private String getMinimalUpgradeCatalogVersion(){
    List<UpgradeCatalog> candidateCatalogs = new ArrayList<>(allUpgradeCatalogs);
    Collections.sort(candidateCatalogs, new AbstractUpgradeCatalog.VersionComparator());

    if (candidateCatalogs.isEmpty()) {
      return null;
    }

    return candidateCatalogs.iterator().next().getTargetVersion();
  }
Returns minimal version of available {@link UpgradeCatalog}

@return string representation of minimal version of {@link UpgradeCatalog}
ambari-server/src/main/java/org/apache/ambari/server/upgrade/SchemaUpgradeHelper.java