apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/ClusterService.java/ClusterService/findClusters
class ClusterService:  public List<Cluster> findClusters(String appId) {
    List<Cluster> clusters = clusterRepository.findByAppId(appId);

    if (clusters == null) {
      return Collections.emptyList();
    }

    // to make sure parent cluster is ahead of branch cluster
    Collections.sort(clusters);

    return clusters;
  }