I agree that SubDAGs are an overly complex abstraction. I think what is needed/useful is a TaskGroup concept. On a high level I think you want this functionality:
- Tasks can be added to a TaskGroup - You *can* have dependencies between Tasks in the same TaskGroup, but *cannot* have dependencies between a Task in a TaskGroup and either a Task in a different TaskGroup or a Task not in any group - You *can* have dependencies between a TaskGroup and either other TaskGroups or Tasks not in any group - The UI will by default render a TaskGroup as a single "object", but which you expand or zoom into in some way - You'd need some way to determine what the "status" of a TaskGroup was at least for UI display purposes Not sure if it would need to be a top level object with its own database table and model or just another attribute on tasks. I think you could build it in a way such that from the schedulers point of view a DAG with TaskGroups doesn't get treated any differently. So it really just becomes a shortcut for setting dependencies between sets of Tasks, and allows the UI to simplify the render of the DAG structure. Chris On Fri, Jun 12, 2020 at 12:12 PM Dan Davydov <ddavy...@twitter.com.invalid> wrote: > Agree with James (and think it's actually the more important issue to fix), > but I am still convinced Ash' idea is the right way forward (just it might > require a bit more work to deprecate than adding visual grouping in the > UI). > > There was a previous thread about this FYI with more context on why subdags > are bad and potential solutions: > https://www.mail-archive.com/dev@airflow.apache.org/msg01202.html . A > solution I outline there to Jame's problem is e.g. enabling the >> operator > for Airflow operators to work with DAGs as well. I see this being separate > from Ash' solution for DAG grouping in the UI but one of the two items > required to replace all existing subdag functionality. > > I've been working with subdags for 3 years and they are always a giant pain > to use. They are a constant source of user confusion and breakages during > upgrades. Would love to see them gone :). > > On Fri, Jun 12, 2020 at 11:11 AM James Coder <jcode...@gmail.com> wrote: > > > I'm not sure I totally agree it's just a UI concept. I use the subdag > > operator to simplify dependencies too. If you have a group of tasks that > > need to finish before another group of tasks start, using a subdag is a > > pretty quick way to set those dependencies and I think also make it > easier > > to follow the dag code. > > > > On Fri, Jun 12, 2020 at 9:53 AM Kyle Hamlin <hamlin...@gmail.com> wrote: > > > > > I second Ash’s grouping concept. > > > > > > On Fri, Jun 12, 2020 at 5:10 AM Ash Berlin-Taylor <a...@apache.org> > > wrote: > > > > > > > Question: > > > > > > > > Do we even need the SubDagOperator anymore? > > > > > > > > Would removing it entirely and just replacing it with a UI grouping > > > > concept be conceptually simpler, less to get wrong, and closer to > what > > > > users actually want to achieve with subdags? > > > > > > > > With your proposed change, tasks in subdags could start running in > > > > parallel (a good change) -- so should we not also just _enitrely_ > > remove > > > > the concept of a sub dag and replace it with something simpler. > > > > > > > > Problems with subdags (I think. I haven't used them extensively so > may > > > > be wrong on some of these): > > > > - They need their own dag_id, but it has(?) to be of the form > > > > `parent_dag_id.subdag_id`. > > > > - They need their own schedule_interval, but it has to match the > parent > > > dag > > > > - Sub dags can be paused on their own. (Does it make sense to do > this? > > > > Pausing just a sub dag would mean the sub dag would never execute, so > > > > the SubDagOperator would fail too. > > > > - You had to choose the executor to operator a subdag with -- always > a > > > > bit of a kludge. > > > > > > > > Thoughts? > > > > > > > > -ash > > > > > > > > On Jun 12 2020, at 12:01 pm, Ash Berlin-Taylor <a...@apache.org> > wrote: > > > > > > > > > Workon sub-dags is much needed, I'm excited to see how this > > progresses. > > > > > > > > > > > > > > >> - *Unpack SubDags during dag parsing*: This rewrites the > > > > *DagBag.bag_dag* > > > > >> method to unpack subdag while parsing, and it will give a flat > > > > >> structure at > > > > >> the task level > > > > > > > > > > The serialized_dag representation already does this I think. At > least > > > if > > > > > I've understood your idea here correctly. > > > > > > > > > > -ash > > > > > > > > > > > > > > > On Jun 12 2020, at 9:51 am, Xinbin Huang <bin.huan...@gmail.com> > > > wrote: > > > > > > > > > >> Hi everyone, > > > > >> > > > > >> Sending a message to everyone and collect feedback on the AIP-34 > on > > > > >> rewriting SubDagOperator. This was previously briefly mentioned in > > the > > > > >> discussion about what needs to be done for Airflow 2.0, and one of > > the > > > > >> ideas is to make SubDagOperator attach tasks back to the root DAG. > > > > >> > > > > >> This AIP-34 focuses on solving SubDagOperator related issues by > > > > reattaching > > > > >> all tasks back to the root dag while respecting dependencies > during > > > > >> parsing. The original grouping effect on the UI will be achieved > > > through > > > > >> grouping related tasks by metadata. > > > > >> > > > > >> This also makes the dag_factory function more reusable because you > > > don't > > > > >> need to have parent_dag_name and child_dag_name in the function > > > > signature > > > > >> anymore. > > > > >> > > > > >> Changes proposed: > > > > >> > > > > >> - *Unpack SubDags during dag parsing*: This rewrites the > > > > *DagBag.bag_dag* > > > > >> method to unpack subdag while parsing, and it will give a flat > > > > >> structure at > > > > >> the task level > > > > >> - *Simplify SubDagOperator*: The new SubDagOperator acts like a > > > > >> container and most of the original methods are removed. The > > > > >> signature is > > > > >> also changed to *subdag_factory *with *subdag_args *and > > > > *subdag_kwargs*. > > > > >> This is similar to the PythonOperator signature. > > > > >> - *Add a TaskGroup model and add current_group & parent_group > > > > attributes > > > > >> to BaseOperator*: This metadata is used to group tasks for > > > > >> rendering at > > > > >> UI level. It may potentially extend further to group arbitrary > > tasks > > > > >> outside the context of subdag to allow group-level operations > > (i.e. > > > > >> stop/trigger a group of task within the dag) > > > > >> - *Webserver UI for SubDag*: Proposed UI modification to allow > > > > >> (un)collapse a group of tasks for a flat structure to pair with > > the > > > > first > > > > >> change instead of the original hierarchical structure. > > > > >> > > > > >> > > > > >> Please see related documents and PRs for details: > > > > >> AIP: > > > > >> > > > > > > > > > > https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-34+Rewrite+SubDagOperator > > > > >> > > > > >> Original Issue: https://github.com/apache/airflow/issues/8078 > > > > >> Draft PR: https://github.com/apache/airflow/pull/9243 > > > > >> > > > > >> Please let me know if there are any aspects that you > agree/disagree > > > > >> with or > > > > >> need more clarification (especially the third change regarding > > > > TaskGroup). > > > > >> Any comments are welcome and I am looking forward to it! > > > > >> > > > > >> Cheers > > > > >> Bin > > > > >> > > > > > > > -- > > > Kyle Hamlin > > > > > >
