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Now it is a bit more messy a at the moment. That is due to the fact that DagRuns are not yet first class citizens. In the past Airflow only had a notion of DagRun: it mentioned them in the code but they did not really exist, instead DAGs were instantiated into "Tasks in Time": TaskInstances. This creates architectural issues but also real life ones. Some of the issues in the current scheduler with "depend_on_past" stem from this, because a Task is not really able to answer "which task instance is the first in time?". It is also the reason why scheduler loops start to increase over time if DAGs get more complex (many tasks). The good news is work is underway to improve this (see the roadmap).

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