code: public static Completable begin(String daoGroup) {
          return SingleRxXian.call(daoGroup, "BeginTransaction").flatMapCompletable(unitResponse -> {
              if (unitResponse.succeeded()) {
                  return Completable.complete();
              } else {
                  throw new RuntimeException("Begin Transaction failure");
              }
          });
      }
docs: Begin the given dao group's node's transaction. If currently the transaction is not existed then create one.
  This method is reentrant and must be paired with {@link #commit(String daoGroup)} method.
  
  @param daoGroup dao group name. This helps to locate which dao application node to sent this request.
  @return deferred transaction beginning result