Misleading scheduler health message when using SequentialExecutor

[eladkal]

Body

In the UI we have a message that warns in case the scheduler isn't running:
The scheduler does not appear to be running. Last heartbeat was received X minutes ago.
https://github.com/apache/airflow/blame/5144bedcee76466a79eea0b2b6137e5899dc73bd/airflow/www/templates/airflow/main.html#L60-L71

This message however is confusing for SequentialExecutor.
In SequentialExecutor we can run only 1 task so while a task is running the scheduler is not sending health checks which results in this message but it make sense that there is no heartbeat because there is a task that occupy the slot.

My suggestion is to make the message be conditional

in case of Sqlite / Sequential executor + no active runs -> It's OK to show the message if scheduler is not live.
in case of Sqlite / Sequential executor + active run -> Message should not appear.

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[bbovenzi]

We should move this logic from main.html and handle it inside the webserver in views.py and pass a single variable of is_scheduler_running to the UI.