I feel we're going a bit off topic here. Although it is good to discuss the
possibilities.
>From my perspective the AIP tries to kill two birds with one stone:
1. Decoupling the web-server from the actual Python DAG files
2. Versioning the DAGs so we can have a historical view of the dags as
they are executed. For example, if you now deploy a new dag, and you rename
an operator, the old name will disappear from the tree view, and you will
get a new row which has no status (until you trigger a run).
They are related but require need different solutions. Personally, I'm most
interested in the first so we can make the webserver (finally) stateless.
The PR tries to remove the need for having the actual python for the
different views. Some of them are trivial, such as the graph and tree view.
Some of them are more tricky, such as the task instance details and the
rendered view because they pull a lot of information from the DAG object,
and it involves jinja templating.
The main goal of the PR is to store this kind of metadata when the
scheduler kicks off the dag. So when a new DagRun is being created, the
latest version of the dag is loaded from the actual python file on disk by
the scheduler. The file is executed and the DAG is being persisted into the
database in a structured way. By moving this into the database, we can
eventually decouple the webserver from the actual dag files, which greatly
simplifies deployment and removes the state since this is now in the (ACID
compliant) database. Furthermore, instead of whitelisting certain classes,
we control the serialization ourselves by knowing what we push to the
database instead of having to push a pickled blob. Besides that, from a
performance perspective, as Max already pointed out, pickling can
potentially be expensive in terms of memory and CPU.
> Since it's pretty clear we need SimpleDAG serialization, and we can see
> through the requirements, people can pretty much get started on this.
I'm afraid if we go this route, then the SimpleDag will be the actual Dag
in the end (but then a slightly smaller and serializable version of it). My
preference would be to simplify the DAG object and get rid of the BaseDag
and SimpleDag to simplify the object hierarchy.
Cheers, Fokko
Op wo 27 feb. 2019 om 21:23 schreef Maxime Beauchemin <
maximebeauche...@gmail.com>:
> 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 <ddavy...@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 <
> > maximebeauche...@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 <yrql...@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 <a...@apache.org>
> > > wrote:
> > > >
> > > > >
> > > > > > On 26 Feb 2019, at 09:37, Kevin Yang <yrql...@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
> > > >
> > >
> >
>
