@Override
    public Collection<String> getRequiredHostGroups(String propertyName,
                                                    String origValue,
                                                    Map<String, Map<String, String>> properties,
                                                    ClusterTopology topology) {
      if (origValue == null) {
        LOG.info("Property {} is null, skipping search for host group placeholder", propertyName);
        return Collections.emptyList();
      }

      //todo: getHostStrings
      Matcher m = HostGroup.HOSTGROUP_REGEX.matcher(origValue);
      Set<String> hostGroups = new HashSet<>();
      while (m.find()) {
        hostGroups.add(m.group(1));
      }
      return hostGroups;
    }
@return the default updater to apply on user defined (blueprint + cluster template, not stack defaults) properties
configuration properties that do not have a registered updater
ambari-server/src/main/java/org/apache/ambari/server/controller/internal/BlueprintConfigurationProcessor.java