public function setPersistence($mode)
    {
        if (!in_array($mode, array(SOAP_PERSISTENCE_SESSION, SOAP_PERSISTENCE_REQUEST))) {
            require_once 'Zend/Soap/Server/Exception.php';
            throw new Zend_Soap_Server_Exception('Invalid persistence mode specified');
        }

        $this->_persistence = $mode;
        return $this;
    }