protected function getLongPollServer() {
        $params = array(
            static::PARAM_GROUP_ID => $this->group_id
        );

        $server = $this->api_client->groups()->getLongPollServer($this->access_token, $params);

        return array(
            static::SERVER_URL       => $server['server'],
            static::SERVER_TIMESTAMP => $server['ts'],
            static::SERVER_KEY       => $server['key'],
        );
    }