private function _updateDbExpiration($sessionId)
    {
        Kwf_Benchmark::count('sessionhdl', '_updateDbExpiration');
        $expiration = $this->_lifeTime + time();
        Kwf_Registry::get('db')->query("UPDATE kwf_sessions SET expiration=? WHERE sessionId=?", array($expiration, $sessionId));

        //we store the time of the new expiration into the Memcache
        $this->_memcacheSet($sessionId, $this->_initSessionData);
    }