diff --git a/server/app.go b/server/app.go
index 12ef2d7..4917afc 100644
--- a/server/app.go
+++ b/server/app.go
@@ -44,8 +44,9 @@ type App struct {
 	rcm sync.Mutex
 	rcs map[*respClient]struct{}
 
-	migrateM       sync.Mutex
-	migrateClients map[string]*goledis.Client
+	migrateM          sync.Mutex
+	migrateClients    map[string]*goledis.Client
+	migrateKeyLockers map[string]*migrateKeyLocker
 }
 
 func netType(s string) string {
@@ -76,6 +77,7 @@ func NewApp(cfg *config.Config) (*App, error) {
 	app.rcs = make(map[*respClient]struct{})
 
 	app.migrateClients = make(map[string]*goledis.Client)
+	app.newMigrateKeyLockers()
 
 	var err error
 
diff --git a/server/cmd_migrate.go b/server/cmd_migrate.go
index ffe47c7..51430d4 100644
--- a/server/cmd_migrate.go
+++ b/server/cmd_migrate.go
@@ -2,10 +2,13 @@ package server
 
 import (
 	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/siddontang/go/hack"
 	goledis "github.com/siddontang/ledisdb/client/goledis"
 	"github.com/siddontang/ledisdb/ledis"
-	"strings"
-	"time"
 )
 
 func dumpCommand(c *client) error {
@@ -110,15 +113,15 @@ func xdump(db *ledis.DB, tp string, key []byte) ([]byte, error) {
 	var err error
 	var data []byte
 	switch strings.ToUpper(tp) {
-	case "KV":
+	case KVName:
 		data, err = db.Dump(key)
-	case "HASH":
+	case HashName:
 		data, err = db.HDump(key)
-	case "LIST":
+	case ListName:
 		data, err = db.LDump(key)
-	case "SET":
+	case SetName:
 		data, err = db.SDump(key)
-	case "ZSET":
+	case ZSetName:
 		data, err = db.ZDump(key)
 	default:
 		err = fmt.Errorf("invalid key type %s", tp)
@@ -129,15 +132,15 @@ func xdump(db *ledis.DB, tp string, key []byte) ([]byte, error) {
 func xdel(db *ledis.DB, tp string, key []byte) error {
 	var err error
 	switch strings.ToUpper(tp) {
-	case "KV":
+	case KVName:
 		_, err = db.Del(key)
-	case "HASH":
+	case HashName:
 		_, err = db.HClear(key)
-	case "LIST":
+	case ListName:
 		_, err = db.LClear(key)
-	case "SET":
+	case SetName:
 		_, err = db.SClear(key)
-	case "ZSET":
+	case ZSetName:
 		_, err = db.ZClear(key)
 	default:
 		err = fmt.Errorf("invalid key type %s", tp)
@@ -147,15 +150,15 @@ func xdel(db *ledis.DB, tp string, key []byte) error {
 
 func xttl(db *ledis.DB, tp string, key []byte) (int64, error) {
 	switch strings.ToUpper(tp) {
-	case "KV":
+	case KVName:
 		return db.TTL(key)
-	case "HASH":
+	case HashName:
 		return db.HTTL(key)
-	case "LIST":
+	case ListName:
 		return db.LTTL(key)
-	case "SET":
+	case SetName:
 		return db.STTL(key)
-	case "ZSET":
+	case ZSetName:
 		return db.ZTTL(key)
 	default:
 		return 0, fmt.Errorf("invalid key type %s", tp)
@@ -164,15 +167,15 @@ func xttl(db *ledis.DB, tp string, key []byte) (int64, error) {
 
 func xscan(db *ledis.DB, tp string, count int) ([][]byte, error) {
 	switch strings.ToUpper(tp) {
-	case "KV":
+	case KVName:
 		return db.Scan(KV, nil, count, false, "")
-	case "HASH":
+	case HashName:
 		return db.Scan(HASH, nil, count, false, "")
-	case "LIST":
+	case ListName:
 		return db.Scan(LIST, nil, count, false, "")
-	case "SET":
+	case SetName:
 		return db.Scan(SET, nil, count, false, "")
-	case "ZSET":
+	case ZSetName:
 		return db.Scan(ZSET, nil, count, false, "")
 	default:
 		return nil, fmt.Errorf("invalid key type %s", tp)
@@ -185,7 +188,7 @@ func xdumpCommand(c *client) error {
 		return ErrCmdParams
 	}
 
-	tp := string(args[0])
+	tp := strings.ToUpper(string(args[0]))
 	key := args[1]
 
 	if data, err := xdump(c.db, tp, key); err != nil {
@@ -211,6 +214,68 @@ func (app *App) getMigrateClient(addr string) *goledis.Client {
 	return mc
 }
 
+type migrateKeyLocker struct {
+	m sync.Mutex
+
+	locks map[string]struct{}
+}
+
+func (m *migrateKeyLocker) Lock(key []byte) bool {
+	m.m.Lock()
+	defer m.m.Unlock()
+
+	k := hack.String(key)
+	_, ok := m.locks[k]
+	if ok {
+		return false
+	}
+	m.locks[k] = struct{}{}
+	return true
+}
+
+func (m *migrateKeyLocker) Unlock(key []byte) {
+	m.m.Lock()
+	defer m.m.Unlock()
+
+	delete(m.locks, hack.String(key))
+}
+
+func newMigrateKeyLocker() *migrateKeyLocker {
+	m := new(migrateKeyLocker)
+
+	m.locks = make(map[string]struct{})
+
+	return m
+}
+
+func (a *App) newMigrateKeyLockers() {
+	a.migrateKeyLockers = make(map[string]*migrateKeyLocker)
+
+	a.migrateKeyLockers[KVName] = newMigrateKeyLocker()
+	a.migrateKeyLockers[HashName] = newMigrateKeyLocker()
+	a.migrateKeyLockers[ListName] = newMigrateKeyLocker()
+	a.migrateKeyLockers[SetName] = newMigrateKeyLocker()
+	a.migrateKeyLockers[ZSetName] = newMigrateKeyLocker()
+}
+
+func (a *App) migrateKeyLock(tp string, key []byte) bool {
+	l, ok := a.migrateKeyLockers[strings.ToUpper(tp)]
+	if !ok {
+		return false
+	}
+
+	return l.Lock(key)
+}
+
+func (a *App) migrateKeyUnlock(tp string, key []byte) {
+	l, ok := a.migrateKeyLockers[strings.ToUpper(tp)]
+	if !ok {
+		return
+	}
+
+	l.Unlock(key)
+}
+
 //XMIGRATEDB host port tp count db timeout
 //select count tp type keys and migrate
 //will block any other write operations
@@ -227,7 +292,7 @@ func xmigratedbCommand(c *client) error {
 		return fmt.Errorf("migrate in same server is not allowed")
 	}
 
-	tp := string(args[2])
+	tp := strings.ToUpper(string(args[2]))
 
 	count, err := ledis.StrInt64(args[3], nil)
 	if err != nil {
@@ -250,13 +315,7 @@ func xmigratedbCommand(c *client) error {
 		return fmt.Errorf("invalid timeout %d", timeout)
 	}
 
-	m, err := c.db.Multi()
-	if err != nil {
-		return err
-	}
-	defer m.Close()
-
-	keys, err := xscan(m.DB, tp, int(count))
+	keys, err := xscan(c.db, tp, int(count))
 	if err != nil {
 		return err
 	} else if len(keys) == 0 {
@@ -270,6 +329,7 @@ func xmigratedbCommand(c *client) error {
 	if err != nil {
 		return err
 	}
+	defer conn.Close()
 
 	//timeout is milliseconds
 	t := time.Duration(timeout) * time.Millisecond
@@ -278,13 +338,24 @@ func xmigratedbCommand(c *client) error {
 		return err
 	}
 
+	migrateNum := int64(0)
 	for _, key := range keys {
-		data, err := xdump(m.DB, tp, key)
-		if err != nil {
-			return err
+		if !c.app.migrateKeyLock(tp, key) {
+			// other may also migrate this key, skip it
+			continue
 		}
 
-		ttl, err := xttl(m.DB, tp, key)
+		defer c.app.migrateKeyUnlock(tp, key)
+
+		data, err := xdump(c.db, tp, key)
+		if err != nil {
+			return err
+		} else if data == nil {
+			// no key now, skip it
+			continue
+		}
+
+		ttl, err := xttl(c.db, tp, key)
 		if err != nil {
 			return err
 		}
@@ -296,13 +367,14 @@ func xmigratedbCommand(c *client) error {
 			return err
 		}
 
-		if err = xdel(m.DB, tp, key); err != nil {
+		if err = xdel(c.db, tp, key); err != nil {
 			return err
 		}
 
+		migrateNum++
 	}
 
-	c.resp.writeInteger(int64(len(keys)))
+	c.resp.writeInteger(migrateNum)
 
 	return nil
 }
@@ -323,7 +395,7 @@ func xmigrateCommand(c *client) error {
 		return fmt.Errorf("migrate in same server is not allowed")
 	}
 
-	tp := string(args[2])
+	tp := strings.ToUpper(string(args[2]))
 	key := args[3]
 	db, err := ledis.StrUint64(args[4], nil)
 	if err != nil {
@@ -339,13 +411,14 @@ func xmigrateCommand(c *client) error {
 		return fmt.Errorf("invalid timeout %d", timeout)
 	}
 
-	m, err := c.db.Multi()
-	if err != nil {
-		return err
+	if !c.app.migrateKeyLock(tp, key) {
+		// other may also migrate this key, skip it
+		return fmt.Errorf("%s %s is in migrating yet", tp, key)
 	}
-	defer m.Close()
 
-	data, err := xdump(m.DB, tp, key)
+	defer c.app.migrateKeyUnlock(tp, key)
+
+	data, err := xdump(c.db, tp, key)
 	if err != nil {
 		return err
 	} else if data == nil {
@@ -353,7 +426,7 @@ func xmigrateCommand(c *client) error {
 		return nil
 	}
 
-	ttl, err := xttl(m.DB, tp, key)
+	ttl, err := xttl(c.db, tp, key)
 	if err != nil {
 		return err
 	}
@@ -364,6 +437,7 @@ func xmigrateCommand(c *client) error {
 	if err != nil {
 		return err
 	}
+	defer conn.Close()
 
 	//timeout is milliseconds
 	t := time.Duration(timeout) * time.Millisecond
@@ -379,7 +453,7 @@ func xmigrateCommand(c *client) error {
 		return err
 	}
 
-	if err = xdel(m.DB, tp, key); err != nil {
+	if err = xdel(c.db, tp, key); err != nil {
 		return err
 	}
 
diff --git a/server/const.go b/server/const.go
index 9804ad7..4d72743 100644
--- a/server/const.go
+++ b/server/const.go
@@ -34,6 +34,14 @@ const (
 	ZSET                = ledis.ZSET
 )
 
+const (
+	KVName   = ledis.KVName
+	ListName = ledis.ListName
+	HashName = ledis.HashName
+	SetName  = ledis.SetName
+	ZSetName = ledis.ZSetName
+)
+
 const (
 	GB uint64 = 1024 * 1024 * 1024
 	MB uint64 = 1024 * 1024