diff --git a/src/datastruct/lock/lock_map.go b/src/datastruct/lock/lock_map.go
new file mode 100644
index 0000000..3ccdd25
--- /dev/null
+++ b/src/datastruct/lock/lock_map.go
@@ -0,0 +1,131 @@
+package lock
+
+import (
+    "fmt"
+    "runtime"
+    "sort"
+    "strconv"
+    "strings"
+    "sync"
+    "time"
+)
+
+type LockMap struct {
+    m sync.Map // key -> mutex
+}
+
+func (lock *LockMap)Lock(key string) {
+    mu := &sync.RWMutex{}
+    existed, loaded := lock.m.LoadOrStore(key, mu)
+    if loaded {
+        mu, _ = existed.(*sync.RWMutex)
+    }
+    mu.Lock()
+}
+
+func (lock *LockMap)RLock(key string) {
+    mu := &sync.RWMutex{}
+    existed, loaded := lock.m.LoadOrStore(key, mu)
+    if loaded {
+        mu, _ = existed.(*sync.RWMutex)
+    }
+    mu.RLock()
+}
+
+func (lock *LockMap)UnLock(key string) {
+    value, ok := lock.m.Load(key)
+    if !ok {
+        return
+    }
+    mu := value.(*sync.RWMutex)
+    mu.Unlock()
+}
+
+func (lock *LockMap)RUnLock(key string) {
+    value, ok := lock.m.Load(key)
+    if !ok {
+        return
+    }
+    mu := value.(*sync.RWMutex)
+    mu.RUnlock()
+}
+
+func (lock *LockMap)Locks(keys ...string) {
+    keySlice := make(sort.StringSlice, len(keys))
+    copy(keySlice, keys)
+    sort.Sort(keySlice)
+    for _, key := range keySlice {
+        lock.Lock(key)
+    }
+}
+
+func (lock *LockMap)RLocks(keys ...string) {
+    keySlice := make(sort.StringSlice, len(keys))
+    copy(keySlice, keys)
+    sort.Sort(keySlice)
+    for _, key := range keySlice {
+        lock.RLock(key)
+    }
+}
+
+
+func (lock *LockMap)UnLocks(keys ...string) {
+    size := len(keys)
+    keySlice := make(sort.StringSlice, size)
+    copy(keySlice, keys)
+    sort.Sort(keySlice)
+    for i := size - 1; i >= 0; i-- {
+        key := keySlice[i]
+        lock.UnLock(key)
+    }
+}
+
+func (lock *LockMap)RUnLocks(keys ...string) {
+    size := len(keys)
+    keySlice := make(sort.StringSlice, size)
+    copy(keySlice, keys)
+    sort.Sort(keySlice)
+    for i := size - 1; i >= 0; i-- {
+        key := keySlice[i]
+        lock.RUnLock(key)
+    }
+}
+
+func (lock *LockMap)Clean(key string) {
+    lock.m.Delete(key)
+}
+
+func (lock *LockMap)Cleans(keys ...string) {
+    for _, key := range keys {
+        lock.Clean(key)
+    }
+}
+
+func GoID() int {
+    var buf [64]byte
+    n := runtime.Stack(buf[:], false)
+    idField := strings.Fields(strings.TrimPrefix(string(buf[:n]), "goroutine "))[0]
+    id, err := strconv.Atoi(idField)
+    if err != nil {
+        panic(fmt.Sprintf("cannot get goroutine id: %v", err))
+    }
+    return id
+}
+
+func debug() {
+    lm := LockMap{}
+    size := 10
+    var wg sync.WaitGroup
+    wg.Add(size)
+    for i := 0; i < size; i++ {
+        go func(i int) {
+            lm.Locks("1", "2")
+            println("go: " + strconv.Itoa(GoID()))
+            time.Sleep(time.Second)
+            println("go: " + strconv.Itoa(GoID()))
+            lm.UnLocks("1", "2")
+            wg.Done()
+        }(i)
+    }
+    wg.Wait()
+}
\ No newline at end of file
diff --git a/src/db/db.go b/src/db/db.go
index 5bbeb5c..6111d67 100644
--- a/src/db/db.go
+++ b/src/db/db.go
@@ -3,12 +3,12 @@ package db
 import (
     "fmt"
     "github.com/HDT3213/godis/src/datastruct/dict"
+    "github.com/HDT3213/godis/src/datastruct/lock"
     "github.com/HDT3213/godis/src/interface/redis"
     "github.com/HDT3213/godis/src/lib/logger"
     "github.com/HDT3213/godis/src/redis/reply"
     "runtime/debug"
     "strings"
-    "sync"
 )
 
 const (
@@ -23,10 +23,6 @@ type DataEntity struct {
     Code uint8
     TTL int64 // ttl in seconds, 0 for unlimited ttl
     Data interface{}
-
-    // dict will ensure thread safety (by using mutex) of its method
-    // use this mutex for complicated command only, eg. rpush, incr ...
-    sync.RWMutex
 }
 
 type DataEntityWithKey struct {
@@ -38,7 +34,12 @@ type DataEntityWithKey struct {
 type CmdFunc func(db *DB, args [][]byte)redis.Reply
 
 type DB struct {
-    Data *dict.Dict // key -> DataEntity
+    // key -> DataEntity
+    Data *dict.Dict
+
+    // dict will ensure thread safety of its method
+    // use this mutex for complicated command only, eg. rpush, incr ...
+    Locks *lock.LockMap
 }
 
 var cmdMap = MakeCmdMap()
@@ -53,6 +54,7 @@ func MakeCmdMap()map[string]CmdFunc {
     cmdMap["psetex"] = PSetEX
     cmdMap["mset"] = MSet
     cmdMap["mget"] = MGet
+    cmdMap["msetnx"] = MSetNX
     cmdMap["get"] = Get
     cmdMap["del"] = Del
 
@@ -75,6 +77,7 @@ func MakeCmdMap()map[string]CmdFunc {
 func MakeDB() *DB {
     return &DB{
         Data: dict.Make(1024),
+        Locks: &lock.LockMap{},
     }
 }
 
@@ -98,3 +101,25 @@ func (db *DB)Exec(args [][]byte)(result redis.Reply) {
     }
     return
 }
+
+func (db *DB)Remove(key string) {
+    db.Data.Remove(key)
+    db.Locks.Clean(key)
+}
+
+func (db *DB)Removes(keys ...string)(deleted int) {
+    db.Locks.Locks(keys...)
+    defer func() {
+        db.Locks.UnLocks(keys...)
+        db.Locks.Cleans(keys...)
+    }()
+    deleted = 0
+    for _, key := range keys {
+        _, exists := db.Data.Get(key)
+        if exists {
+            db.Data.Remove(key)
+            deleted++
+        }
+    }
+    return deleted
+}
diff --git a/src/db/get.go b/src/db/get.go
deleted file mode 100644
index 297c459..0000000
--- a/src/db/get.go
+++ /dev/null
@@ -1,27 +0,0 @@
-package db
-
-import (
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func Get(db *DB, args [][]byte)redis.Reply {
-    if len(args) != 1 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'get' command")
-    }
-    key := string(args[0])
-    val, ok := db.Data.Get(key)
-    if !ok {
-        return &reply.NullBulkReply{}
-    }
-    entity, _ := val.(*DataEntity)
-    if entity.Code == StringCode {
-        bytes, ok := entity.Data.([]byte)
-        if !ok {
-            return &reply.UnknownErrReply{}
-        }
-        return reply.MakeBulkReply(bytes)
-    } else {
-        return &reply.WrongTypeErrReply{}
-    }
-}
\ No newline at end of file
diff --git a/src/db/del.go b/src/db/keys.go
similarity index 70%
rename from src/db/del.go
rename to src/db/keys.go
index e90a33a..259909e 100644
--- a/src/db/del.go
+++ b/src/db/keys.go
@@ -14,14 +14,6 @@ func Del(db *DB, args [][]byte)redis.Reply {
         keys[i] = string(v)
     }
 
-    deleted := 0
-    for _, key := range keys {
-        _, exists := db.Data.Get(key)
-        if exists {
-            db.Data.Remove(key)
-            deleted++
-        }
-    }
-
+    deleted := db.Removes(keys...)
     return reply.MakeIntReply(int64(deleted))
 }
diff --git a/src/db/lindex.go b/src/db/lindex.go
deleted file mode 100644
index bfd431f..0000000
--- a/src/db/lindex.go
+++ /dev/null
@@ -1,49 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-    "strconv"
-)
-
-func LIndex(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
-    }
-    key := string(args[0])
-    index64, err := strconv.ParseInt(string(args[1]), 10, 64)
-    if err != nil {
-        return reply.MakeErrReply("ERR value is not an integer or out of range")
-    }
-    index := int(index64)
-
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return &reply.NullBulkReply{}
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    entity.RLock()
-    defer entity.RUnlock()
-
-    // check type
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-
-    list, _ := entity.Data.(*List.LinkedList)
-    size := list.Len() // assert: size > 0
-    if index < -1 * size {
-        return &reply.NullBulkReply{}
-    } else if index < 0 {
-        index = size + index
-    } else if index >= size {
-        return &reply.NullBulkReply{}
-    }
-
-    val, _ := list.Get(index).([]byte)
-    return reply.MakeBulkReply(val)
-}
diff --git a/src/db/list.go b/src/db/list.go
new file mode 100644
index 0000000..51fdfe1
--- /dev/null
+++ b/src/db/list.go
@@ -0,0 +1,476 @@
+package db
+
+import (
+    List "github.com/HDT3213/godis/src/datastruct/list"
+    "github.com/HDT3213/godis/src/interface/redis"
+    "github.com/HDT3213/godis/src/redis/reply"
+    "strconv"
+)
+
+func LIndex(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
+    }
+    key := string(args[0])
+    index64, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    index := int(index64)
+
+    // get entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return &reply.NullBulkReply{}
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+
+    // check type
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    list, _ := entity.Data.(*List.LinkedList)
+    size := list.Len() // assert: size > 0
+    if index < -1 * size {
+        return &reply.NullBulkReply{}
+    } else if index < 0 {
+        index = size + index
+    } else if index >= size {
+        return &reply.NullBulkReply{}
+    }
+
+    val, _ := list.Get(index).([]byte)
+    return reply.MakeBulkReply(val)
+}
+
+func LLen(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'llen' command")
+    }
+    key := string(args[0])
+
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return reply.MakeIntReply(0)
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+
+    // check type
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    list, _ := entity.Data.(*List.LinkedList)
+    size := int64(list.Len())
+    return reply.MakeIntReply(size)
+}
+
+func LPop(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
+    }
+    key := string(args[0])
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return &reply.NullBulkReply{}
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+
+    // check type
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // remove
+    list, _ := entity.Data.(*List.LinkedList)
+    val, _ := list.Remove(0).([]byte)
+    if list.Len() == 0 {
+        db.Remove(key)
+    }
+    return reply.MakeBulkReply(val)
+}
+
+func LPush(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lpush' command")
+    }
+    key := string(args[0])
+    values := args[1:]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        entity = &DataEntity{
+            Code: ListCode,
+            Data: &List.LinkedList{},
+        }
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+
+    // check type
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // insert
+    list, _ := entity.Data.(*List.LinkedList)
+    for _, value := range values {
+        list.Insert(0, value)
+    }
+    db.Data.Put(key, entity)
+
+    return reply.MakeIntReply(int64(list.Len()))
+}
+
+func LPushX(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lpush' command")
+    }
+    key := string(args[0])
+    values := args[1:]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return reply.MakeIntReply(0)
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // insert
+    list, _ := entity.Data.(*List.LinkedList)
+    for _, value := range values {
+        list.Insert(0, value)
+    }
+    db.Data.Put(key, entity)
+
+    return reply.MakeIntReply(int64(list.Len()))
+}
+
+func LRange(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lrange' command")
+    }
+    key := string(args[0])
+    start64, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    start := int(start64)
+    stop64, err := strconv.ParseInt(string(args[2]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    stop := int(stop64)
+
+    // get data
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return &reply.EmptyMultiBulkReply{}
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // compute index
+    list, _ := entity.Data.(*List.LinkedList)
+    size := list.Len() // assert: size > 0
+    if start < -1 * size {
+        start = 0
+    } else if start < 0 {
+        start = size + start
+    } else if start >= size {
+        return &reply.EmptyMultiBulkReply{}
+    }
+    if stop < -1 * size {
+        stop = 0
+    } else if stop < 0 {
+        stop = size + stop + 1
+    } else if stop < size {
+        stop = stop + 1
+    } else {
+        stop = size
+    }
+    if stop < start {
+        stop = start
+    }
+
+    // assert: start in [0, size - 1], stop in [start, size]
+    slice := list.Range(start, stop)
+    result := make([][]byte, len(slice))
+    for i, raw := range slice {
+        bytes, _ := raw.([]byte)
+        result[i] = bytes
+    }
+    return reply.MakeMultiBulkReply(result)
+}
+
+func LRem(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lrem' command")
+    }
+    key := string(args[0])
+    count64, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    count := int(count64)
+    value := args[2]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return reply.MakeIntReply(0)
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    list, _ := entity.Data.(*List.LinkedList)
+    var removed int
+    if count == 0 {
+        removed = list.RemoveAllByVal(value)
+    } else if count > 0 {
+        removed = list.RemoveByVal(value, count)
+    } else {
+        removed = list.ReverseRemoveByVal(value, -count)
+    }
+
+    if list.Len() == 0 {
+        db.Remove(key)
+    }
+
+    return reply.MakeIntReply(int64(removed))
+}
+
+func LSet(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lset' command")
+    }
+    key := string(args[0])
+    index64, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    index := int(index64)
+    value := args[2]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return reply.MakeErrReply("ERR no such key")
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    list, _ := entity.Data.(*List.LinkedList)
+    size := list.Len() // assert: size > 0
+    if index < -1 * size {
+        return reply.MakeErrReply("ERR index out of range")
+    } else if index < 0 {
+        index = size + index
+    } else if index >= size {
+        return reply.MakeErrReply("ERR index out of range")
+    }
+
+    list.Set(index, value)
+    return &reply.OkReply{}
+}
+
+func RPop(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
+    }
+    key := string(args[0])
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return &reply.NullBulkReply{}
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    list, _ := entity.Data.(*List.LinkedList)
+    val, _ := list.RemoveLast().([]byte)
+    if list.Len() == 0 {
+        db.Remove(key)
+    }
+    return reply.MakeBulkReply(val)
+}
+
+func RPopLPush(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'rpoplpush' command")
+    }
+    sourceKey := string(args[0])
+    destKey := string(args[1])
+
+    // lock
+    db.Locks.Locks(sourceKey, destKey)
+    defer db.Locks.UnLocks(sourceKey, destKey)
+
+    // get source entity
+    rawEntity, exists := db.Data.Get(sourceKey)
+    var sourceEntity *DataEntity
+    if !exists {
+        return &reply.NullBulkReply{}
+    } else {
+        sourceEntity, _ = rawEntity.(*DataEntity)
+    }
+    sourceList, _ := sourceEntity.Data.(*List.LinkedList)
+
+    // get dest entity
+    rawEntity, exists = db.Data.Get(destKey)
+    var destEntity *DataEntity
+    if !exists {
+        destEntity = &DataEntity{
+            Code: ListCode,
+            Data: &List.LinkedList{},
+        }
+        db.Data.Put(destKey, destEntity)
+    } else {
+        destEntity, _ = rawEntity.(*DataEntity)
+    }
+    destList, _ := destEntity.Data.(*List.LinkedList)
+
+    // pop and push
+    val, _ := sourceList.RemoveLast().([]byte)
+    destList.Insert(0, val)
+
+    if sourceList.Len() == 0 {
+        db.Remove(sourceKey)
+    }
+
+    return reply.MakeBulkReply(val)
+}
+
+func RPush(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'rpush' command")
+    }
+    key := string(args[0])
+    values := args[1:]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        entity = &DataEntity{
+            Code: ListCode,
+            Data: &List.LinkedList{},
+        }
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // put list
+    list, _ := entity.Data.(*List.LinkedList)
+    for _, value := range values {
+        list.Add(value)
+    }
+    db.Data.Put(key, entity)
+
+    return reply.MakeIntReply(int64(list.Len()))
+}
+
+func RPushX(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'rpush' command")
+    }
+    key := string(args[0])
+    values := args[1:]
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    rawEntity, exists := db.Data.Get(key)
+    var entity *DataEntity
+    if !exists {
+        return reply.MakeIntReply(0)
+    } else {
+        entity, _ = rawEntity.(*DataEntity)
+    }
+    if entity.Code != ListCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // put list
+    list, _ := entity.Data.(*List.LinkedList)
+    for _, value := range values {
+        list.Add(value)
+    }
+    db.Data.Put(key, entity)
+
+    return reply.MakeIntReply(int64(list.Len()))
+}
\ No newline at end of file
diff --git a/src/db/llen.go b/src/db/llen.go
deleted file mode 100644
index 3809286..0000000
--- a/src/db/llen.go
+++ /dev/null
@@ -1,34 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func LLen(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 1 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'llen' command")
-    }
-    key := string(args[0])
-
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return reply.MakeIntReply(0)
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    entity.RLock()
-    defer entity.RUnlock()
-
-    // check type
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-
-    list, _ := entity.Data.(*List.LinkedList)
-    size := int64(list.Len())
-    return reply.MakeIntReply(size)
-}
\ No newline at end of file
diff --git a/src/db/lpop.go b/src/db/lpop.go
deleted file mode 100644
index bf6d937..0000000
--- a/src/db/lpop.go
+++ /dev/null
@@ -1,38 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func LPop(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 1 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
-    }
-    key := string(args[0])
-
-    // get data
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return &reply.NullBulkReply{}
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    // check type
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-
-    list, _ := entity.Data.(*List.LinkedList)
-    val, _ := list.Remove(0).([]byte)
-    if list.Len() == 0 {
-        db.Data.Remove(key)
-    }
-    return reply.MakeBulkReply(val)
-}
diff --git a/src/db/lpush.go b/src/db/lpush.go
deleted file mode 100644
index 8fd7ee2..0000000
--- a/src/db/lpush.go
+++ /dev/null
@@ -1,73 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func LPush(db *DB, args [][]byte)redis.Reply {
-    if len(args) < 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lpush' command")
-    }
-    key := string(args[0])
-    values := args[1:]
-
-    // get or init entity
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        entity = &DataEntity{
-            Code: ListCode,
-            Data: &List.LinkedList{},
-        }
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-
-    // insert
-    list, _ := entity.Data.(*List.LinkedList)
-    for _, value := range values {
-        list.Insert(0, value)
-    }
-    db.Data.Put(key, entity)
-
-    return reply.MakeIntReply(int64(list.Len()))
-}
-
-func LPushX(db *DB, args [][]byte)redis.Reply {
-    if len(args) < 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lpush' command")
-    }
-    key := string(args[0])
-    values := args[1:]
-
-    // get or init entity
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return reply.MakeIntReply(0)
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    // insert
-    list, _ := entity.Data.(*List.LinkedList)
-    for _, value := range values {
-        list.Insert(0, value)
-    }
-    db.Data.Put(key, entity)
-
-    return reply.MakeIntReply(int64(list.Len()))
-}
\ No newline at end of file
diff --git a/src/db/lrange.go b/src/db/lrange.go
deleted file mode 100644
index 56e01b1..0000000
--- a/src/db/lrange.go
+++ /dev/null
@@ -1,72 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-    "strconv"
-)
-
-func LRange(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 3 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lrange' command")
-    }
-    key := string(args[0])
-    start64, err := strconv.ParseInt(string(args[1]), 10, 64)
-    if err != nil {
-        return reply.MakeErrReply("ERR value is not an integer or out of range")
-    }
-    start := int(start64)
-    stop64, err := strconv.ParseInt(string(args[2]), 10, 64)
-    if err != nil {
-        return reply.MakeErrReply("ERR value is not an integer or out of range")
-    }
-    stop := int(stop64)
-
-    // get data
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return &reply.EmptyMultiBulkReply{}
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.RLock()
-    defer entity.RUnlock()
-
-    // compute index
-    list, _ := entity.Data.(*List.LinkedList)
-    size := list.Len() // assert: size > 0
-    if start < -1 * size {
-        start = 0
-    } else if start < 0 {
-        start = size + start
-    } else if start >= size {
-        return &reply.EmptyMultiBulkReply{}
-    }
-    if stop < -1 * size {
-        stop = 0
-    } else if stop < 0 {
-        stop = size + stop + 1
-    } else if stop < size {
-        stop = stop + 1
-    } else {
-        stop = size
-    }
-    if stop < start {
-        stop = start
-    }
-
-    // assert: start in [0, size - 1], stop in [start, size]
-    slice := list.Range(start, stop)
-    result := make([][]byte, len(slice))
-    for i, raw := range slice {
-        bytes, _ := raw.([]byte)
-        result[i] = bytes
-    }
-    return reply.MakeMultiBulkReply(result)
-}
diff --git a/src/db/lrem.go b/src/db/lrem.go
deleted file mode 100644
index 42cfb54..0000000
--- a/src/db/lrem.go
+++ /dev/null
@@ -1,52 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-    "strconv"
-)
-
-func LRem(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 3 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lrem' command")
-    }
-    key := string(args[0])
-    count64, err := strconv.ParseInt(string(args[1]), 10, 64)
-    if err != nil {
-        return reply.MakeErrReply("ERR value is not an integer or out of range")
-    }
-    count := int(count64)
-    value := args[2]
-
-    // get data entity
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return reply.MakeIntReply(0)
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    list, _ := entity.Data.(*List.LinkedList)
-    var removed int
-    if count == 0 {
-        removed = list.RemoveAllByVal(value)
-    } else if count > 0 {
-        removed = list.RemoveByVal(value, count)
-    } else {
-        removed = list.ReverseRemoveByVal(value, -count)
-    }
-
-    if list.Len() == 0 {
-        db.Data.Remove(key)
-    }
-
-    return reply.MakeIntReply(int64(removed))
-}
\ No newline at end of file
diff --git a/src/db/lset.go b/src/db/lset.go
deleted file mode 100644
index 14623f8..0000000
--- a/src/db/lset.go
+++ /dev/null
@@ -1,49 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-    "strconv"
-)
-
-func LSet(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 3 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lset' command")
-    }
-    key := string(args[0])
-    index64, err := strconv.ParseInt(string(args[1]), 10, 64)
-    if err != nil {
-        return reply.MakeErrReply("ERR value is not an integer or out of range")
-    }
-    index := int(index64)
-    value := args[2]
-
-    // get data
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return reply.MakeErrReply("ERR no such key")
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    list, _ := entity.Data.(*List.LinkedList)
-    size := list.Len() // assert: size > 0
-    if index < -1 * size {
-        return reply.MakeErrReply("ERR index out of range")
-    } else if index < 0 {
-        index = size + index
-    } else if index >= size {
-        return reply.MakeErrReply("ERR index out of range")
-    }
-
-    list.Set(index, value)
-    return &reply.OkReply{}
-}
\ No newline at end of file
diff --git a/src/db/mget.go b/src/db/mget.go
deleted file mode 100644
index 572ec5e..0000000
--- a/src/db/mget.go
+++ /dev/null
@@ -1,34 +0,0 @@
-package db
-
-import (
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func MGet(db *DB, args [][]byte)redis.Reply {
-    if len(args) == 0 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'mget' command")
-    }
-    keys := make([]string, len(args))
-    for i, v := range args {
-        keys[i] = string(v)
-    }
-
-    result := make([][]byte, len(args))
-    for i, key := range keys {
-        val, exists := db.Data.Get(key)
-        if !exists {
-            result[i] = nil
-            continue
-        }
-        entity, _ := val.(*DataEntity)
-        if entity.Code != StringCode {
-            result[i] = nil
-            continue
-        }
-        bytes, _ := entity.Data.([]byte)
-        result[i] = bytes
-    }
-
-    return reply.MakeMultiBulkReply(result)
-}
\ No newline at end of file
diff --git a/src/db/mset.go b/src/db/mset.go
deleted file mode 100644
index 01e0253..0000000
--- a/src/db/mset.go
+++ /dev/null
@@ -1,33 +0,0 @@
-package db
-
-import (
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-
-func MSet(db *DB, args [][]byte)redis.Reply {
-    if len(args) % 2 != 0 || len(args) == 0 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'mset' command")
-    }
-    size := len(args) / 2
-    entities := make([]*DataEntityWithKey, size)
-    for i := 0; i < size; i++ {
-        key := string(args[2 * i])
-        value := args[2 * i + 1]
-        entity := &DataEntityWithKey{
-            DataEntity: DataEntity{
-                Code: StringCode,
-                Data: value,
-            },
-            Key: key,
-        }
-        entities[i] = entity
-    }
-
-    for _, entity := range entities {
-        db.Data.Put(entity.Key, &entity.DataEntity)
-    }
-
-    return &reply.OkReply{}
-}
diff --git a/src/db/rpop.go b/src/db/rpop.go
deleted file mode 100644
index c3355e5..0000000
--- a/src/db/rpop.go
+++ /dev/null
@@ -1,36 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func RPop(db *DB, args [][]byte)redis.Reply {
-    // parse args
-    if len(args) != 1 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'lindex' command")
-    }
-    key := string(args[0])
-
-    // get data
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return &reply.NullBulkReply{}
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    list, _ := entity.Data.(*List.LinkedList)
-    val, _ := list.RemoveLast().([]byte)
-    if list.Len() == 0 {
-        db.Data.Remove(key)
-    }
-    return reply.MakeBulkReply(val)
-}
diff --git a/src/db/rpoplpush.go b/src/db/rpoplpush.go
deleted file mode 100644
index a71473a..0000000
--- a/src/db/rpoplpush.go
+++ /dev/null
@@ -1,49 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func RPopLPush(db *DB, args [][]byte)redis.Reply {
-    if len(args) != 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'rpoplpush' command")
-    }
-    sourceKey := string(args[0])
-    destKey := string(args[1])
-
-    // get source entity
-    rawEntity, exists := db.Data.Get(sourceKey)
-    var sourceEntity *DataEntity
-    if !exists {
-        return &reply.NullBulkReply{}
-    } else {
-        sourceEntity, _ = rawEntity.(*DataEntity)
-    }
-    sourceList, _ := sourceEntity.Data.(*List.LinkedList)
-    sourceEntity.Lock()
-    defer sourceEntity.Unlock()
-
-    // get dest entity
-    rawEntity, exists = db.Data.Get(destKey)
-    var destEntity *DataEntity
-    if !exists {
-        destEntity = &DataEntity{
-            Code: ListCode,
-            Data: &List.LinkedList{},
-        }
-        db.Data.Put(destKey, destEntity)
-    } else {
-        destEntity, _ = rawEntity.(*DataEntity)
-    }
-    destList, _ := destEntity.Data.(*List.LinkedList)
-    destEntity.Lock()
-    defer destEntity.Unlock()
-
-    // pop and push
-    val, _ := sourceList.RemoveLast().([]byte)
-    destList.Insert(0, val)
-
-    return reply.MakeBulkReply(val)
-}
diff --git a/src/db/rpush.go b/src/db/rpush.go
deleted file mode 100644
index 5283a50..0000000
--- a/src/db/rpush.go
+++ /dev/null
@@ -1,72 +0,0 @@
-package db
-
-import (
-    List "github.com/HDT3213/godis/src/datastruct/list"
-    "github.com/HDT3213/godis/src/interface/redis"
-    "github.com/HDT3213/godis/src/redis/reply"
-)
-
-func RPush(db *DB, args [][]byte)redis.Reply {
-    if len(args) < 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'rpush' command")
-    }
-    key := string(args[0])
-    values := args[1:]
-
-    // get or init entity
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        entity = &DataEntity{
-            Code: ListCode,
-            Data: &List.LinkedList{},
-        }
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    // put list
-    list, _ := entity.Data.(*List.LinkedList)
-    for _, value := range values {
-        list.Add(value)
-    }
-    db.Data.Put(key, entity)
-
-    return reply.MakeIntReply(int64(list.Len()))
-}
-
-func RPushX(db *DB, args [][]byte)redis.Reply {
-    if len(args) < 2 {
-        return reply.MakeErrReply("ERR wrong number of arguments for 'rpush' command")
-    }
-    key := string(args[0])
-    values := args[1:]
-
-    // get or init entity
-    rawEntity, exists := db.Data.Get(key)
-    var entity *DataEntity
-    if !exists {
-        return reply.MakeIntReply(0)
-    } else {
-        entity, _ = rawEntity.(*DataEntity)
-    }
-    if entity.Code != ListCode {
-        return &reply.WrongTypeErrReply{}
-    }
-    entity.Lock()
-    defer entity.Unlock()
-
-    // put list
-    list, _ := entity.Data.(*List.LinkedList)
-    for _, value := range values {
-        list.Add(value)
-    }
-    db.Data.Put(key, entity)
-
-    return reply.MakeIntReply(int64(list.Len()))
-}
\ No newline at end of file
diff --git a/src/db/ping.go b/src/db/server.go
similarity index 99%
rename from src/db/ping.go
rename to src/db/server.go
index 91809c0..8c6c5fe 100644
--- a/src/db/ping.go
+++ b/src/db/server.go
@@ -1,8 +1,8 @@
 package db
 
 import (
-    "github.com/HDT3213/godis/src/redis/reply"
     "github.com/HDT3213/godis/src/interface/redis"
+    "github.com/HDT3213/godis/src/redis/reply"
 )
 
 func Ping(db *DB, args [][]byte)redis.Reply {
@@ -13,4 +13,4 @@ func Ping(db *DB, args [][]byte)redis.Reply {
     } else {
         return reply.MakeErrReply("ERR wrong number of arguments for 'ping' command")
     }
-}
+}
\ No newline at end of file
diff --git a/src/db/set.go b/src/db/string.go
similarity index 59%
rename from src/db/set.go
rename to src/db/string.go
index 2a4db91..4495886 100644
--- a/src/db/set.go
+++ b/src/db/string.go
@@ -7,6 +7,27 @@ import (
     "strings"
 )
 
+func Get(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'get' command")
+    }
+    key := string(args[0])
+    val, ok := db.Data.Get(key)
+    if !ok {
+        return &reply.NullBulkReply{}
+    }
+    entity, _ := val.(*DataEntity)
+    if entity.Code == StringCode {
+        bytes, ok := entity.Data.([]byte)
+        if !ok {
+            return &reply.UnknownErrReply{}
+        }
+        return reply.MakeBulkReply(bytes)
+    } else {
+        return &reply.WrongTypeErrReply{}
+    }
+}
+
 const (
     upsertPolicy = iota // default
     insertPolicy // set nx
@@ -156,4 +177,98 @@ func PSetEX(db *DB, args [][]byte)redis.Reply {
     }
     db.Data.PutIfExists(key, entity)
     return &reply.OkReply{}
+}
+
+func MSet(db *DB, args [][]byte)redis.Reply {
+    if len(args) % 2 != 0 || len(args) == 0 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'mset' command")
+    }
+    size := len(args) / 2
+    entities := make([]*DataEntityWithKey, size)
+    for i := 0; i < size; i++ {
+        key := string(args[2 * i])
+        value := args[2 * i + 1]
+        entity := &DataEntityWithKey{
+            DataEntity: DataEntity{
+                Code: StringCode,
+                Data: value,
+            },
+            Key: key,
+        }
+        entities[i] = entity
+    }
+
+    for _, entity := range entities {
+        db.Data.Put(entity.Key, &entity.DataEntity)
+    }
+
+    return &reply.OkReply{}
+}
+
+func MGet(db *DB, args [][]byte)redis.Reply {
+    if len(args) == 0 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'mget' command")
+    }
+    keys := make([]string, len(args))
+    for i, v := range args {
+        keys[i] = string(v)
+    }
+
+    result := make([][]byte, len(args))
+    for i, key := range keys {
+        val, exists := db.Data.Get(key)
+        if !exists {
+            result[i] = nil
+            continue
+        }
+        entity, _ := val.(*DataEntity)
+        if entity.Code != StringCode {
+            result[i] = nil
+            continue
+        }
+        bytes, _ := entity.Data.([]byte)
+        result[i] = bytes
+    }
+
+    return reply.MakeMultiBulkReply(result)
+}
+
+func MSetNX(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) % 2 != 0 || len(args) == 0 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'msetnx' command")
+    }
+    size := len(args) / 2
+    entities := make([]*DataEntityWithKey, size)
+    keys := make([]string, size)
+    for i := 0; i < size; i++ {
+        key := string(args[2 * i])
+        value := args[2 * i + 1]
+        entity := &DataEntityWithKey{
+            DataEntity: DataEntity{
+                Code: StringCode,
+                Data: value,
+            },
+            Key: key,
+        }
+        entities[i] = entity
+        keys[i] = key
+    }
+
+    // lock keys
+    db.Locks.Locks(keys...)
+    defer db.Locks.UnLocks(keys...)
+
+    for _, key := range keys {
+        _, exists := db.Data.Get(key)
+        if exists {
+            return reply.MakeIntReply(0)
+        }
+    }
+
+    for _, entity := range entities {
+        db.Data.Put(entity.Key, &entity.DataEntity)
+    }
+
+    return reply.MakeIntReply(1)
 }
\ No newline at end of file