diff --git a/discovery/README.md b/discovery/README.md
index 39e46774ab..7215c57487 100644
--- a/discovery/README.md
+++ b/discovery/README.md
@@ -87,7 +87,7 @@ simply implements this interface:
 type DiscoveryService interface {
      Initialize(string, int) error
      Fetch() ([]string, error)
-     Watch(func([]*Node))
+     Watch(WatchCallback)
      Register(string) error
 }
 ```
diff --git a/discovery/discovery.go b/discovery/discovery.go
index 34c81e833f..84d339a3a3 100644
--- a/discovery/discovery.go
+++ b/discovery/discovery.go
@@ -24,10 +24,12 @@ func (n Node) String() string {
 	return n.url
 }
 
+type WatchCallback func(nodes []*Node)
+
 type DiscoveryService interface {
 	Initialize(string, int) error
 	Fetch() ([]*Node, error)
-	Watch(func(nodes []*Node))
+	Watch(WatchCallback)
 	Register(string) error
 }
 
diff --git a/discovery/etcd/etcd.go b/discovery/etcd/etcd.go
index 35dd594f3d..80f3f87460 100644
--- a/discovery/etcd/etcd.go
+++ b/discovery/etcd/etcd.go
@@ -63,14 +63,14 @@ func (s *EtcdDiscoveryService) Fetch() ([]*discovery.Node, error) {
 	return nodes, nil
 }
 
-func (s *EtcdDiscoveryService) Watch(updateNodes func(nodes []*discovery.Node)) {
+func (s *EtcdDiscoveryService) Watch(callback discovery.WatchCallback) {
 	watchChan := make(chan *etcd.Response)
 	go s.client.Watch(s.path, 0, true, watchChan, nil)
 	for _ = range watchChan {
 		log.Debugf("[ETCD] Watch triggered")
 		nodes, err := s.Fetch()
 		if err == nil {
-			updateNodes(nodes)
+			callback(nodes)
 		}
 	}
 }
diff --git a/discovery/file/file.go b/discovery/file/file.go
index 2f8da0bb89..39ac7af32b 100644
--- a/discovery/file/file.go
+++ b/discovery/file/file.go
@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"strings"
 	"time"
+
 	"github.com/docker/swarm/discovery"
 )
 
@@ -43,11 +44,11 @@ func (s *FileDiscoveryService) Fetch() ([]*discovery.Node, error) {
 	return nodes, nil
 }
 
-func (s *FileDiscoveryService) Watch(updateNodes func(nodes []*discovery.Node)) {
+func (s *FileDiscoveryService) Watch(callback discovery.WatchCallback) {
 	for _ = range time.Tick(time.Duration(s.heartbeat) * time.Second) {
 		nodes, err := s.Fetch()
 		if err == nil {
-			updateNodes(nodes)
+			callback(nodes)
 		}
 	}
 }
diff --git a/discovery/token/token.go b/discovery/token/token.go
index 7bc9cae1c0..3fb0995413 100644
--- a/discovery/token/token.go
+++ b/discovery/token/token.go
@@ -66,11 +66,11 @@ func (s *TokenDiscoveryService) Fetch() ([]*discovery.Node, error) {
 	return nodes, nil
 }
 
-func (s *TokenDiscoveryService) Watch(updateNodes func(nodes []*discovery.Node)) {
+func (s *TokenDiscoveryService) Watch(callback discovery.WatchCallback) {
 	for _ = range time.Tick(time.Duration(s.heartbeat) * time.Second) {
 		nodes, err := s.Fetch()
 		if err == nil {
-			updateNodes(nodes)
+			callback(nodes)
 		}
 	}
 }