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From Boris Tyukin <bo...@boristyukin.com>
Subject Re: variable scope with dynamic dags
Date Wed, 22 Mar 2017 15:12:11 GMT
hi Jeremiah, thanks for the explanation!

i am very new to Python so was surprised that it works and my external
dictionary object was still accessible to all dags generated. I think it
makes sense but I would like to confirm one thing and I do not know how to
test it myself.

do you think that large dictionary object will still be loaded to memory
only once even if I generate 200 dags that will be accessing it? so
basically they will just use a reference to it or they would create a copy
of the same 60Mb structure.

I hope my question makes sense :)

On Wed, Mar 22, 2017 at 10:54 AM, Jeremiah Lowin <jlowin@apache.org> wrote:

> At the risk of oversimplifying things, your DAG definition file is loaded
> *every* time a DAG (or any task in that DAG) is run. Think of it as a
> literal Python import of your dag-defining module: any variables are loaded
> along with the DAGs, which are then executed. That's why your dict is
> always available. This will work with Celery since it follows the same
> approach, parsing your DAG file to run each task.
>
> (By the way, this is why it's critical that all parts of your Airflow
> infrastructure have access to the same DAGS_FOLDER)
>
> Now it is true that the DagBag loads DAG objects but think of it as more of
> an "index" so that the scheduler/webserver know what DAGs are available.
> When it's time to actually run one of those DAGs, the executor loads it
> from the underlying source file.
>
> Jeremiah
>
> On Wed, Mar 22, 2017 at 8:45 AM Boris Tyukin <boris@boristyukin.com>
> wrote:
>
> > Hi,
> >
> > I have a weird question but it bugs my mind. I have some like below to
> > generate dags dynamically, using Max's example code from FAQ.
> >
> > It works fine but I have one large dict (let's call it my_outer_dict)
> that
> > takes over 60Mb in memory and I need to access it from all generated
> dags.
> > Needless to say, i do not want to recreate that dict for every dag as I
> > want to load it to memory only once.
> >
> > To my surprise, if i define that dag outside of my dag definition code, I
> > can still access it.
> >
> > Can someone explain why and where is it stored? I thought only dag
> > definitions are loaded to dagbag and not the variables outside it.
> >
> > Is it even a good practice and will it work still if I switch to celery
> > executor?
> >
> >
> > def get_dag(i):
> >     dag_id = 'foo_{}'.format(i)
> > dag = DAG(dag_id)
> > ....
> > print my_outer_dict
> >
> > my_outer_dict = {}
> > for i in range(10):
> > dag = get_dag(i)
> >     globals()[dag.dag_id] = dag
> >
>

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