diff --git a/leveldb.go b/leveldb.go
index b245e41..5db6caa 100644
--- a/leveldb.go
+++ b/leveldb.go
@@ -6,6 +6,7 @@ package main
 
 import (
 	"log"
+	"sync"
 
 	leveldb "github.com/jmhodges/levigo"
 )
@@ -13,6 +14,7 @@ import (
 type LevelDBBackend struct {
 	cache *leveldb.Cache
 	db    *leveldb.DB
+	mutex sync.RWMutex
 }
 
 type LevelDBSession struct {
@@ -36,6 +38,18 @@ func NewLeveDBBackend(path string, cacheSize int) (ldb *LevelDBBackend, err erro
 	return
 }
 
+func (ldb *LevelDBBackend) doRead(f func(db *leveldb.DB)) {
+	ldb.mutex.RLock()
+	f(ldb.db)
+	ldb.mutex.RUnlock()
+}
+
+func (ldb *LevelDBBackend) doWrite(f func(db *leveldb.DB)) {
+	ldb.mutex.Lock()
+	f(ldb.db)
+	ldb.mutex.Unlock()
+}
+
 func (ldb *LevelDBBackend) NewSession() (Session, error) {
 	return &LevelDBSession{ldb, nil}, nil
 }
@@ -54,9 +68,19 @@ func (ldb *LevelDBBackend) Shutdown() error {
 }
 
 func (ldbs *LevelDBSession) Fetch(hash, key []byte) (value []byte, err error) {
-	ro := leveldb.NewReadOptions()
-	value, err = ldbs.backend.db.Get(ro, key)
-	ro.Close()
+
+	var pos int64
+	if pos, err = bytes2pos(key); err != nil {
+		return
+	}
+	// Re-code it to make LevelDB happy.
+	key = pos2bytes(pos)
+
+	ldbs.backend.doRead(func(db *leveldb.DB) {
+		ro := leveldb.NewReadOptions()
+		value, err = ldbs.backend.db.Get(ro, key)
+		ro.Close()
+	})
 	return
 }
 
@@ -64,11 +88,11 @@ func (ldbs *LevelDBSession) InTransaction() bool {
 	return ldbs.tx != nil
 }
 
-func (ldbs *LevelDBSession) keyExists(key []byte) (exists bool, err error) {
+func keyExists(db *leveldb.DB, key []byte) (exists bool, err error) {
 	ro := leveldb.NewReadOptions()
 	defer ro.Close()
 	var data []byte
-	if data, err = ldbs.backend.db.Get(ro, key); err != nil {
+	if data, err = db.Get(ro, key); err != nil {
 		return
 	}
 	exists = data != nil
@@ -84,18 +108,20 @@ func (ldbs *LevelDBSession) Store(hash, key, value []byte) (exists bool, err err
 	// Re-code it to make LevelDB happy.
 	key = pos2bytes(pos)
 
-	if exists, err = ldbs.keyExists(key); err != nil {
-		return
-	}
+	ldbs.backend.doWrite(func(db *leveldb.DB) {
+		if exists, err = keyExists(db, key); err != nil {
+			return
+		}
 
-	if ldbs.tx != nil {
-		ldbs.tx.Put(key, value)
-		return
-	}
+		if ldbs.tx != nil {
+			ldbs.tx.Put(key, value)
+			return
+		}
 
-	wo := leveldb.NewWriteOptions()
-	err = ldbs.backend.db.Put(wo, key, value)
-	wo.Close()
+		wo := leveldb.NewWriteOptions()
+		err = ldbs.backend.db.Put(wo, key, value)
+		wo.Close()
+	})
 
 	return
 }
@@ -112,8 +138,12 @@ func (ldbs *LevelDBSession) CommitTransaction() (err error) {
 		return
 	}
 	ldbs.tx = nil
-	wo := leveldb.NewWriteOptions()
-	err = ldbs.backend.db.Write(wo, tx)
-	wo.Close()
+	ldbs.backend.doWrite(func(db *leveldb.DB) {
+		wo := leveldb.NewWriteOptions()
+		wo.SetSync(true)
+		err = db.Write(wo, tx)
+		wo.Close()
+		tx.Close()
+	})
 	return
 }