diff --git a/tiramisu/storage/sqlite3/storage.py b/tiramisu/storage/sqlite3/storage.py
index 09eb2b4..897de6d 100644
--- a/tiramisu/storage/sqlite3/storage.py
+++ b/tiramisu/storage/sqlite3/storage.py
@@ -22,6 +22,10 @@ import sqlite3
 from ...error import ConflictError
 
 
+global CONN
+CONN = None
+
+
 class Setting:
     """:param extension: database file extension (by default: db)
     :param dir_database: root database directory (by default: /tmp)
@@ -35,6 +39,12 @@ class Setting:
         self.dir_database = '/tmp'
         self.name = 'tiramisu'
 
+    def __setattr__(self, key, value):
+        if CONN is not None:
+            raise Exception(_('cannot change setting when connexion is already '
+                              'opened'))
+        super().__setattr__(key, value)
+
 
 SETTING = Setting()
 
@@ -66,8 +76,6 @@ def delete_session(session_id,
         CONN.commit()
     cursor.close()
 
-global CONN
-CONN = None
 
 class Storage(object):
     __slots__ = ('_conn', '_cursor', 'persistent', 'session_id', 'session_name', 'created')