Copy informer example from @wfarr 's kubernetes/client-go#30 and addr…

…ess remaining code review comments.

staging/src/k8s.io/client-go/examples/informer/README.md

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+# Informer Example
+
+Informers provide a high-level API for creating custom controllers for Kubernetes resources.
+
+This particular example demonstrates:
+
+* How to write an Informer against a core resource type and handle to create/update/delete events
+
+## Running
+
+``` go
+# assumes you have a working kubeconfig, not required if operating in-cluster
+go run *.go -kubeconfig=$HOME/.kube/config
+```
+
+## Use Cases
+
+* Capturing resource events for logging to external systems (e.g. monitor non-"Normal" events and publish metrics to a time series database)
+* Creating lifecycle controllers for ThirdPartyResources (eg. coordinate create/update/delete of an external datastore represented via a ThirdPartyResource type)

staging/src/k8s.io/client-go/examples/informer/main.go

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+/*
+Copyright 2017 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.
+*/
+
+// Note: the example only works with the code within the same release/branch.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"k8s.io/apimachinery/pkg/fields"
+	"k8s.io/client-go/kubernetes"
+	"k8s.io/client-go/pkg/api"
+	"k8s.io/client-go/pkg/api/v1"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/tools/cache"
+	"k8s.io/client-go/tools/clientcmd"
+
+	// Only required to authenticate against GKE clusters
+	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp"
+)
+
+func main() {
+	kubeconfig := flag.String("kubeconfig", "", "Path to a kube config. Only required if out-of-cluster.")
+	flag.Parse()
+
+	// Create the client config. Use kubeconfig if given, otherwise assume in-cluster.
+	config, err := buildConfig(*kubeconfig)
+	if err != nil {
+		panic(err)
+	}
+
+	clientset, err := kubernetes.NewForConfig(config)
+	if err != nil {
+		panic(err)
+	}
+
+	stop := make(chan struct{}, 1)
+	source := cache.NewListWatchFromClient(
+		clientset.Core().RESTClient(),
+		"pods",
+		api.NamespaceAll,
+		fields.Everything())
+
+	store, controller := cache.NewInformer(
+		source,
+
+		// The object type.
+		&v1.Pod{},
+
+		// resyncPeriod
+		// Every resyncPeriod, all resources in the cache will retrigger events.
+		// Set to 0 to disable the resync.
+		time.Second*0,
+
+		// Your custom resource event handlers.
+		cache.ResourceEventHandlerFuncs{
+			// Takes a single argument of type interface{}.
+			// Called on controller startup and when new resources are created.
+			AddFunc: create,
+
+			// Takes two arguments of type interface{}.
+			// Called on resource update and every resyncPeriod on existing resources.
+			UpdateFunc: update,
+
+			// Takes a single argument of type interface{}.
+			// Called on resource deletion.
+			DeleteFunc: delete,
+		})
+
+	// the controller run starts the event processing loop
+	go controller.Run(stop)
+
+	// wait until a store finished its initial synchronization
+	cache.WaitForCacheSync(stop, controller.HasSynced)
+
+	// store can be used to List and Get
+	// NEVER modify objects from the store. It's a read-only, local cache.
+	fmt.Println("listing pods from store:")
+	for _, obj := range store.List() {
+		pod := obj.(*v1.Pod)
+
+		// This will likely be empty the first run, but may not
+		fmt.Printf("%s\n", pod.ObjectMeta.Name)
+	}
+
+	// and now we block on a signal
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
+	s := <-signals
+	fmt.Printf("received signal %#v, exiting...\n", s)
+	close(stop)
+	os.Exit(0)
+}
+
+// Handler functions as per the controller above.
+// Note the coercion of the interface{} into a pointer of the expected type.
+
+func create(obj interface{}) {
+	pod := obj.(*v1.Pod)
+
+	fmt.Println("POD CREATED:", podWithNamespace(pod))
+}
+
+func update(old, new interface{}) {
+	oldPod := old.(*v1.Pod)
+	newPod := new.(*v1.Pod)
+
+	fmt.Printf("POD UPDATED:\n  old: %s\n  new: %s\n", podWithNamespace(oldPod), podWithNamespace(newPod))
+}
+
+func delete(obj interface{}) {
+	pod := obj.(*v1.Pod)
+
+	fmt.Println("POD DELETED:", podWithNamespace(pod))
+}
+
+// convenience functions
+
+func podWithNamespace(pod *v1.Pod) string {
+	return fmt.Sprintf("%s/%s", pod.Namespace, pod.Name)
+}
+
+func buildConfig(kubeconfig string) (*rest.Config, error) {
+	if kubeconfig != "" {
+		return clientcmd.BuildConfigFromFlags("", kubeconfig)
+	}
+	return rest.InClusterConfig()
+}