GitHub user potiuk added a comment to the discussion: Occasional Airflow tasks: 
Upstream tasks succeed and downstream tasks are not scheduled

I guess you are talking about "mini-scheduler" only - your downstream tasks are 
still getting scheduled but not immediately after task completes. Your 
downstream tasks will be scheduled by the next scheduler loop I presume.

If so. Yes. It might happen. "Mini-scheduler" is an optimisation that is not 
guaranteed to work. In fact in Airlfow 3 we are removing it altogether because 
it has proven to be problematic in many ways - one of the problems was that it 
causes excessive locking on the database - where the database can be locked by 
- sometimes - many tasks completing at about the same time. So I believe in 
Airflow 2.8 we implemented a strategy to skip some of that if the contention is 
too high. Also this has been further improved in 2.9 and 2.10 so you will 
likely get a bit better behaviour when you upgrade. 

But still - there is no guarantee it will happen immediately after task 
completes.

GitHub link: 
https://github.com/apache/airflow/discussions/45540#discussioncomment-11795489

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