apollo-adminservice/src/main/java/com/ctrip/framework/apollo/adminservice/controller/NamespaceBranchController.java/NamespaceBranchController/checkNamespace
class NamespaceBranchController:  private void checkNamespace(String appId, String clusterName, String namespaceName) {
    Namespace parentNamespace = namespaceService.findOne(appId, clusterName, namespaceName);
    if (parentNamespace == null) {
      throw new BadRequestException(
          "Namespace not exist. AppId = %s, ClusterName = %s, NamespaceName = %s", appId,
          clusterName, namespaceName);
    }
  }