[AIRFLOW-1424] add a next_scheduler_run property to DAGs

[ultrabug]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1424

Description

• Here are some details about my PR, including screenshots of any UI changes:

The scheduler's DAG run creation logic can be tricky and one is
easily confused with the start_date + interval
and period end scheduling way of thinking.

It would ease airflow's usage to add a next_scheduler_run property to DAGs
so that we can very easily see the (un)famous period end after which
the scheduler will create a new DAG run for our workflows.

These patches add such a field to the dag table and the DagModel
property + scheduler logic to update it when calculated.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Tests are proposed along this patches.

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[codecov-io]

Codecov Report

Merging #2457 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2457      +/-   ##
==========================================
+ Coverage    69.4%   69.42%   +0.02%     
==========================================
  Files         146      146              
  Lines       11289    11297       +8     
==========================================
+ Hits         7835     7843       +8     
  Misses       3454     3454

Impacted Files	Coverage Δ
airflow/models.py	87.27% <100%> (+0.03%)	⬆️
airflow/jobs.py	76.37% <100%> (+0.04%)	⬆️

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[bolkedebruin]

Please explain where the user wants to see this and how this is exposed? We already make the following schedule available in templates for operators as "next_execution_date"

[ultrabug]

Thanks for your reply @bolkedebruin.

The 2 main reasons are:

• Make it possible for this valuable information to be exposed on the web interface (see screenshot) so you can access it in a non programmatic way
• This information is very handy when you deploy your DAG for a later execution and want to make sure of the exact time after which it will be triggered.

To sum it up, having the "next_execution_date" in the code is cool but your code has to be executed in the first hand to get this information + it is not informative on a user / monitoring perspective.

Adding this on the dag table allows to immediately have this information available on the web UI like this:

[saguziel]

The dagbag has access to the webserver

[bolkedebruin]

and thus can derive this programmatically. No need for a scheduler and database change.

[ultrabug]

I'm sorry if this sounds obvious to you but would you mind giving a bit more details please guys?

Let's say I want this information displayed on the web UI, how would I proceed and avoid that schema change?

[ultrabug]

Also, would you be interested/okay for a PR adding this field in the web UI please?

[ultrabug]

I am closing this PR in favour to #2460 which implements the desired information without a schema change.