fix: Persist EventsTimetable's description during serialization

[kacpermuda]

When using EventsTimetable, the schedule in Airflow UI is not shown correctly/. My guess is that the EventsTimetable description (and therefore summary) not being serialized is to blame.

In Airflow 3 we are relying on timetable.summary when displaying schedule in the UI. Currently, even if user provide a description Airflow UI will always display "X events" as timetable_summary, instead of description provided by the user.

In Airflow 2 we kept the timetable summary as DAG attribute and serialize it separately, so it always showed up in UI correctly.

This is also causing discrepancies in OpenLineage events, for DagRun events emitted from scheduler (for Airflow 2 and 3), we are receiving "X events", and for task events emitted from worker, the user description.

Example DAG to test it:

import pendulum
from datetime import datetime

from airflow import DAG
from airflow.timetables.events import EventsTimetable
from airflow.providers.standard.operators.bash import BashOperator


DAG_ID = "timetable_dag"

with DAG(
    dag_id=DAG_ID,
    start_date=datetime(2021, 1, 1),
    schedule=EventsTimetable(
        description="Example description",
        event_dates=[pendulum.datetime(2095, 3, 1, tz="America/Chicago")]
    ),
    catchup=False,
) as dag:
    nothing_task = BashOperator(task_id="do_nothing_task", bash_command="sleep 1;")

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[kacpermuda]

Can we also backport it to 2.X as well if merged?

[uranusjr]

This fix is awkward. I'd prefer the serialisation logic to just store the provided value, and supply it correctly on deserialisation. This shouldn't require an extra flag.

[kacpermuda]

It's implemented this way because when providing description to the EventsTimetable, both _summary and description attributes are set to the provided description. When not provided, the _summary and description are generated, and different from each other. So if we serialize description regardless if it was provided or not, after deserializing the resulting summary will be different than before serialization if the description was not provided by the user. We would end up displaying the description (longer text) in the Airflow UI if we did that. We can also add the summary argument explicitly, but that was a bigger change imho.

[uranusjr]

Why not fix the value during deserialisation? Like

@classmethod
def deserialize(cls, data) -> Timetable:
    timetable = cls(...)
    timetable._summary = ...
    return timetable

[kacpermuda]

Thanks for the advice, that make sense. Adjusted the code

[kacpermuda]

Anything else that needs to be adjusted?

[kacpermuda]

Should we also backport it to 2.11 or fix it there? I think the difference is not visible on the UI but f.e. when listener is accessing dag timetable summary.

[uranusjr]

I don’t have an opinion either way. This is not very meaningful from the user’s perspective, but also very easy to backport so why not.

[kacpermuda]

Okay, I think it will be useful, so if it's not a big effort then let's do it. Does this PR need a specific label only or there is something else that needs to be done?

[uranusjr]

I think this should work, the rest will be automated after this is merged.

[kacpermuda]

Should we merge it?

[github-actions]

Backport successfully created: v3-0-test

Status	Branch	Result
✅	v3-0-test