Option 2 clarification doc

This is a working document to clarify proposal option number 2 originally presented in AIP-83 amendment to support classic Airflow authoring style.

Proposal: restore uniqueness but make logical_date nullable and / or optional

Uncontroversial components:

• restore the uniqueness constraint on logical date
• make logical date nullable
• we will not immediately add timetables that don't use logical date

Quetions of controversy

Question 1. Default behavior when manually triggering a dag

Shall we make logical_date null by default, or only allow a user to optionally, explicitly, set logical_date to null?

Option A: logical date will continue to be populated by default for manual runs, but user can optionally set null when creating.

Option B: logical date will be null by default, though user can select one when triggering a run.

Question 2. Default behavior for asset-triggered dags

Option A: leave the existing behavior unchanged.  Asset-triggered dag runs would continue to have a logical date.

Option B: asset-triggered dag runs would no longer have a logical date, i.e. it would be null