protected function _getHandlerKey(Zend_Http_Response $response = null)
    {
        if ($response) {
            $code = $response->getStatus();
            if ($response->isSuccessful() || $response->isRedirect()) {
                return 'success';
            } elseif ($code < 500 && $code >= 400) {
                return 'client_error';
            } elseif ($code >= 500) {
                return 'server_error';
            }
            // @codeCoverageIgnoreStart
        }
        // @codeCoverageIgnoreEnd
        return 'no_response';
    }