If you configured an ExternalTaskSensor in Airflow to wait until a task in another DAG finishes (or the entire DAG finishes), you have probably noticed that it behaves surprisingly. Most data engineers assume that it is a bug, and their sensor gets stuck for whatever reason. I have to admit that I find it annoying because the documentation of the ExternalTaskSensor does not warn us about it.
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ExternalTaskSensor works only if the other DAG has the same execution date equal to the expected execution date. Sure, you can specify the execution time delta, but it is not a time range.
For example:
- The execution date of the DAG that contains the sensor is: 2020-10-01 14:00:00
- The execution delta = 3 hours
In this situation, the other DAG’s execution date must be 2020-10-01 11:00:00.
I think that most data engineers expect that delta = 3 hours means that the other DAG may execute at any time between 11:00 and 14:00.
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