	go kubeInformerFactory.Start(stopCh)

	return w, nil
}

// ---------------------------- boilerplate ------------------------------------------------------

// createApiserverClient creates a new Kubernetes REST client. apiserverHost is
// the URL of the API server in the format protocol://address:port/pathPrefix,
// kubeConfig is the location of a kubeconfig file. If defined, the kubeconfig
// file is loaded first, the URL of the API server read from the file is then
// optionally overridden by the value of apiserverHost.
// If neither apiserverHost nor kubeConfig is passed in, we assume the
// controller runs inside Kubernetes and fallback to the in-cluster config. If
// the in-cluster config is missing or fails, we fallback to the default config.
func createApiserverClient(apiserverHost, kubeConfig string) (*kubernetes.Clientset, error) {
	cfg, err := clientcmd.BuildConfigFromFlags(apiserverHost, kubeConfig)
	if err != nil {
		return nil, err
	}

	klog.Infof("Creating API client for %s", cfg.Host)

	client, err := kubernetes.NewForConfig(cfg)
	if err != nil {
		return nil, err
	}

	var v *discovery.Info

	// The client may fail to connect to the API server in the first request.
	// https://github.com/kubernetes/ingress-nginx/issues/1968
	defaultRetry := wait.Backoff{
		Steps:    10,
		Duration: 1 * time.Second,
		Factor:   1.5,
		Jitter:   0.1,
	}

	var lastErr error
	retries := 0
	klog.V(2).Info("Trying to discover Kubernetes version")
	err = wait.ExponentialBackoff(defaultRetry, func() (bool, error) {
		v, err = client.Discovery().ServerVersion()

		if err == nil {
			return true, nil
		}

		lastErr = err
		klog.V(2).Infof("Unexpected error discovering Kubernetes version (attempt %v): %v", retries, err)
		retries++
		return false, nil
	})

	// err is returned in case of timeout in the exponential backoff (ErrWaitTimeout)
	if err != nil {
		return nil, lastErr
	}

	// this should not happen, warn the user
	if retries > 0 {
		klog.Warningf("Initial connection to the Kubernetes API server was retried %d times.", retries)
	}

	klog.Infof("Running in Kubernetes cluster version v%v.%v (%v) - git (%v) commit %v - platform %v",
		v.Major, v.Minor, v.GitVersion, v.GitTreeState, v.GitCommit, v.Platform)

	return client, nil
}

// Handler for fatal init errors. Prints a verbose error message and exits.
func handleFatalInitError(err error) {
	klog.Fatalf("Error while initiating a connection to the Kubernetes API server. "+
		"This could mean the cluster is misconfigured (e.g. it has invalid API server certificates "+
		"or Service Accounts configuration). Reason: %s\n"+
		"Refer to the troubleshooting guide for more information: "+
		"https://kubernetes.github.io/ingress-nginx/troubleshooting/",
		err)
}

0. kubectl proxy
1. run the watcher go run watch-single-object.go
2. create the configmap and watch the change

echo "
kind: ConfigMap
apiVersion: v1
metadata:
  name: test
data:
  X: '1'
" | kubectl apply -f -

3. change the configmap values

kubectl patch configmap/test --type merge -p '{"data":{"X":"something else"}}'

4. if the same patch is applied multiple times, we should not see any update in the console