apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/ClusterService.java/ClusterService/findParentClusters
class ClusterService:  public List<Cluster> findParentClusters(String appId) {
    if (Strings.isNullOrEmpty(appId)) {
      return Collections.emptyList();
    }

    List<Cluster> clusters = clusterRepository.findByAppIdAndParentClusterId(appId, 0L);
    if (clusters == null) {
      return Collections.emptyList();
    }

    Collections.sort(clusters);

    return clusters;
  }