code: public Observable<ServiceResponse<Void>> beginDownloadWithServiceResponseAsync(String resourceGroupName, String virtualWANName, GetVpnSitesConfigurationRequest request) {
          if (this.client.subscriptionId() == null) {
              throw new IllegalArgumentException("Parameter this.client.subscriptionId() is required and cannot be null.");
          }
          if (resourceGroupName == null) {
              throw new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null.");
          }
          if (virtualWANName == null) {
              throw new IllegalArgumentException("Parameter virtualWANName is required and cannot be null.");
          }
          if (request == null) {
              throw new IllegalArgumentException("Parameter request is required and cannot be null.");
          }
          Validator.validate(request);
          final String apiVersion = "2018-07-01";
          return service.beginDownload(this.client.subscriptionId(), resourceGroupName, virtualWANName, request, apiVersion, this.client.acceptLanguage(), this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<Void>>>() {
                  @Override
                  public Observable<ServiceResponse<Void>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<Void> clientResponse = beginDownloadDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
docs: Gives the sas-url to download the configurations for vpn-sites in a resource group.
  
  @param resourceGroupName The resource group name.
  @param virtualWANName The name of the VirtualWAN for which configuration of all vpn-sites is needed.
  @param request Parameters supplied to download vpn-sites configuration.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceResponse} object if successful.