[GitHub] [airflow] eladkal commented on issue #17585: New Tasks to an existing DAG do not get scheduled if they have depends_on_past = True

[GitBox <gi...@apache.org>]

eladkal commented on issue #17585:
URL: https://github.com/apache/airflow/issues/17585#issuecomment-902014936


   > Add another task with depends_on_past=True to the existing DAG
   
   I think the issue here is that you assume that when you add a new task Airflow knows that it starts from the date you added it thus you expect depends_on_past=True to work but this is not the case. You must provide a `start_date` to this new task then it will work as expected.
   
   ```
   from airflow import DAG
   from airflow.operators.dummy import DummyOperator
   from datetime import datetime
   
   default_args = {
       'owner': 'Elad',
       'start_date': datetime(2021, 8, 19),
   }
   
   with DAG('my_dag2', default_args=default_args, schedule_interval='*/5 * * * *') as dag:
       a = DummyOperator(task_id="1st")
       b = DummyOperator(task_id="2nd", wait_for_downstream=True)
       a >> b
   
   with DAG('my_dag3', default_args=default_args, schedule_interval='*/5 * * * *') as dag:
       a = DummyOperator(task_id="1st")
       b = DummyOperator(task_id="2nd", wait_for_downstream=True, start_date=datetime(2021, 8, 19, 15, 15))
       a >> b
   ```
   
   as you can see `my_dag3` works fine yet `my_dag2` is stuck:
   
   ![Screen Shot 2021-08-19 at 18 29 18](https://user-images.githubusercontent.com/45845474/130097259-52b79d35-0186-446e-a668-aa12f145c642.png)
   
   ![Screen Shot 2021-08-19 at 18 29 11](https://user-images.githubusercontent.com/45845474/130097294-7996b42f-2b24-4db3-abcc-486f15e86f51.png)
   
   
   


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