apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/util/WatchKeysUtil.java/WatchKeysUtil/findPublicConfigWatchKeys
class WatchKeysUtil:  private Multimap<String, String> findPublicConfigWatchKeys(String applicationId,
                                                             String clusterName,
                                                             Set<String> namespaces,
                                                             String dataCenter) {
    Multimap<String, String> watchedKeysMap = HashMultimap.create();
    List<AppNamespace> appNamespaces = appNamespaceService.findPublicNamespacesByNames(namespaces);

    for (AppNamespace appNamespace : appNamespaces) {
      //check whether the namespace's appId equals to current one
      if (Objects.equals(applicationId, appNamespace.getAppId())) {
        continue;
      }

      String publicConfigAppId = appNamespace.getAppId();

      watchedKeysMap.putAll(appNamespace.getName(),
          assembleWatchKeys(publicConfigAppId, clusterName, appNamespace.getName(), dataCenter));
    }

    return watchedKeysMap;
  }