apollo-portal/src/main/java/com/ctrip/framework/apollo/openapi/server/service/ServerClusterOpenApiService.java/ServerClusterOpenApiService/getCluster
class ServerClusterOpenApiService:  @Override
  public OpenClusterDTO getCluster(String appId, String env, String clusterName) {
    ClusterDTO clusterDTO = clusterService.loadCluster(appId, Env.valueOf(env), clusterName);
    return clusterDTO == null ? null : OpenApiBeanUtils.transformFromClusterDTO(clusterDTO);
  }