apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/service/ItemService.java/ItemService/compare
class ItemService:  public List<ItemDiffs> compare(List<NamespaceIdentifier> comparedNamespaces, List<ItemDTO> sourceItems) {

    List<ItemDiffs> result = new LinkedList<>();

    for (NamespaceIdentifier namespace : comparedNamespaces) {

      ItemDiffs itemDiffs = new ItemDiffs(namespace);
      try {
        itemDiffs.setDiffs(parseChangeSets(namespace, sourceItems));
      } catch (BadRequestException e) {
        itemDiffs.setDiffs(new ItemChangeSets());
        itemDiffs.setExtInfo("该集群下没有名为 " + namespace.getNamespaceName() + " 的namespace");
      }
      result.add(itemDiffs);
    }

    return result;
  }