apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/ClusterService.java/ClusterService/findChildClusters
class ClusterService:  public List<Cluster> findChildClusters(String appId, String parentClusterName) {
    Cluster parentCluster = findOne(appId, parentClusterName);
    if (parentCluster == null) {
      throw new BadRequestException("parent cluster not exist");
    }

    return clusterRepository.findByParentClusterId(parentCluster.getId());
  }