diff --git a/ledis/t_ttl.go b/ledis/t_ttl.go
new file mode 100644
index 0000000..167acce
--- /dev/null
+++ b/ledis/t_ttl.go
@@ -0,0 +1,208 @@
+package ledis
+
+import (
+	"encoding/binary"
+	"errors"
+	"github.com/siddontang/go-leveldb/leveldb"
+	"time"
+)
+
+var mapExpMetaType = map[byte]byte{
+	kvExpType: kvExpMetaType,
+	lExpType:  lExpMetaType,
+	hExpType:  hExpMetaType,
+	zExpType:  zExpMetaType}
+
+type retireCallback func(*tx, []byte) int64
+
+type Elimination struct {
+	db         *DB
+	exp2Tx     map[byte]*tx
+	exp2Retire map[byte]retireCallback
+}
+
+var errExpType = errors.New("invalid expire type")
+
+func (db *DB) expEncodeTimeKey(expType byte, key []byte, when int64) []byte {
+	// format : db[8] / expType[8] / when[64] / key[...]
+	buf := make([]byte, len(key)+10)
+
+	buf[0] = db.index
+	buf[1] = expType
+	pos := 2
+
+	binary.BigEndian.PutUint64(buf[pos:], uint64(when))
+	pos += 8
+
+	copy(buf[pos:], key)
+
+	return buf
+}
+
+func (db *DB) expEncodeMetaKey(expType byte, key []byte) []byte {
+	// format : db[8] / expType[8] / key[...]
+	buf := make([]byte, len(key)+2)
+
+	buf[0] = db.index
+	buf[1] = expType
+	pos := 2
+
+	copy(buf[pos:], key)
+
+	return buf
+}
+
+// usage : separate out the original key
+func (db *DB) expDecodeMetaKey(mk []byte) []byte {
+	if len(mk) <= 2 {
+		//	check db ? check type ?
+		return nil
+	}
+
+	return mk[2:]
+}
+
+func (db *DB) expire(t *tx, expType byte, key []byte, duration int64) {
+	db.expireAt(t, expType, key, time.Now().Unix()+duration)
+}
+
+func (db *DB) expireAt(t *tx, expType byte, key []byte, when int64) {
+	mk := db.expEncodeMetaKey(expType+1, key)
+	tk := db.expEncodeTimeKey(expType, key, when)
+
+	t.Put(tk, mk)
+	t.Put(mk, PutInt64(when))
+}
+
+func (db *DB) ttl(expType byte, key []byte) (t int64, err error) {
+	mk := db.expEncodeMetaKey(expType+1, key)
+
+	if t, err = Int64(db.db.Get(mk)); err != nil || t == 0 {
+		t = -1
+	} else {
+		t -= time.Now().Unix()
+		if t <= 0 {
+			t = -1
+		}
+		// if t == -1 : to remove ????
+	}
+
+	return t, err
+}
+
+func (db *DB) rmExpire(t *tx, expType byte, key []byte) {
+	mk := db.expEncodeMetaKey(expType+1, key)
+	when, err := Int64(db.db.Get(mk))
+	if err == nil && when > 0 {
+		tk := db.expEncodeTimeKey(expType, key, when)
+
+		t.Delete(mk)
+		t.Delete(tk)
+	}
+}
+
+func (db *DB) expFlush(t *tx, expType byte) (err error) {
+	expMetaType, ok := mapExpMetaType[expType]
+	if !ok {
+		return errExpType
+	}
+
+	drop := 0
+
+	minKey := make([]byte, 2)
+	minKey[0] = db.index
+	minKey[1] = expType
+
+	maxKey := make([]byte, 2)
+	maxKey[0] = db.index
+	maxKey[1] = expMetaType + 1
+
+	it := db.db.Iterator(minKey, maxKey, leveldb.RangeROpen, 0, -1)
+	for ; it.Valid(); it.Next() {
+		t.Delete(it.Key())
+		drop++
+		if drop&1023 == 0 {
+			if err = t.Commit(); err != nil {
+				return
+			}
+		}
+	}
+
+	err = t.Commit()
+	return
+}
+
+//////////////////////////////////////////////////////////
+//
+//////////////////////////////////////////////////////////
+
+func newEliminator(db *DB) *Elimination {
+	eli := new(Elimination)
+	eli.db = db
+	eli.exp2Tx = make(map[byte]*tx)
+	eli.exp2Retire = make(map[byte]retireCallback)
+	return eli
+}
+
+func (eli *Elimination) regRetireContext(expType byte, t *tx, onRetire retireCallback) {
+	eli.exp2Tx[expType] = t
+	eli.exp2Retire[expType] = onRetire
+}
+
+//	call by outside ... (from *db to another *db)
+func (eli *Elimination) active() {
+	now := time.Now().Unix()
+	db := eli.db
+	dbGet := db.db.Get
+	expKeys := make([][]byte, 0, 1024)
+	expTypes := [...]byte{kvExpType, lExpType, hExpType, zExpType}
+
+	for _, et := range expTypes {
+		//	search those keys' which expire till the moment
+		minKey := db.expEncodeTimeKey(et, nil, 0)
+		maxKey := db.expEncodeTimeKey(et, nil, now+1)
+		expKeys = expKeys[0:0]
+
+		t, _ := eli.exp2Tx[et]
+		onRetire, _ := eli.exp2Retire[et]
+		if t == nil || onRetire == nil {
+			// todo : log error
+			continue
+		}
+
+		it := db.db.Iterator(minKey, maxKey, leveldb.RangeROpen, 0, -1)
+		for it.Valid() {
+			for i := 1; i < 512 && it.Valid(); i++ {
+				expKeys = append(expKeys, it.Key(), it.Value())
+				it.Next()
+			}
+
+			var cnt int = len(expKeys)
+			if cnt == 0 {
+				continue
+			}
+
+			t.Lock()
+			var mk, ek, k []byte
+			for i := 0; i < cnt; i += 2 {
+				ek, mk = expKeys[i], expKeys[i+1]
+				if exp, err := Int64(dbGet(mk)); err == nil {
+					// check expire again
+					if exp > now {
+						continue
+					}
+
+					// delete keys
+					k = db.expDecodeMetaKey(mk)
+					onRetire(t, k)
+					t.Delete(ek)
+					t.Delete(mk)
+				}
+			}
+			t.Commit()
+			t.Unlock()
+		} // end : it
+	} // end : expType
+
+	return
+}
diff --git a/ledis/t_ttl_test.go b/ledis/t_ttl_test.go
new file mode 100644
index 0000000..0483ae7
--- /dev/null
+++ b/ledis/t_ttl_test.go
@@ -0,0 +1,172 @@
+package ledis
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+)
+
+var m sync.Mutex
+
+func TestKvExpire(t *testing.T) {
+	db := getTestDB()
+	m.Lock()
+	defer m.Unlock()
+
+	k := []byte("ttl_a")
+	ek := []byte("ttl_b")
+	db.Set(k, []byte("1"))
+
+	if ok, _ := db.Expire(k, 10); ok != 1 {
+		t.Fatal(ok)
+	}
+
+	//	err - expire on an inexisting key
+	if ok, _ := db.Expire(ek, 10); ok != 0 {
+		t.Fatal(ok)
+	}
+
+	//	err - duration is zero
+	if ok, err := db.Expire(k, 0); err == nil || ok != 0 {
+		t.Fatal(fmt.Sprintf("res = %d, err = %s", ok, err))
+	}
+
+	//	err - duration is negative
+	if ok, err := db.Expire(k, -10); err == nil || ok != 0 {
+		t.Fatal(fmt.Sprintf("res = %d, err = %s", ok, err))
+	}
+}
+
+func TestKvExpireAt(t *testing.T) {
+	db := getTestDB()
+	m.Lock()
+	defer m.Unlock()
+
+	k := []byte("ttl_a")
+	ek := []byte("ttl_b")
+	db.Set(k, []byte("1"))
+
+	now := time.Now().Unix()
+
+	if ok, _ := db.ExpireAt(k, now+5); ok != 1 {
+		t.Fatal(ok)
+	}
+
+	//	err - expire on an inexisting key
+	if ok, _ := db.ExpireAt(ek, now+5); ok != 0 {
+		t.Fatal(ok)
+	}
+
+	//	err - expire with the current time
+	if ok, err := db.ExpireAt(k, now); err == nil || ok != 0 {
+		t.Fatal(fmt.Sprintf("res = %d, err = %s", ok, err))
+	}
+
+	//	err - expire with the time before
+	if ok, err := db.ExpireAt(k, now-5); err == nil || ok != 0 {
+		t.Fatal(fmt.Sprintf("res = %d, err = %s", ok, err))
+	}
+}
+
+func TestKvTtl(t *testing.T) {
+	db := getTestDB()
+	m.Lock()
+	defer m.Unlock()
+
+	k := []byte("ttl_a")
+	ek := []byte("ttl_b")
+
+	db.Set(k, []byte("1"))
+	db.Expire(k, 2)
+
+	if tRemain, _ := db.Ttl(k); tRemain != 2 {
+		t.Fatal(tRemain)
+	}
+
+	//	err - check ttl on an inexisting key
+	if tRemain, _ := db.Ttl(ek); tRemain != -1 {
+		t.Fatal(tRemain)
+	}
+
+	db.Del(k)
+	if tRemain, _ := db.Ttl(k); tRemain != -1 {
+		t.Fatal(tRemain)
+	}
+}
+
+func TestKvExpCompose(t *testing.T) {
+	db := getTestDB()
+	m.Lock()
+	defer m.Unlock()
+
+	k0 := []byte("ttl_a")
+	k1 := []byte("ttl_b")
+	k2 := []byte("ttl_c")
+
+	db.Set(k0, k0)
+	db.Set(k1, k1)
+	db.Set(k2, k2)
+
+	db.Expire(k0, 5)
+	db.Expire(k1, 2)
+	db.Expire(k2, 60)
+
+	if tRemain, _ := db.Ttl(k0); tRemain != 5 {
+		t.Fatal(tRemain)
+	}
+	if tRemain, _ := db.Ttl(k1); tRemain != 2 {
+		t.Fatal(tRemain)
+	}
+	if tRemain, _ := db.Ttl(k2); tRemain != 60 {
+		t.Fatal(tRemain)
+	}
+
+	// after 1 sec
+	time.Sleep(1 * time.Second)
+	if tRemain, _ := db.Ttl(k0); tRemain != 4 {
+		t.Fatal(tRemain)
+	}
+	if tRemain, _ := db.Ttl(k1); tRemain != 1 {
+		t.Fatal(tRemain)
+	}
+
+	// after 2 sec
+	time.Sleep(2 * time.Second)
+	if tRemain, _ := db.Ttl(k1); tRemain != -1 {
+		t.Fatal(tRemain)
+	}
+	if v, _ := db.Get(k1); v != nil {
+		t.Fatal(v)
+	}
+
+	if tRemain, _ := db.Ttl(k0); tRemain != 2 {
+		t.Fatal(tRemain)
+	}
+	if v, _ := db.Get(k0); v == nil {
+		t.Fatal(v)
+	}
+
+	// refresh the expiration of key
+	if tRemain, _ := db.Ttl(k2); !(0 < tRemain && tRemain < 60) {
+		t.Fatal(tRemain)
+	}
+
+	if ok, _ := db.Expire(k2, 100); ok != 1 {
+		t.Fatal(false)
+	}
+
+	if tRemain, _ := db.Ttl(k2); tRemain != 100 {
+		t.Fatal(tRemain)
+	}
+
+	//	expire an inexisting key
+	if ok, _ := db.Expire(k1, 10); ok == 1 {
+		t.Fatal(false)
+	}
+	if tRemain, _ := db.Ttl(k1); tRemain != -1 {
+		t.Fatal(tRemain)
+	}
+
+	return
+}