diff --git a/database/database.go b/database/database.go
index a790af7..0e0a7d5 100644
--- a/database/database.go
+++ b/database/database.go
@@ -20,7 +20,7 @@ import (
 
 // MultiDB is a set of multiple database set
 type MultiDB struct {
-	dbSet []*DB
+	dbSet []*atomic.Value // *DB
 
 	// handle publish/subscribe
 	hub *pubsub.Hub
@@ -39,11 +39,13 @@ func NewStandaloneServer() *MultiDB {
 	if config.Properties.Databases == 0 {
 		config.Properties.Databases = 16
 	}
-	mdb.dbSet = make([]*DB, config.Properties.Databases)
+	mdb.dbSet = make([]*atomic.Value, config.Properties.Databases)
 	for i := range mdb.dbSet {
 		singleDB := makeDB()
 		singleDB.index = i
-		mdb.dbSet[i] = singleDB
+		holder := &atomic.Value{}
+		holder.Store(singleDB)
+		mdb.dbSet[i] = holder
 	}
 	mdb.hub = pubsub.MakeHub()
 	validAof := false
@@ -56,8 +58,7 @@ func NewStandaloneServer() *MultiDB {
 		}
 		mdb.aofHandler = aofHandler
 		for _, db := range mdb.dbSet {
-			// avoid closure
-			singleDB := db
+			singleDB := db.Load().(*DB)
 			singleDB.addAof = func(line CmdLine) {
 				mdb.aofHandler.AddAof(singleDB.index, line)
 			}
@@ -77,9 +78,11 @@ func NewStandaloneServer() *MultiDB {
 // MakeBasicMultiDB create a MultiDB only with basic abilities for aof rewrite and other usages
 func MakeBasicMultiDB() *MultiDB {
 	mdb := &MultiDB{}
-	mdb.dbSet = make([]*DB, config.Properties.Databases)
+	mdb.dbSet = make([]*atomic.Value, config.Properties.Databases)
 	for i := range mdb.dbSet {
-		mdb.dbSet[i] = makeBasicDB()
+		holder := &atomic.Value{}
+		holder.Store(makeBasicDB())
+		mdb.dbSet[i] = holder
 	}
 	return mdb
 }
@@ -139,6 +142,14 @@ func (mdb *MultiDB) Exec(c redis.Connection, cmdLine [][]byte) (result redis.Rep
 		return RewriteAOF(mdb, cmdLine[1:])
 	} else if cmdName == "flushall" {
 		return mdb.flushAll()
+	} else if cmdName == "flushdb" {
+		if !validateArity(1, cmdLine) {
+			return protocol.MakeArgNumErrReply(cmdName)
+		}
+		if c.InMultiState() {
+			return protocol.MakeErrReply("ERR command 'FlushDB' cannot be used in MULTI")
+		}
+		return mdb.flushDB(c.GetDBIndex())
 	} else if cmdName == "save" {
 		return SaveRDB(mdb, cmdLine[1:])
 	} else if cmdName == "bgsave" {
@@ -161,10 +172,10 @@ func (mdb *MultiDB) Exec(c redis.Connection, cmdLine [][]byte) (result redis.Rep
 
 	// normal commands
 	dbIndex := c.GetDBIndex()
-	if dbIndex >= len(mdb.dbSet) {
-		return protocol.MakeErrReply("ERR DB index is out of range")
+	selectedDB, errReply := mdb.selectDB(dbIndex)
+	if errReply != nil {
+		return errReply
 	}
-	selectedDB := mdb.dbSet[dbIndex]
 	return selectedDB.Exec(c, cmdLine)
 }
 
@@ -194,9 +205,29 @@ func execSelect(c redis.Connection, mdb *MultiDB, args [][]byte) redis.Reply {
 	return protocol.MakeOkReply()
 }
 
+func (mdb *MultiDB) flushDB(dbIndex int) redis.Reply {
+	if dbIndex >= len(mdb.dbSet) || dbIndex < 0 {
+		return protocol.MakeErrReply("ERR DB index is out of range")
+	}
+	newDB := makeDB()
+	mdb.loadDB(dbIndex, newDB)
+	return &protocol.OkReply{}
+}
+
+func (mdb *MultiDB) loadDB(dbIndex int, newDB *DB) redis.Reply {
+	if dbIndex >= len(mdb.dbSet) || dbIndex < 0 {
+		return protocol.MakeErrReply("ERR DB index is out of range")
+	}
+	oldDB := mdb.mustSelectDB(dbIndex)
+	newDB.index = dbIndex
+	newDB.addAof = oldDB.addAof // inherit oldDB
+	mdb.dbSet[dbIndex].Store(newDB)
+	return &protocol.OkReply{}
+}
+
 func (mdb *MultiDB) flushAll() redis.Reply {
-	for _, db := range mdb.dbSet {
-		db.Flush()
+	for i := range mdb.dbSet {
+		mdb.flushDB(i)
 	}
 	if mdb.aofHandler != nil {
 		mdb.aofHandler.AddAof(0, utils.ToCmdLine("FlushAll"))
@@ -204,48 +235,56 @@ func (mdb *MultiDB) flushAll() redis.Reply {
 	return &protocol.OkReply{}
 }
 
-func (mdb *MultiDB) selectDB(dbIndex int) *DB {
-	if dbIndex >= len(mdb.dbSet) {
-		panic("ERR DB index is out of range")
+func (mdb *MultiDB) selectDB(dbIndex int) (*DB, *protocol.StandardErrReply) {
+	if dbIndex >= len(mdb.dbSet) || dbIndex < 0 {
+		return nil, protocol.MakeErrReply("ERR DB index is out of range")
 	}
-	return mdb.dbSet[dbIndex]
+	return mdb.dbSet[dbIndex].Load().(*DB), nil
+}
+
+func (mdb *MultiDB) mustSelectDB(dbIndex int) *DB {
+	selectedDB, err := mdb.selectDB(dbIndex)
+	if err != nil {
+		panic(err)
+	}
+	return selectedDB
 }
 
 // ForEach traverses all the keys in the given database
 func (mdb *MultiDB) ForEach(dbIndex int, cb func(key string, data *database.DataEntity, expiration *time.Time) bool) {
-	mdb.selectDB(dbIndex).ForEach(cb)
+	mdb.mustSelectDB(dbIndex).ForEach(cb)
 }
 
 // ExecMulti executes multi commands transaction Atomically and Isolated
 func (mdb *MultiDB) ExecMulti(conn redis.Connection, watching map[string]uint32, cmdLines []CmdLine) redis.Reply {
-	if conn.GetDBIndex() >= len(mdb.dbSet) {
-		return protocol.MakeErrReply("ERR DB index is out of range")
+	selectedDB, errReply := mdb.selectDB(conn.GetDBIndex())
+	if errReply != nil {
+		return errReply
 	}
-	db := mdb.dbSet[conn.GetDBIndex()]
-	return db.ExecMulti(conn, watching, cmdLines)
+	return selectedDB.ExecMulti(conn, watching, cmdLines)
 }
 
 // RWLocks lock keys for writing and reading
 func (mdb *MultiDB) RWLocks(dbIndex int, writeKeys []string, readKeys []string) {
-	mdb.selectDB(dbIndex).RWLocks(writeKeys, readKeys)
+	mdb.mustSelectDB(dbIndex).RWLocks(writeKeys, readKeys)
 }
 
 // RWUnLocks unlock keys for writing and reading
 func (mdb *MultiDB) RWUnLocks(dbIndex int, writeKeys []string, readKeys []string) {
-	mdb.selectDB(dbIndex).RWUnLocks(writeKeys, readKeys)
+	mdb.mustSelectDB(dbIndex).RWUnLocks(writeKeys, readKeys)
 }
 
 // GetUndoLogs return rollback commands
 func (mdb *MultiDB) GetUndoLogs(dbIndex int, cmdLine [][]byte) []CmdLine {
-	return mdb.selectDB(dbIndex).GetUndoLogs(cmdLine)
+	return mdb.mustSelectDB(dbIndex).GetUndoLogs(cmdLine)
 }
 
 // ExecWithLock executes normal commands, invoker should provide locks
 func (mdb *MultiDB) ExecWithLock(conn redis.Connection, cmdLine [][]byte) redis.Reply {
-	if conn.GetDBIndex() >= len(mdb.dbSet) {
-		panic("ERR DB index is out of range")
+	db, errReply := mdb.selectDB(conn.GetDBIndex())
+	if errReply != nil {
+		return errReply
 	}
-	db := mdb.dbSet[conn.GetDBIndex()]
 	return db.execWithLock(cmdLine)
 }
 
@@ -297,6 +336,6 @@ func BGSaveRDB(db *MultiDB, args [][]byte) redis.Reply {
 
 // GetDBSize returns keys count and ttl key count
 func (mdb *MultiDB) GetDBSize(dbIndex int) (int, int) {
-	db := mdb.selectDB(dbIndex)
+	db := mdb.mustSelectDB(dbIndex)
 	return db.data.Len(), db.ttlMap.Len()
 }
diff --git a/database/keys.go b/database/keys.go
index c9095d6..dbb2210 100644
--- a/database/keys.go
+++ b/database/keys.go
@@ -51,6 +51,7 @@ func execExists(db *DB, args [][]byte) redis.Reply {
 }
 
 // execFlushDB removes all data in current db
+// deprecated, use MultiDB.flushDB
 func execFlushDB(db *DB, args [][]byte) redis.Reply {
 	db.Flush()
 	db.addAof(utils.ToCmdLine3("flushdb", args...))
@@ -316,7 +317,7 @@ func undoExpire(db *DB, args [][]byte) []CmdLine {
 // This command copies the value stored at the source key to the destination key.
 func execCopy(mdb *MultiDB, conn redis.Connection, args [][]byte) redis.Reply {
 	dbIndex := conn.GetDBIndex()
-	db := mdb.dbSet[dbIndex] // Current DB
+	db := mdb.mustSelectDB(dbIndex) // Current DB
 	replaceFlag := false
 	srcKey := string(args[0])
 	destKey := string(args[1])
@@ -356,7 +357,7 @@ func execCopy(mdb *MultiDB, conn redis.Connection, args [][]byte) redis.Reply {
 		return protocol.MakeIntReply(0)
 	}
 
-	destDB := mdb.dbSet[dbIndex]
+	destDB := mdb.mustSelectDB(dbIndex)
 	if _, exists = destDB.GetEntity(destKey); exists != false {
 		// If destKey exists and there is no "replace" option
 		if replaceFlag == false {
diff --git a/database/rdb.go b/database/rdb.go
index e80bbb4..b628780 100644
--- a/database/rdb.go
+++ b/database/rdb.go
@@ -31,7 +31,7 @@ func loadRdbFile(mdb *MultiDB) {
 
 func dumpRDB(dec *core.Decoder, mdb *MultiDB) error {
 	return dec.Parse(func(o rdb.RedisObject) bool {
-		db := mdb.selectDB(o.GetDBIndex())
+		db := mdb.mustSelectDB(o.GetDBIndex())
 		switch o.GetType() {
 		case rdb.StringType:
 			str := o.(*rdb.StringObject)
diff --git a/database/replication.go b/database/replication.go
index 72d7cfd..ccc59d9 100644
--- a/database/replication.go
+++ b/database/replication.go
@@ -303,9 +303,9 @@ func (mdb *MultiDB) doPsync() error {
 	}
 	logger.Info("full resync from master: " + mdb.replication.replId)
 	logger.Info("current offset:", mdb.replication.replOffset)
-	for i, newDB := range rdbHolder.dbSet {
-		oldDB := mdb.selectDB(i)
-		oldDB.Load(newDB)
+	for i, h := range rdbHolder.dbSet {
+		newDB := h.Load().(*DB)
+		mdb.loadDB(i, newDB)
 	}
 	// there is no CRLF between RDB and following AOF, reset stream to avoid parser error
 	mdb.replication.masterChan = parser.ParseStream(mdb.replication.masterConn)
diff --git a/database/replication_test.go b/database/replication_test.go
index c2c19a5..537631b 100644
--- a/database/replication_test.go
+++ b/database/replication_test.go
@@ -8,17 +8,20 @@ import (
 	"github.com/hdt3213/godis/redis/connection"
 	"github.com/hdt3213/godis/redis/protocol"
 	"github.com/hdt3213/godis/redis/protocol/asserts"
+	"sync/atomic"
 	"testing"
 	"time"
 )
 
 func TestReplication(t *testing.T) {
 	mdb := &MultiDB{}
-	mdb.dbSet = make([]*DB, 16)
+	mdb.dbSet = make([]*atomic.Value, 16)
 	for i := range mdb.dbSet {
 		singleDB := makeDB()
 		singleDB.index = i
-		mdb.dbSet[i] = singleDB
+		holder := &atomic.Value{}
+		holder.Store(singleDB)
+		mdb.dbSet[i] = holder
 	}
 	mdb.replication = initReplStatus()
 	masterCli, err := client.MakeClient("127.0.0.1:6379")
diff --git a/database/single_db.go b/database/single_db.go
index e5cf34f..5e402a5 100644
--- a/database/single_db.go
+++ b/database/single_db.go
@@ -10,7 +10,6 @@ import (
 	"github.com/hdt3213/godis/lib/timewheel"
 	"github.com/hdt3213/godis/redis/protocol"
 	"strings"
-	"sync"
 	"time"
 )
 
@@ -33,9 +32,7 @@ type DB struct {
 	// dict.Dict will ensure concurrent-safety of its method
 	// use this mutex for complicated command only, eg. rpush, incr ...
 	locker *lock.Locks
-	// stop all data access for execFlushDB
-	stopWorld sync.WaitGroup
-	addAof    func(CmdLine)
+	addAof func(CmdLine)
 }
 
 // ExecFunc is interface for command executor
@@ -102,14 +99,6 @@ func (db *DB) Exec(c redis.Connection, cmdLine [][]byte) redis.Reply {
 			return protocol.MakeArgNumErrReply(cmdName)
 		}
 		return Watch(db, c, cmdLine[1:])
-	} else if cmdName == "flushdb" {
-		if !validateArity(1, cmdLine) {
-			return protocol.MakeArgNumErrReply(cmdName)
-		}
-		if c.InMultiState() {
-			return protocol.MakeErrReply("ERR command 'FlushDB' cannot be used in MULTI")
-		}
-		return execFlushDB(db, cmdLine[1:])
 	}
 	if c != nil && c.InMultiState() {
 		EnqueueCmd(c, cmdLine)
@@ -164,8 +153,6 @@ func validateArity(arity int, cmdArgs [][]byte) bool {
 
 // GetEntity returns DataEntity bind to given key
 func (db *DB) GetEntity(key string) (*database.DataEntity, bool) {
-	db.stopWorld.Wait()
-
 	raw, ok := db.data.Get(key)
 	if !ok {
 		return nil, false
@@ -179,25 +166,21 @@ func (db *DB) GetEntity(key string) (*database.DataEntity, bool) {
 
 // PutEntity a DataEntity into DB
 func (db *DB) PutEntity(key string, entity *database.DataEntity) int {
-	db.stopWorld.Wait()
 	return db.data.Put(key, entity)
 }
 
 // PutIfExists edit an existing DataEntity
 func (db *DB) PutIfExists(key string, entity *database.DataEntity) int {
-	db.stopWorld.Wait()
 	return db.data.PutIfExists(key, entity)
 }
 
 // PutIfAbsent insert an DataEntity only if the key not exists
 func (db *DB) PutIfAbsent(key string, entity *database.DataEntity) int {
-	db.stopWorld.Wait()
 	return db.data.PutIfAbsent(key, entity)
 }
 
 // Remove the given key from db
 func (db *DB) Remove(key string) {
-	db.stopWorld.Wait()
 	db.data.Remove(key)
 	db.ttlMap.Remove(key)
 	taskKey := genExpireTask(key)
@@ -206,7 +189,6 @@ func (db *DB) Remove(key string) {
 
 // Removes the given keys from db
 func (db *DB) Removes(keys ...string) (deleted int) {
-	db.stopWorld.Wait()
 	deleted = 0
 	for _, key := range keys {
 		_, exists := db.data.Get(key)
@@ -219,24 +201,14 @@ func (db *DB) Removes(keys ...string) (deleted int) {
 }
 
 // Flush clean database
+// deprecated
+// for test only
 func (db *DB) Flush() {
-	db.stopWorld.Add(1)
-	defer db.stopWorld.Done()
-
 	db.data.Clear()
 	db.ttlMap.Clear()
 	db.locker = lock.Make(lockerSize)
 }
 
-func (db *DB) Load(db2 *DB) {
-	db.stopWorld.Add(1)
-	defer db.stopWorld.Done()
-
-	db.data = db2.data
-	db.ttlMap = db2.ttlMap
-	db.locker = lock.Make(lockerSize)
-}
-
 /* ---- Lock Function ----- */
 
 // RWLocks lock keys for writing and reading
@@ -257,7 +229,6 @@ func genExpireTask(key string) string {
 
 // Expire sets ttlCmd of key
 func (db *DB) Expire(key string, expireTime time.Time) {
-	db.stopWorld.Wait()
 	db.ttlMap.Put(key, expireTime)
 	taskKey := genExpireTask(key)
 	timewheel.At(expireTime, taskKey, func() {
@@ -280,7 +251,6 @@ func (db *DB) Expire(key string, expireTime time.Time) {
 
 // Persist cancel ttlCmd of key
 func (db *DB) Persist(key string) {
-	db.stopWorld.Wait()
 	db.ttlMap.Remove(key)
 	taskKey := genExpireTask(key)
 	timewheel.Cancel(taskKey)
@@ -288,7 +258,6 @@ func (db *DB) Persist(key string) {
 
 // IsExpired check whether a key is expired
 func (db *DB) IsExpired(key string) bool {
-	db.stopWorld.Wait()
 	rawExpireTime, ok := db.ttlMap.Get(key)
 	if !ok {
 		return false
@@ -304,7 +273,6 @@ func (db *DB) IsExpired(key string) bool {
 /* --- add version --- */
 
 func (db *DB) addVersion(keys ...string) {
-	db.stopWorld.Wait()
 	for _, key := range keys {
 		versionCode := db.GetVersion(key)
 		db.versionMap.Put(key, versionCode+1)
@@ -313,7 +281,6 @@ func (db *DB) addVersion(keys ...string) {
 
 // GetVersion returns version code for given key
 func (db *DB) GetVersion(key string) uint32 {
-	db.stopWorld.Wait()
 	entity, ok := db.versionMap.Get(key)
 	if !ok {
 		return 0
@@ -323,7 +290,6 @@ func (db *DB) GetVersion(key string) uint32 {
 
 // ForEach traverses all the keys in the database
 func (db *DB) ForEach(cb func(key string, data *database.DataEntity, expiration *time.Time) bool) {
-	db.stopWorld.Wait()
 	db.data.ForEach(func(key string, raw interface{}) bool {
 		entity, _ := raw.(*database.DataEntity)
 		var expiration *time.Time