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From Gabriel Silk <gs...@dropbox.com.INVALID>
Subject Re: Database referral integrity
Date Wed, 10 Apr 2019 21:31:46 GMT
The main concern I have with FK's is the potential performance impact.

When evaluating Airflow for use at Dropbox, I ran benchmarks that showed
several bottlenecks in the query patterns being used by the scheduler,
which led me to open https://issues.apache.org/jira/browse/AIRFLOW-2430 and
submit a PR.

If we move forward with FKs, we need to do a robust set of benchmarks to
understand how this will affect users who are running large Airflow
clusters.

On Wed, Apr 10, 2019 at 1:05 PM Alex Guziel <alex.guziel@airbnb.com.invalid>
wrote:

> I'm not a huge fan of having foreign keys. I know Airbnb has and definitely
> still has problems with DB load. I don't see any real convincing arguments
> for how adding referential integrity will improve Airflow meaningfully
> (yet).
>
> On Wed, Apr 10, 2019 at 12:38 PM Bas Harenslak <
> basharenslak@godatadriven.com> wrote:
>
> > In my experience it could go either way; in some cases FKs could impair
> > the performance and in some cases FKs can help the query optimizer
> improve
> > query performance. Each case is different and without testing it’s just
> > guessing.
> >
> > I’m in favour of adding FKs and value referential integrity over
> > performance. If you’re sacrificing integrity for performance you’re doing
> > either advanced funky stuff or the wrong thing. I haven’t seen the
> database
> > being a bottleneck in Airflow, even with large setups (+-5000 DAGs). So
> why
> > not add FKs and performance test some Airflow queries, to know for sure
> :-)
> >
> > Bas
> >
> > On 10 Apr 2019, at 20:39, Bolke de Bruin <bdbruin@gmail.com<mailto:
> > bdbruin@gmail.com>> wrote:
> >
> > Please not that Fks can be quite slow...
> >
> > Verstuurd vanaf mijn iPad
> >
> > Op 10 apr. 2019 om 19:55 heeft Ash Berlin-Taylor <ash@apache.org<mailto:
> > ash@apache.org>> het volgende geschreven:
> >
> > 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 <fokko@driesprong.frl
> <mailto:
> > fokko@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 <
> > maximebeauchemin@gmail.com<mailto:maximebeauchemin@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 <fokko@driesprong.frl
> > <mailto:fokko@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 <bdbruin@gmail.com
> > <mailto:bdbruin@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 <ash@apache.org<mailto:
> > ash@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 <mail@stefan-seelmann.de
> <mailto:
> > mail@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
> >
> >
> >
> >
> >
> >
> >
> >
>

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