code: public static boolean available(String groupName) {
          return LocalUnitsManager.getGroupByName(groupName) != null ||
                  (GroupDiscovery.singleton != null && GroupDiscovery.singleton.firstInstance(groupName) != null);
      }
docs: check whether the specified unit is available.
  tips: don't use exception catching to get the result, cause this is bad performance.
  We check local first and then remote.