code: public Observable<JSONObject> sendInBackground() {
      try {
        Map<String, Object> map = postDataMap();
        return PaasClient.getPushClient().sendPushRequest(map);
      } catch (Exception ex) {
        return Observable.error(ex);
      }
    }
docs: Sends this push notification in a background thread. This is preferable to using send(), unless
  your code is already running from a background thread.