diff --git a/core/discov/subscriber.go b/core/discov/subscriber.go
index 1e1bf508..8d7c5ce1 100644
--- a/core/discov/subscriber.go
+++ b/core/discov/subscriber.go
@@ -36,6 +36,10 @@ func NewSubscriber(endpoints []string, key string, opts ...SubOption) (*Subscrib
 	return sub, nil
 }
 
+func (s *Subscriber) AddListener(listener func()) {
+	s.items.addListener(listener)
+}
+
 func (s *Subscriber) Values() []string {
 	return s.items.getValues()
 }
@@ -54,6 +58,7 @@ type container struct {
 	mapping   map[string]string
 	snapshot  atomic.Value
 	dirty     *syncx.AtomicBool
+	listeners []func()
 	lock      sync.Mutex
 }
 
@@ -68,10 +73,12 @@ func newContainer(exclusive bool) *container {
 
 func (c *container) OnAdd(kv internal.KV) {
 	c.addKv(kv.Key, kv.Val)
+	c.notifyChange()
 }
 
 func (c *container) OnDelete(kv internal.KV) {
 	c.removeKey(kv.Key)
+	c.notifyChange()
 }
 
 // addKv adds the kv, returns if there are already other keys associate with the value
@@ -98,6 +105,12 @@ func (c *container) addKv(key, value string) ([]string, bool) {
 	}
 }
 
+func (c *container) addListener(listener func()) {
+	c.lock.Lock()
+	c.listeners = append(c.listeners, listener)
+	c.lock.Unlock()
+}
+
 func (c *container) doRemoveKey(key string) {
 	server, ok := c.mapping[key]
 	if !ok {
@@ -139,6 +152,16 @@ func (c *container) getValues() []string {
 	return vals
 }
 
+func (c *container) notifyChange() {
+	c.lock.Lock()
+	listeners := append(([]func())(nil), c.listeners...)
+	c.lock.Unlock()
+
+	for _, listener := range listeners {
+		listener()
+	}
+}
+
 // removeKey removes the kv, returns true if there are still other keys associate with the value
 func (c *container) removeKey(key string) {
 	c.lock.Lock()
diff --git a/core/discov/subscriber_test.go b/core/discov/subscriber_test.go
index 6a147c3a..2c2d590b 100644
--- a/core/discov/subscriber_test.go
+++ b/core/discov/subscriber_test.go
@@ -168,8 +168,13 @@ func TestContainer(t *testing.T) {
 	for _, test := range tests {
 		for _, exclusive := range exclusives {
 			t.Run(test.name, func(t *testing.T) {
+				var changed bool
 				c := newContainer(exclusive)
+				c.addListener(func() {
+					changed = true
+				})
 				assert.Nil(t, c.getValues())
+				assert.False(t, changed)
 
 				for _, order := range test.do {
 					if order.act == actionAdd {
@@ -185,6 +190,7 @@ func TestContainer(t *testing.T) {
 					}
 				}
 
+				assert.True(t, changed)
 				assert.True(t, c.dirty.True())
 				assert.ElementsMatch(t, test.expect, c.getValues())
 				assert.False(t, c.dirty.True())
diff --git a/rpcx/internal/balancer/roundrobin.go b/rpcx/internal/balancer/roundrobin.go
index 7a7a4bf1..bb173972 100644
--- a/rpcx/internal/balancer/roundrobin.go
+++ b/rpcx/internal/balancer/roundrobin.go
@@ -2,15 +2,16 @@ package balancer
 
 import (
 	"context"
+	"math/rand"
+	"sync"
+	"time"
 
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/balancer/base"
 	"google.golang.org/grpc/resolver"
 )
 
-const (
-	Name = "roundrobin"
-)
+const Name = "roundrobin"
 
 func init() {
 	balancer.Register(newBuilder())
@@ -24,13 +25,38 @@ func newBuilder() balancer.Builder {
 }
 
 func (b *roundRobinPickerBuilder) Build(readySCs map[resolver.Address]balancer.SubConn) balancer.Picker {
-	panic("implement me")
+	rand.Seed(time.Now().UnixNano())
+	picker := &roundRobinPicker{
+		index: rand.Int(),
+	}
+
+	for addr, conn := range readySCs {
+		picker.conns = append(picker.conns, &subConn{
+			addr: addr,
+			conn: conn,
+		})
+	}
+
+	return picker
 }
 
 type roundRobinPicker struct {
+	conns []*subConn
+	index int
+	lock  sync.Mutex
 }
 
 func (p *roundRobinPicker) Pick(ctx context.Context, info balancer.PickInfo) (
 	conn balancer.SubConn, done func(balancer.DoneInfo), err error) {
-	panic("implement me")
+	p.lock.Lock()
+	defer p.lock.Unlock()
+
+	p.index = (p.index + 1) % len(p.conns)
+	return p.conns[p.index].conn, func(info balancer.DoneInfo) {
+	}, nil
+}
+
+type subConn struct {
+	addr resolver.Address
+	conn balancer.SubConn
 }
diff --git a/rpcx/internal/resolver/resolver.go b/rpcx/internal/resolver/resolver.go
index 2f1a14a4..4f2e4a7e 100644
--- a/rpcx/internal/resolver/resolver.go
+++ b/rpcx/internal/resolver/resolver.go
@@ -3,6 +3,7 @@ package resolver
 import "google.golang.org/grpc/resolver"
 
 type discovResolver struct {
+	cc resolver.ClientConn
 }
 
 func (r discovResolver) ResolveNow(options resolver.ResolveNowOptions) {