code: void markFailed() {
      // It's possible that the dag is killed before this method is called.
      assertRunningOrKilling();
      changeStatus(Status.FAILURE);
      for (final Node child : this.children) {
        child.cancel();
      }
      //todo: HappyRay support failure options "Finish Current Running" and "Cancel All"
      this.dag.updateDagStatus();
    }
docs: Transitions the node to the failure state.