use consul's watch

Signed-off-by: JChien <jeffchien13@gmail.com>

discovery/consul/consul.go

@@ -6,14 +6,16 @@ import (
 	"strings"
 	"time"
 
+	log "github.com/Sirupsen/logrus"
 	consul "github.com/armon/consul-api"
 	"github.com/docker/swarm/discovery"
 )
 
 type ConsulDiscoveryService struct {
-	heartbeat uint64
+	heartbeat time.Duration
 	client    *consul.Client
 	prefix    string
+	lastIndex uint64
 }
 
 func init() {
@@ -36,14 +38,18 @@ func (s *ConsulDiscoveryService) Initialize(uris string, heartbeat int) error {
 		return err
 	}
 	s.client = client
-	s.heartbeat = uint64(heartbeat)
+	s.heartbeat = time.Duration(heartbeat) * time.Second
 	s.prefix = path + "/"
 	kv := s.client.KV()
 	p := &consul.KVPair{Key: s.prefix, Value: nil}
-	_, err = kv.Put(p, nil)
+	if _, err = kv.Put(p, nil); err != nil {
+		return err
+	}
+	_, meta, err := kv.Get(s.prefix, nil)
 	if err != nil {
 		return err
 	}
+	s.lastIndex = meta.LastIndex
 	return nil
 }
 func (s *ConsulDiscoveryService) Fetch() ([]*discovery.Node, error) {
@@ -65,9 +71,7 @@ func (s *ConsulDiscoveryService) Fetch() ([]*discovery.Node, error) {
 }
 
 func (s *ConsulDiscoveryService) Watch(callback discovery.WatchCallback) {
-	c := time.Tick(time.Duration(s.heartbeat) * time.Second)
-	for {
-		<-c
+	for _ = range s.waitForChange() {
 		nodes, err := s.Fetch()
 		if err == nil {
 			callback(nodes)
@@ -79,8 +83,26 @@ func (s *ConsulDiscoveryService) Register(addr string) error {
 	kv := s.client.KV()
 	p := &consul.KVPair{Key: path.Join(s.prefix, addr), Value: []byte(addr)}
 	_, err := kv.Put(p, nil)
-	if err != nil {
-		return err
-	}
-	return nil
+	return err
+}
+
+func (s *ConsulDiscoveryService) waitForChange() <-chan uint64 {
+	c := make(chan uint64)
+	go func() {
+		for {
+			kv := s.client.KV()
+			option := &consul.QueryOptions{
+				WaitIndex: s.lastIndex,
+				WaitTime:  s.heartbeat}
+			_, meta, err := kv.List(s.prefix, option)
+			if err != nil {
+				log.Errorln(err)
+				break
+			}
+			s.lastIndex = meta.LastIndex
+			c <- s.lastIndex
+		}
+		close(c)
+	}()
+	return c
 }