retryWhen(errors => {
          return errors.pipe(
            switchMap((httpErrorResponse: HttpErrorResponse) => {

              if (httpErrorResponse.status === 503) {
                return Observable.of(true);
              }
              return Observable.throwError(httpErrorResponse);
            }),
            take(NUMBER_OF_RETRIES),
            concat(Observable.throwError(`please wait, server error 503`)),
          );
        },
      )