diff --git a/src/lib/collection/concurrent/dict/dict.go b/src/datastruct/dict/dict.go
similarity index 88%
rename from src/lib/collection/concurrent/dict/dict.go
rename to src/datastruct/dict/dict.go
index c56fc0a..cf6f826 100644
--- a/src/lib/collection/concurrent/dict/dict.go
+++ b/src/datastruct/dict/dict.go
@@ -113,6 +113,23 @@ func (d *Dict)PutIfAbsent(key string, val interface{})int {
 
     if existed {
         return 0
+    } else {
+        // insert
+        shard.table[key] = val
+        return 1
+    }
+}
+
+// return the number of updated key-value
+func (d *Dict)PutIfExists(key string, val interface{})int {
+    shard := d.shards[d.spread(key)]
+    shard.mutex.Lock()
+    defer shard.mutex.Unlock()
+
+    _, existed := shard.table[key]
+
+    if !existed {
+        return 0
     } else {
         // update
         shard.table[key] = val
diff --git a/src/lib/collection/concurrent/dict/dict_test.go b/src/datastruct/dict/dict_test.go
similarity index 81%
rename from src/lib/collection/concurrent/dict/dict_test.go
rename to src/datastruct/dict/dict_test.go
index 3c44725..78df2e4 100644
--- a/src/lib/collection/concurrent/dict/dict_test.go
+++ b/src/datastruct/dict/dict_test.go
@@ -73,6 +73,28 @@ func TestPutIfAbsent(t *testing.T) {
     }
 }
 
+func TestPutIfExists(t *testing.T) {
+    d := Make(0)
+
+    // insert
+    ret := d.PutIfExists("a", 1)
+    if ret != 0 { // insert
+        t.Error("put test failed: expected result 0, actual: " + strconv.Itoa(ret))
+    }
+
+    d.Put("a", 1)
+    ret = d.PutIfExists("a", 2)
+    val, ok := d.Get("a")
+    if ok {
+        intVal, _ := val.(int)
+        if intVal != 2 {
+            t.Error("put test failed: expected 2, actual: " + strconv.Itoa(intVal))
+        }
+    } else {
+        t.Error("put test failed: expected true, actual: false")
+    }
+}
+
 func TestRemove(t *testing.T) {
     d := Make(0)
 
diff --git a/src/lib/collection/list/linked.go b/src/datastruct/list/linked.go
similarity index 100%
rename from src/lib/collection/list/linked.go
rename to src/datastruct/list/linked.go
diff --git a/src/lib/collection/list/linked_test.go b/src/datastruct/list/linked_test.go
similarity index 100%
rename from src/lib/collection/list/linked_test.go
rename to src/datastruct/list/linked_test.go
diff --git a/src/db/db.go b/src/db/db.go
index 8df8940..b96d111 100644
--- a/src/db/db.go
+++ b/src/db/db.go
@@ -3,53 +3,73 @@ package db
 import (
     "strings"
     "github.com/HDT3213/godis/src/redis/reply"
-    "github.com/HDT3213/godis/src/db/db"
     "fmt"
     "runtime/debug"
     "github.com/HDT3213/godis/src/lib/logger"
     "github.com/HDT3213/godis/src/interface/redis"
+    "github.com/HDT3213/godis/src/datastruct/dict"
 )
 
-// args don't include cmd line
-type CmdFunc func(args [][]byte)redis.Reply
+const (
+    StringCode = iota // Data is []byte
+    ListCode // *list.LinkedList
+    SetCode
+    DictCode // *dict.Dict
+    SortedSetCode
+)
 
-
-type DB struct {
-    cmdMap map[string]CmdFunc
+type DataEntity struct {
+    Code uint8
+    TTL int64 // ttl in seconds, 0 for unlimited ttl
+    Data interface{}
 }
 
-type UnknownErrReply struct {}
+// args don't include cmd line
+type CmdFunc func(db *DB, args [][]byte)redis.Reply
 
-func (r *UnknownErrReply)ToBytes()[]byte {
-    return []byte("-Err unknown\r\n")
+type DB struct {
+    Data *dict.Dict // key -> DataEntity
+}
+
+var cmdMap = MakeCmdMap()
+
+func MakeCmdMap()map[string]CmdFunc {
+    cmdMap := make(map[string]CmdFunc)
+    cmdMap["ping"] = Ping
+
+    cmdMap["set"] = Set
+    cmdMap["setnx"] = SetNX
+    cmdMap["setex"] = SetEX
+    cmdMap["psetex"] = PSetEX
+
+    cmdMap["get"] = Get
+
+    return cmdMap
+}
+
+func MakeDB() *DB {
+    return &DB{
+        Data: dict.Make(1024),
+    }
 }
 
 func (db *DB)Exec(args [][]byte)(result redis.Reply) {
     defer func() {
         if err := recover(); err != nil {
             logger.Warn(fmt.Sprintf("error occurs: %v\n%s", err, string(debug.Stack())))
-            result = &UnknownErrReply{}
+            result = &reply.UnknownErrReply{}
         }
     }()
 
     cmd := strings.ToLower(string(args[0]))
-    cmdFunc, ok := db.cmdMap[cmd]
+    cmdFunc, ok := cmdMap[cmd]
     if !ok {
         return reply.MakeErrReply("ERR unknown command '" + cmd + "'")
     }
     if len(args) > 1 {
-        result = cmdFunc(args[1:])
+        result = cmdFunc(db, args[1:])
     } else {
-        result = cmdFunc([][]byte{})
+        result = cmdFunc(db, [][]byte{})
     }
     return
 }
-
-func MakeDB() *DB {
-    cmdMap := make(map[string]CmdFunc)
-    cmdMap["ping"] = db.Ping
-
-    return &DB{
-        cmdMap:cmdMap,
-    }
-}
diff --git a/src/db/db/ping.go b/src/db/db/ping.go
deleted file mode 100644
index ae6a745..0000000
--- a/src/db/db/ping.go
+++ /dev/null
@@ -1,28 +0,0 @@
-package db
-
-import (
-    "github.com/HDT3213/godis/src/redis/reply"
-    "github.com/HDT3213/godis/src/interface/redis"
-)
-
-type PongReply struct {}
-
-func (r *PongReply)ToBytes()[]byte {
-    return []byte("+PONG\r\n")
-}
-
-type ArgNumErrReply struct {}
-
-func (r *ArgNumErrReply)ToBytes()[]byte {
-    return []byte("-ERR wrong number of arguments for 'ping' command\r\n")
-}
-
-func Ping(args [][]byte)redis.Reply {
-    if len(args) == 0 {
-        return &PongReply{}
-    } else if len(args) == 1 {
-        return reply.MakeStatusReply("\"" + string(args[0]) + "\"")
-    } else {
-        return &ArgNumErrReply{}
-    }
-}
diff --git a/src/db/get.go b/src/db/get.go
new file mode 100644
index 0000000..297c459
--- /dev/null
+++ b/src/db/get.go
@@ -0,0 +1,27 @@
+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/ping.go b/src/db/ping.go
new file mode 100644
index 0000000..91809c0
--- /dev/null
+++ b/src/db/ping.go
@@ -0,0 +1,16 @@
+package db
+
+import (
+    "github.com/HDT3213/godis/src/redis/reply"
+    "github.com/HDT3213/godis/src/interface/redis"
+)
+
+func Ping(db *DB, args [][]byte)redis.Reply {
+    if len(args) == 0 {
+        return &reply.PongReply{}
+    } else if len(args) == 1 {
+        return reply.MakeStatusReply("\"" + string(args[0]) + "\"")
+    } else {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'ping' command")
+    }
+}
diff --git a/src/db/set.go b/src/db/set.go
new file mode 100644
index 0000000..2a4db91
--- /dev/null
+++ b/src/db/set.go
@@ -0,0 +1,159 @@
+package db
+
+import (
+    "github.com/HDT3213/godis/src/interface/redis"
+    "github.com/HDT3213/godis/src/redis/reply"
+    "strconv"
+    "strings"
+)
+
+const (
+    upsertPolicy = iota // default
+    insertPolicy // set nx
+    updatePolicy // set ex
+)
+
+const unlimitedTTL int64 = 0
+
+// SET key value [EX seconds] [PX milliseconds] [NX|XX]
+func Set(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'set' command")
+    }
+    key := string(args[0])
+    value := args[1]
+    policy := upsertPolicy
+    ttl := unlimitedTTL
+
+    // parse options
+    if len(args) > 2 {
+        for i := 2; i < len(args); i++ {
+            arg := strings.ToUpper(string(args[i]))
+            if arg == "NX" { // insert
+                if policy == updatePolicy {
+                    return &reply.SyntaxErrReply{}
+                }
+                policy = insertPolicy
+            } else if arg == "XX" { // update policy
+                if policy == insertPolicy {
+                    return &reply.SyntaxErrReply{}
+                }
+                policy = updatePolicy
+            } else if arg == "EX" { // ttl in seconds
+                if ttl != unlimitedTTL {
+                    return &reply.SyntaxErrReply{}
+                }
+                if i + 1 >= len(args) {
+                    return &reply.SyntaxErrReply{}
+                }
+                ttlArg, err := strconv.ParseInt(string(args[i+1]), 10, 64)
+                if err != nil {
+                    return &reply.SyntaxErrReply{}
+                }
+                if ttlArg <= 0 {
+                    return reply.MakeErrReply("ERR invalid expire time in set")
+                }
+                ttl = ttlArg * 1000
+                i++ // skip next arg
+            } else if arg == "PX" { // ttl in milliseconds
+                if ttl != unlimitedTTL {
+                    return &reply.SyntaxErrReply{}
+                }
+                if i + 1 >= len(args) {
+                    return &reply.SyntaxErrReply{}
+                }
+                ttlArg, err := strconv.ParseInt(string(args[i+1]), 10, 64)
+                if err != nil {
+                    return &reply.SyntaxErrReply{}
+                }
+                if ttlArg <= 0 {
+                    return reply.MakeErrReply("ERR invalid expire time in set")
+                }
+                ttl = ttlArg
+                i++ // skip next arg
+            } else {
+                return &reply.SyntaxErrReply{}
+            }
+        }
+    }
+
+
+    entity := &DataEntity{
+        Code: StringCode,
+        TTL: ttl,
+        Data: value,
+    }
+
+    switch policy {
+    case upsertPolicy:
+        db.Data.Put(key, entity)
+    case insertPolicy:
+        db.Data.PutIfAbsent(key, entity)
+    case updatePolicy:
+        db.Data.PutIfExists(key, entity)
+    }
+    return &reply.OkReply{}
+}
+
+func SetNX(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 2 {
+        reply.MakeErrReply("ERR wrong number of arguments for 'setnx' command")
+    }
+    key := string(args[0])
+    value := args[1]
+    entity := &DataEntity{
+        Code: StringCode,
+        Data: value,
+    }
+    result := db.Data.PutIfAbsent(key, entity)
+    return reply.MakeIntReply(int64(result))
+}
+
+func SetEX(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'setex' command")
+    }
+    key := string(args[0])
+    value := args[1]
+
+    ttlArg, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return &reply.SyntaxErrReply{}
+    }
+    if ttlArg <= 0 {
+        return reply.MakeErrReply("ERR invalid expire time in setex")
+    }
+    ttl := ttlArg * 1000
+
+    entity := &DataEntity{
+        Code: StringCode,
+        TTL: ttl,
+        Data: value,
+    }
+    db.Data.PutIfExists(key, entity)
+    return &reply.OkReply{}
+}
+
+func PSetEX(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'psetex' command")
+    }
+    key := string(args[0])
+    value := args[1]
+
+    ttl, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return &reply.SyntaxErrReply{}
+    }
+    if ttl <= 0 {
+        return reply.MakeErrReply("ERR invalid expire time in psetex")
+    }
+
+    entity := &DataEntity{
+        Code: StringCode,
+        TTL: ttl,
+        Data: value,
+    }
+    db.Data.PutIfExists(key, entity)
+    return &reply.OkReply{}
+}
\ No newline at end of file
diff --git a/src/interface/db/db.go b/src/interface/db/db.go
index ca21a6e..3670681 100644
--- a/src/interface/db/db.go
+++ b/src/interface/db/db.go
@@ -5,7 +5,3 @@ import "github.com/HDT3213/godis/src/interface/redis"
 type DB interface {
     Exec([][]byte)redis.Reply
 }
-
-type DataEntity interface {
-
-}
\ No newline at end of file
diff --git a/src/redis/reply/consts.go b/src/redis/reply/consts.go
new file mode 100644
index 0000000..853fa57
--- /dev/null
+++ b/src/redis/reply/consts.go
@@ -0,0 +1,25 @@
+package reply
+
+type PongReply struct {}
+
+var PongBytes = []byte("+PONG\r\n")
+
+func (r *PongReply)ToBytes()[]byte {
+    return PongBytes
+}
+
+type OkReply struct {}
+
+var OkBytes = []byte("+OK\r\n")
+
+func (r *OkReply)ToBytes()[]byte {
+    return OkBytes
+}
+
+var nullBulkBytes = []byte("$-1\r\n")
+
+type NullBulkReply struct {}
+
+func (r *NullBulkReply)ToBytes()[]byte {
+    return nullBulkBytes
+}
\ No newline at end of file
diff --git a/src/redis/reply/errors.go b/src/redis/reply/errors.go
new file mode 100644
index 0000000..b8ae694
--- /dev/null
+++ b/src/redis/reply/errors.go
@@ -0,0 +1,37 @@
+package reply
+
+// UnknownErr
+type UnknownErrReply struct {}
+
+var unknownErrBytes = []byte("-Err unknown\r\n")
+
+func (r *UnknownErrReply)ToBytes()[]byte {
+    return unknownErrBytes
+}
+
+// ArgNumErr
+type ArgNumErrReply struct {
+    Cmd string
+}
+
+func (r *ArgNumErrReply)ToBytes()[]byte {
+    return []byte("-ERR wrong number of arguments for '" + r.Cmd + "' command\r\n")
+}
+
+// SyntaxErr
+type SyntaxErrReply struct {}
+
+var syntaxErrBytes = []byte("-Err syntax error\r\n")
+
+func (r *SyntaxErrReply)ToBytes()[]byte {
+    return syntaxErrBytes
+}
+
+// WrongTypeErr
+type WrongTypeErrReply struct {}
+
+var wrongTypeErrBytes = []byte("-WRONGTYPE Operation against a key holding the wrong kind of value\r\n")
+
+func (r *WrongTypeErrReply)ToBytes()[]byte {
+    return wrongTypeErrBytes
+}