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From Maxime Beauchemin <maximebeauche...@gmail.com>
Subject Re: [DISCUSS] AIP-12 Persist DAG into DB
Date Wed, 27 Feb 2019 20:23:25 GMT
I fully agree on all your points Dan.

First about PEX vs Docker, Docker is a clear choice here. It's a superset
of what PEX can do (PEX is limited to python env) and a great standard that
has awesome tooling around it, works natively in k8s, which is becoming the
preferred executor. PEX is a bit of a hack and has failed to become a
standard. I don't think anyone would argue for PEX over Docker in almost
any context where this question would show up nowadays (beyond Airflow too).

And pickles have a lot of disadvantages over docker:
* some objects are not picklable (JinjaTemplates!)
* lack of visibility / tooling on building them, you might make one call
and get a 500mb pickle, and have not clue why it's so big
* unlike docker, they are impossible to introspect (as far as I know), you
have to intercept the __deepcopy__  method, good luck!
* pickles require a very similar environment to be rehydrated (same GCC
version, same modules/version available?)
* weird side effects and unknown boundaries. If I pickled a DAG on an older
version of airflow and code logic got included, and restore it on a newer,
could the new host be oddly downgraded as a result? when restored, did it
affect the whole environment / other DAGs?

My vote goes towards something like SimpleDAG serialization (for web server
and similar use cases) + docker images built with top of a lightweight SDK
as the way to go.

Since it's pretty clear we need SimpleDAG serialization, and we can see
through the requirements, people can pretty much get started on this.

The question of how to bring native Docker support to Airflow is a bit more
complex. I think the k8s executor has support for custom DAG containers but
idk how it works as I haven't look deeply into this recently. Docker
support in Airflow is a tricky and important question, and it offers an
opportunity to provide solutions to long standing issues around versioning,
test/dev user workflows, and more.

Related docker questions:
* what's the docker interface contract? what entry point have to exist in
the image?
* does airflow provide tooling to help bake images that enforce that
contract?
* do we need docker tag semantics in the DSL? does it look something like
`DAG(id='mydag', docker_tag=hub.docker.com://org/repo/tag', ...)`
* is docker optional or required? (probably optional at first)
* should docker support be k8s-executor centric? work the same across
executor? are we running docker-in-docker as a result / penalty in k8s?

Max

On Wed, Feb 27, 2019 at 11:38 AM Dan Davydov <ddavydov@twitter.com.invalid>
wrote:

> >
> > * on the topic of serialization, let's be clear whether we're talking
> about
> > unidirectional serialization and *not* deserialization back to the
> object.
> > This works for making the web server stateless, but isn't a solution
> around
> > how DAG definition get shipped around on the cluster (which would be nice
> > to have from a system standpoint, but we'd have to break lots of dynamic
> > features, things like callbacks and attaching complex objects to DAGs,
> ...)
>
> I feel these dynamic features are not worth the tradeoffs, and in most
> cases have alternatives, e.g. on_failure_callback can be replaced by a task
> with a ONE_FAILURE trigger rule, which gives additional advantages that
> first-class Airflow tasks have like retries. That being said, we should
> definitely do our due diligence weighing the trade-offs and coming up with
> alternatives for any feature we disable (jinja templating related to
> webserver rendering, callbacks, etc). I remember speaking to Alex about
> this and he agreed that the consistency/auditing/isolation guarantees were
> worth losing some features, I think Paul did as well. Certainly we will
> need to have a discussion/vote with the rest of the committers.
>
> My initial thinking is that both the DAG topology serialization (i.e.
> generating and storing SimpleDag in the DB for each DAG), and linking each
> DAG with a pex/docker image/etc as well as authentication tokens should
> happen at the same place, probably the client runs some command that will
> generate SimpleDag as well as a container, and then sends it to some
> Airflow Service that stores all of this information appropriately. Then
> Scheduler/Webserver/Worker consume the stored SimpleDAgs, and Workers
> consume the containers in addition.
>
> * docker as "serialization" is interesting, I looked into "pex" format in
> > the past. It's pretty cool to think of DAGs as micro docker application
> > that get shipped around and executed. The challenge with this is that it
> > makes it hard to control Airflow's core. Upgrading Airflow becomes [also]
> > about upgrading the DAG docker images. We had similar concerns with
> "pex".
> > The data platform team looses their handle on the core, or has to get in
> > the docker building business, which is atypical. For an upgrade, you'd
> have
> > to ask/force the people who own the DAG dockers to upgrade their images,
>
> The container vs Airflow versioning problem I believe is just an API
> versioning problem. I.e. you don't necessarily have to rebuild all
> containers when you bump version of airflow as long as the API is backwards
> compatible). I think this is reasonable for a platform like Airflow, and
> not sure there is a great way to avoid it if we want other nice system
> guarantees (e.g. reproducibility).
>
> Contract could be like "we'll only run
> > your Airflow-docker-dag container if it's in a certain version range" or
> > something like that. I think it's a cool idea. It gets intricate for the
> > stateless web server though, it's a bit of a mind bender :) You could ask
> > the docker to render the page (isn't that crazy?!) or ask the docker for
> a
> > serialized version of the DAG that allows you to render the page (similar
> > to point 1).
>
> If the webserver uses the SimpleDag representation that is generated at the
> time of DAG creation, then you can avoid having Docker needing to provide
> this serialized version, i.e. you push the responsibility to the client to
> have the right dependencies in order to build the DAG which I feel is good.
> One tricky thing I can think of is if you have special UI elements related
> to the operator type of a task (I saw a PR out for this recently), you
> would need to solve the API versioning problem separately for this as well
> (i.e. make sure the serialized DAG representation works with the version of
> the newest operator UI).
>
> * About storing in the db, for efficiency, the pk should be the SHA of the
> > deterministic serialized DAG. Only store a new entry if the DAG has
> > changed, and stamp the DagRun to a FK of that serialized DAG table. If
> > people have shapeshifting DAG within DagRuns we just do best effort, show
> > them the last one or something like that
>
> If we link each dagrun to it's "container" and "serialized representation"
> then the web UI can actually iterate through each dagrun and even render
> changes in topology. I think at least for v1 we can just use the current
> solution as you mentioned (best effort using the latest version).
>
> * everyone hates pickles (including me), but it really almost works, might
> > be worth revisiting, or at least I think it's good for me to list out the
> > blockers:
> >     * JinjaTemplate objects are not serializable for some odd obscure
> > reason, I think the community can solve that easily, if someone wants a
> > full brain dump on this I can share what I know
>
> What was the preference for using Pickle over Docker/PEX for serialization?
> I think we discussed this a long time ago with Paul but I forget the
> rationale and it would be good to have the information shared publicly too.
> One big problem is you don't get isolation at the binary dependency level,
> i.e. .so/.dll dependencies, along with all of the other problems you
> listed.
>
> On Tue, Feb 26, 2019 at 8:55 PM Maxime Beauchemin <
> maximebeauchemin@gmail.com> wrote:
>
> > Related thoughts:
> >
> > * on the topic of serialization, let's be clear whether we're talking
> about
> > unidirectional serialization and *not* deserialization back to the
> object.
> > This works for making the web server stateless, but isn't a solution
> around
> > how DAG definition get shipped around on the cluster (which would be nice
> > to have from a system standpoint, but we'd have to break lots of dynamic
> > features, things like callbacks and attaching complex objects to DAGs,
> ...)
> >
> > * docker as "serialization" is interesting, I looked into "pex" format in
> > the past. It's pretty cool to think of DAGs as micro docker application
> > that get shipped around and executed. The challenge with this is that it
> > makes it hard to control Airflow's core. Upgrading Airflow becomes [also]
> > about upgrading the DAG docker images. We had similar concerns with
> "pex".
> > The data platform team looses their handle on the core, or has to get in
> > the docker building business, which is atypical. For an upgrade, you'd
> have
> > to ask/force the people who own the DAG dockers to upgrade their images,
> > else they won't run or something. Contract could be like "we'll only run
> > your Airflow-docker-dag container if it's in a certain version range" or
> > something like that. I think it's a cool idea. It gets intricate for the
> > stateless web server though, it's a bit of a mind bender :) You could ask
> > the docker to render the page (isn't that crazy?!) or ask the docker for
> a
> > serialized version of the DAG that allows you to render the page (similar
> > to point 1).
> >
> > * About storing in the db, for efficiency, the pk should be the SHA of
> the
> > deterministic serialized DAG. Only store a new entry if the DAG has
> > changed, and stamp the DagRun to a FK of that serialized DAG table. If
> > people have shapeshifting DAG within DagRuns we just do best effort, show
> > them the last one or something like that
> >
> > * everyone hates pickles (including me), but it really almost works,
> might
> > be worth revisiting, or at least I think it's good for me to list out the
> > blockers:
> >     * JinjaTemplate objects are not serializable for some odd obscure
> > reason, I think the community can solve that easily, if someone wants a
> > full brain dump on this I can share what I know
> >     * Size: as you pickle something, someone might have attached things
> > that recurse into hundreds of GBs-size pickle. Like some
> > on_failure_callback may bring in the whole Slack api library. That can be
> > solved or mitigated in different ways. At some point I thought I'd have a
> > DAG.validate() method that makes sure that the DAG can be pickled, and
> > serialized to a reasonable size pickle. I also think we'd have to make
> sure
> > operators are defined as more "abstract" otherwise the pickle includes
> > things like the whole pyhive lib and all sorts of other deps. It could be
> > possible to limit what gets attached to the pickle (whitelist classes),
> and
> > dehydrate objects during serialization / and rehydrate them on the other
> > size (assuming classes are on the worker too). If that sounds crazy to
> you,
> > it's because it is.
> >
> > * the other crazy idea is thinking of git repo (the code itself) as the
> > serialized DAG. There are git filesystem in userspace [fuse] that allow
> > dynamically accessing the git history like it's just a folder, as in
> > `REPO/{ANY_GIT_REF}/dags/mydag.py` . Beautifully hacky. A company with a
> > blue logo with a big F on it that I used to work at did that. Talking
> about
> > embracing config-as-code! The DagRun can just stamp the git SHA it's
> > running with.
> >
> > Sorry about the confusion, config as code gets tricky around the corners.
> > But it's all worth it, right? Right!? :)
> >
> > On Tue, Feb 26, 2019 at 3:09 AM Kevin Yang <yrqls21@gmail.com> wrote:
> >
> > > My bad, I was misunderstanding a bit and mixing up two issues. I was
> > > thinking about the multiple runs for one DagRun issue( e.g. after we
> > clear
> > > the DagRun).
> > >
> > > This is an orthogonal issue. So the current implementation can work in
> > the
> > > long term plan.
> > >
> > > Cheers,
> > > Kevin Y
> > >
> > > On Tue, Feb 26, 2019 at 2:34 AM Ash Berlin-Taylor <ash@apache.org>
> > wrote:
> > >
> > > >
> > > > > On 26 Feb 2019, at 09:37, Kevin Yang <yrqls21@gmail.com> wrote:
> > > > >
> > > > > Now since we're already trying to have multiple graphs for one
> > > > > execution_date, maybe we should just have multiple DagRun.
> > > >
> > > > I thought that there is exactly 1 graph for a DAG run - dag_run has a
> > > > "graph_id" column
> > >
> >
>

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