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From Kevin Yang <yrql...@gmail.com>
Subject Re: Airflow DAG Serialisation
Date Sat, 27 Jul 2019 09:42:56 GMT
Nice job Zhou!

Really excited, exactly what we wanted for the webserver scaling issue.
Want to add another big drive for Airbnb to start think about this
previously to support the effort: it can not only bring consistency between
webservers but also bring consistency between webserver and
scheduler/workers. It may be less of a problem if total DAG parsing time is
small, but for us the total DAG parsing time is 15+ mins and we had to set
the webserver( gunicorn subprocesses) restart interval to 20 mins, which
leads to a worst case 15+20+15=50 mins delay between scheduler start to
schedule things and users can see their deployed DAGs/changes...

I'm not so sure about the scheduler performance improvement: currently we
already feed the main scheduler process with SimpleDag through
DagFileProcessorManager running in a subprocess--in the future we feed it
with data from DB, which is likely slower( tho the diff should have
negligible impact to the scheduler performance). In fact if we'd keep the
existing behavior, try schedule only fresh parsed DAGs, then we may need to
deal with some consistency issue--dag processor and the scheduler race for
updating the flag indicating if the DAG is newly parsed. No big deal there
but just some thoughts on the top of my head and hopefully can be helpful.

And good idea on pre-rendering the template, believe template rendering was
the biggest concern in the previous discussion. We've also chose the
pre-rendering+JSON approach in our smart sensor API
<https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-17+Airflow+sensor+optimization>
and
seems to be working fine--a supporting case for ur proposal ;) There's a WIP
PR <https://github.com/apache/airflow/pull/5499> for it just in case you
are interested--maybe we can even share some logics.

Thumbs-up again for this and please don't heisitate to reach out if you
want to discuss further with us or need any help from us.


Cheers,
Kevin Y

On Sat, Jul 27, 2019 at 12:54 AM Driesprong, Fokko <fokko@driesprong.frl>
wrote:

> Looks great Zhou,
>
> I have one thing that pops in my mind while reading the AIP; should keep
> the caching on the webserver level. As the famous quote goes: *"There are
> only two hard things in Computer Science: cache invalidation and naming
> things." -- Phil Karlton*
>
> Right now, the fundamental change that is being proposed in the AIP is
> fetching the DAGs from the database in a serialized format, and not parsing
> the Python files all the time. This will give already a great performance
> improvement on the webserver side because it removes a lot of the
> processing. However, since we're still fetching the DAGs from the database
> in a regular interval, cache it in the local process, so we still have the
> two issues that Airflow is suffering from right now:
>
>    1. No snappy UI because it is still polling the database in a regular
>    interval.
>    2. Inconsistency between webservers because they might poll in a
>    different interval, I think we've all seen this:
>    https://www.youtube.com/watch?v=sNrBruPS3r4
>
> As I also mentioned in the Slack channel, I strongly feel that we should be
> able to render most views from the tables in the database, so without
> touching the blob. For specific views, we could just pull the blob from the
> database. In this case we always have the latest version, and we tackle the
> second point above.
>
> To tackle the first one, I also have an idea. We should change the DAG
> parser from a loop to something that uses inotify
> https://pypi.org/project/inotify_simple/. This will change it from polling
> to an event-driven design, which is much more performant and less resource
> hungry. But this would be an AIP on its own.
>
> Again, great design and a comprehensive AIP, but I would include the
> caching on the webserver to greatly improve the user experience in the UI.
> Looking forward to the opinion of others on this.
>
> Cheers, Fokko
>
>
>
>
>
>
>
>
> Op za 27 jul. 2019 om 01:44 schreef Zhou Fang <zhoufang@google.com.invalid
> >:
>
> > Hi Kaxi,
> >
> > Just sent out the AIP:
> >
> >
> https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-24+DAG+Persistence+in+DB+using+JSON+for+Airflow+Webserver+and+%28optional%29+Scheduler
> >
> > Thanks!
> > Zhou
> >
> >
> > On Fri, Jul 26, 2019 at 1:33 PM Zhou Fang <zhoufang@google.com> wrote:
> >
> > > Hi Kaxil,
> > >
> > > We are also working on persisting DAGs into DB using JSON for Airflow
> > > webserver in Google Composer. We target at minimizing the change to the
> > > current Airflow code. Happy to get synced on this!
> > >
> > > Here is our progress:
> > > (1) Serializing DAGs using Pickle to be used in webserver
> > > It has been launched in Composer. I am working on the PR to upstream
> it:
> > > https://github.com/apache/airflow/pull/5594
> > > Currently it does not support non-Airflow operators and we are working
> on
> > > a fix.
> > >
> > > (2) Caching Pickled DAGs in DB to be used by webserver
> > > We have a proof-of-concept implementation, working on an AIP now.
> > >
> > > (3) Using JSON instead of Pickle in (1) and (2)
> > > Decided to use JSON because Pickle is not secure and human readable.
> The
> > > serialization approach is very similar to (1).
> > >
> > > I will update the RP (https://github.com/apache/airflow/pull/5594) to
> > > replace Pickle by JSON, and send our design of (2) as an AIP next week.
> > > Glad to check together whether our implementation makes sense and do
> > > improvements on that.
> > >
> > > Thanks!
> > > Zhou
> > >
> > >
> > > On Fri, Jul 26, 2019 at 7:37 AM Kaxil Naik <kaxilnaik@gmail.com>
> wrote:
> > >
> > >> Hi all,
> > >>
> > >> We, at Astronomer, are going to spend time working on DAG
> Serialisation.
> > >> There are 2 AIPs that are somewhat related to what we plan to work on:
> > >>
> > >>    - AIP-18 Persist all information from DAG file in DB
> > >>    <
> > >>
> >
> https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-18+Persist+all+information+from+DAG+file+in+DB
> > >> >
> > >>    - AIP-19 Making the webserver stateless
> > >>    <
> > >>
> >
> https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-19+Making+the+webserver+stateless
> > >> >
> > >>
> > >> We plan to use JSON as the Serialisation format and store it as a blob
> > in
> > >> metadata DB.
> > >>
> > >> *Goals:*
> > >>
> > >>    - Make Webserver Stateless
> > >>    - Use the same version of the DAG across Webserver & Scheduler
> > >>    - Keep backward compatibility and have a flag (globally & at DAG
> > level)
> > >>    to turn this feature on/off
> > >>    - Enable DAG Versioning (extended Goal)
> > >>
> > >>
> > >> We will be preparing a proposal (AIP) after some research and some
> > initial
> > >> work and open it for the suggestions of the community.
> > >>
> > >> We already had some good brain-storming sessions with Twitter folks
> > (DanD
> > >> &
> > >> Sumit), folks from GoDataDriven (Fokko & Bas) & Alex (from Uber)
which
> > >> will
> > >> be a good starting point for us.
> > >>
> > >> If anyone in the community is interested in it or has some experience
> > >> about
> > >> the same and want to collaborate please let me know and join
> > >> #dag-serialisation channel on Airflow Slack.
> > >>
> > >> Regards,
> > >> Kaxil
> > >>
> > >
> >
>

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