diff --git a/README.md b/README.md
index 3efcbc4..9f4599c 100644
--- a/README.md
+++ b/README.md
@@ -142,8 +142,8 @@ I suggest focusing on the following directories:
     - set: a hash set based on map
     - sortedset: a sorted set implements based on skiplist
 - database: the core of storage engine
-    - server.go: a standalone redis server, with multiple database 
-    - db.go: data structure and base functions of single database
+    - database.go: a standalone redis server, with multiple database 
+    - single_db.go: data structure and base functions of single database
     - exec.go: the gateway of database
     - router.go: the command table
     - keys.go: handlers for keys commands
diff --git a/README_CN.md b/README_CN.md
index f7956e5..4feda44 100644
--- a/README_CN.md
+++ b/README_CN.md
@@ -132,7 +132,8 @@ MSET (10 keys): 65487.89 requests per second
     - set： 基于hash表的集合
     - sortedset: 基于跳表实现的有序集合
 - database: 存储引擎核心
-    - db.go: 单个 database 的数据结构和基本功能
+    - database.go: 支持多数据库的单机版 redis 服务实例
+    - single_db.go: 单个 database 的数据结构和基本功能
     - router.go: 将命令路由给响应的处理函数
     - keys.go: del、ttl、expire 等通用命令实现
     - string.go: get、set 等字符串命令实现
diff --git a/database/server.go b/database/database.go
similarity index 98%
rename from database/server.go
rename to database/database.go
index cf17be8..8936f68 100644
--- a/database/server.go
+++ b/database/database.go
@@ -87,8 +87,7 @@ func (mdb *MultiDB) Exec(c redis.Connection, cmdLine [][]byte) (result redis.Rep
 		return protocol.MakeErrReply("NOAUTH Authentication required")
 	}
 
-	// todo: merge special commands into router
-	// special commands
+	// special commands which cannot execute within transaction
 	if cmdName == "subscribe" {
 		if len(cmdLine) < 2 {
 			return protocol.MakeArgNumErrReply("subscribe")
diff --git a/database/keys.go b/database/keys.go
index d739224..344e2bd 100644
--- a/database/keys.go
+++ b/database/keys.go
@@ -321,6 +321,5 @@ func init() {
 	RegisterCommand("Type", execType, readFirstKey, nil, 2)
 	RegisterCommand("Rename", execRename, prepareRename, undoRename, 3)
 	RegisterCommand("RenameNx", execRenameNx, prepareRename, undoRename, 3)
-	RegisterCommand("FlushDB", execFlushDB, noPrepare, nil, -1)
 	RegisterCommand("Keys", execKeys, noPrepare, nil, 2)
 }
diff --git a/database/db.go b/database/single_db.go
similarity index 90%
rename from database/db.go
rename to database/single_db.go
index 588b63a..d4c600a 100644
--- a/database/db.go
+++ b/database/single_db.go
@@ -80,6 +80,7 @@ func makeBasicDB() *DB {
 
 // Exec executes command within one database
 func (db *DB) Exec(c redis.Connection, cmdLine [][]byte) redis.Reply {
+	// transaction control commands and other commands which cannot execute within transaction
 	cmdName := strings.ToLower(string(cmdLine[0]))
 	if cmdName == "multi" {
 		if len(cmdLine) != 1 {
@@ -101,6 +102,14 @@ 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)
@@ -129,6 +138,20 @@ func (db *DB) execNormalCommand(cmdLine [][]byte) redis.Reply {
 	return fun(db, cmdLine[1:])
 }
 
+// execWithLock executes normal commands, invoker should provide locks
+func (db *DB) execWithLock(cmdLine [][]byte) redis.Reply {
+	cmdName := strings.ToLower(string(cmdLine[0]))
+	cmd, ok := cmdTable[cmdName]
+	if !ok {
+		return protocol.MakeErrReply("ERR unknown command '" + cmdName + "'")
+	}
+	if !validateArity(cmd.arity, cmdLine) {
+		return protocol.MakeArgNumErrReply(cmdName)
+	}
+	fun := cmd.executor
+	return fun(db, cmdLine[1:])
+}
+
 func validateArity(arity int, cmdArgs [][]byte) bool {
 	argNum := len(cmdArgs)
 	if arity >= 0 {
diff --git a/database/transaction.go b/database/transaction.go
index 520f9c6..788c66e 100644
--- a/database/transaction.go
+++ b/database/transaction.go
@@ -164,20 +164,6 @@ func (db *DB) GetUndoLogs(cmdLine [][]byte) []CmdLine {
 	return undo(db, cmdLine[1:])
 }
 
-// execWithLock executes normal commands, invoker should provide locks
-func (db *DB) execWithLock(cmdLine [][]byte) redis.Reply {
-	cmdName := strings.ToLower(string(cmdLine[0]))
-	cmd, ok := cmdTable[cmdName]
-	if !ok {
-		return protocol.MakeErrReply("ERR unknown command '" + cmdName + "'")
-	}
-	if !validateArity(cmd.arity, cmdLine) {
-		return protocol.MakeArgNumErrReply(cmdName)
-	}
-	fun := cmd.executor
-	return fun(db, cmdLine[1:])
-}
-
 // GetRelatedKeys analysis related keys
 func GetRelatedKeys(cmdLine [][]byte) ([]string, []string) {
 	cmdName := strings.ToLower(string(cmdLine[0]))