public function destroy($sessionId)
    {
        $this->_memcache->delete(Kwf_Cache_Simple::getUniquePrefix().'sess-'.$sessionId);
        $this->_memcache->delete(Kwf_Cache_Simple::getUniquePrefix().'sess-db-'.$sessionId);
        Kwf_Registry::get('db')->query("DELETE FROM kwf_sessions WHERE sessionId=?", $sessionId); 
        return true;
    }