Re: [PR] Fix misleading timetable Variable example in best practices [airflow]

["ashb (via GitHub)" <gi...@apache.org>]

ashb commented on code in PR #73945:
URL: https://github.com/apache/airflow/pull/73945#discussion_r4229051549


##########
airflow-core/docs/best-practices.rst:
##########
@@ -463,9 +463,15 @@ for any variable that contains sensitive data.
 
 Timetables
 ----------
-Avoid using Airflow Variables/Connections or accessing Airflow database at the top level of your timetable code.
-Database access should be delayed until the execution time of the Dag. This means that you should not have variables/connections retrieval
-as argument to your timetable class initialization or have Variable/connection at the top level of your custom timetable module.
+Avoid using Airflow Variables/Connections or accessing Airflow database anywhere in your timetable code: at the
+top level of the module, in ``__init__``, and in scheduling methods such as ``next_dagrun_info``. The scheduler
+rebuilds your timetable from the serialized Dag by calling ``deserialize``, which calls ``__init__`` again, so
+this code runs in the scheduler, outside of any task. Retrieving a Variable or Connection there fails and can
+crash the Dag processor or the scheduler.

Review Comment:
   We should probably fix that -- it shouldn't crash the whole process ideally. Fancy another PR?



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