I am all for FKs. How do you think we should handle the case of "Xcom but missing TIs" (or similar) that we might run into on installs when a user runs `airflow upgradedb`?
-a > On 10 Apr 2019, at 18:44, Driesprong, Fokko <fo...@driesprong.frl> wrote: > > Reviving this discussion again :-) > > Lately, I was digging into the PR of Julian regarding adding FK's to the > database: https://github.com/apache/airflow/pull/4922 > > After digging into the details, I've noticed that the current situation > regarding referential integrity is bad. It is not uncommon the have > DagRun's without having the DAG in the database. For example, you can do a > backfill job before the scheduler persisted the DAG in the database. I also > think this is often the case in the UI, where we the nuke when some of the > models haven't been persisted in the database. Therefore I'd like to > suggest to enforce consistency by foreign keys. This will prevent us from > having DagRuns without DAGs, but also removing xcom objects of > TaskInstances that are already removed. > > To create an overview of the FK's, I've created subtasks the ticket of > Peter: https://jira.apache.org/jira/browse/AIRFLOW-3904 > > WDYT? > > Cheers, Fokko > > > > Op di 18 sep. 2018 om 21:51 schreef Maxime Beauchemin < > maximebeauche...@gmail.com>: > >> The database migration creating the FK will/would need to have something >> that either creates dummy missing PKs first, or delete the orphaned keys to >> insure the operation of creating the FK doesn't error out. Seems like >> adding dummy keys is a better approach. Then you'll have to make sure that >> everywhere where FKs are created that there are no edge cases of missing >> PKs. Then some delete operations in some cases may have to "cascade" >> properly. >> >> The Django Admin had this nice confirm screen on delete that would show you >> clearly the scope of the cascading operation when deleting objects. To my >> knowledge Flask-Admin and FAB don't have such a feature. Personally I >> wouldn't allow cascade unless such a feature is implemented in some way. >> Note that SQLAlchemy has builtin semantics for specifying how/whether >> cascading should take place. >> >> Personally I think referential integrity is overrated in some cases, >> especially when using meaningful "business keys" (as opposed to >> auto-increted "surrogate" keys) as PKs. It also slows down insert >> operations. For data warehousing (this clearly doesn't apply to the Airflow >> metadata store), the best practice on most db engine is to **not** enforce >> FK constraints as it slows down inserts in fact tables and straight out >> prevents bulk loading. >> >> Another approach is to avoid deleting in general, especially referenced >> fks, and setting up some activity/visibility flag to false instead. >> >> Max >> >> On Tue, Sep 18, 2018 at 10:47 AM Driesprong, Fokko <fo...@driesprong.frl> >> wrote: >> >>> I'm in favor of having referential integrity. It will add some load in >>> having to enforce the referential integrity, but it will also make sure >>> that the database stays clean. Also in Airflow we use transactions which >>> will make sure that the integrity checks are not validated on every >>> statement, but after the commit. I'm happy to help with this as well. >>> >>> Cheers, Fokko >>> >>> Op di 18 sep. 2018 om 11:07 schreef Bolke de Bruin <bdbr...@gmail.com>: >>> >>>> Adding these kind of checks which work for integrity well make database >>>> access pretty slow. In addition it isnt there because in the past there >>> was >>>> no strong connection between for example tasks and dagruns, it was more >>> or >>>> less just coincidental. There also so some bisecting tools that >> probably >>>> have difficulty functioning in a new regime. In other words it is not >> an >>>> easy change and it will have operational challenges. >>>> >>>>> On 18 Sep 2018, at 11:03, Ash Berlin-Taylor <a...@apache.org> wrote: >>>>> >>>>> Ooh good spot. >>>>> >>>>> Yes I would be in favour of adding these, but as you say we need to >>>> thing about how we might migrate old data. >>>>> >>>>> Doing this at 2.0.0 and providing a cleanup script (or doing it as >> part >>>> of the migration?) is probably the way to go. >>>>> >>>>> -ash- >>>>> >>>>>> On 17 Sep 2018, at 19:56, Stefan Seelmann <m...@stefan-seelmann.de> >>>> wrote: >>>>>> >>>>>> Hi, >>>>>> >>>>>> looking into the DB schema there is almost no referral integrity >>>>>> enforced at the database level. Many foreign key constraints between >>>>>> dag, dag_run, task_instance, xcom, dag_pickle, log, etc would make >>> sense >>>>>> IMO. >>>>>> >>>>>> Is there a particular reason why that's not implemented? >>>>>> >>>>>> Introducing it now will be hard, probably any real-world setup has >>> some >>>>>> violations. But I'm still in favor of this additional safety net. >>>>>> >>>>>> Kind Regards, >>>>>> Stefan >>>>> >>>> >>>> >>> >>
