watch pods instead of polling

discovery.go

@@ -3,12 +3,17 @@ package main
 import (
 	"context"
 
+	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/watch"
 	"k8s.io/client-go/kubernetes"
+	"k8s.io/client-go/tools/cache"
+	toolsWatch "k8s.io/client-go/tools/watch"
 )
 
 type Discovery interface {
 	Discover() ([]string, error)
+	Watch() (<-chan string, error)
 }
 
 type K8sDiscovery struct {
@@ -33,6 +38,32 @@ func (k *K8sDiscovery) DiscoverInNamespace(namespace string) ([]string, error) {
 	return hosts, nil
 }
 
+func (k *K8sDiscovery) Watch() (<-chan string, error) {
+	timeout := int64(30)
+	watcherFn := func(options metav1.ListOptions) (watch.Interface, error) {
+		return k.client.CoreV1().Pods("").Watch(k.ctx, metav1.ListOptions{
+			LabelSelector:  "actor-pool=cart",
+			TimeoutSeconds: &timeout,
+		})
+	}
+	watcher, err := toolsWatch.NewRetryWatcher("1", &cache.ListWatch{WatchFunc: watcherFn})
+	if err != nil {
+		return nil, err
+	}
+	ch := make(chan string)
+	go func() {
+		for event := range watcher.ResultChan() {
+			if event.Type != watch.Added {
+				continue
+			}
+			pod := event.Object.(*v1.Pod)
+
+			ch <- pod.Status.PodIP
+		}
+	}()
+	return ch, nil
+}
+
 func NewK8sDiscovery(client *kubernetes.Clientset) *K8sDiscovery {
 	return &K8sDiscovery{
 		ctx:    context.Background(),