Heya This might be stupid question sweat smile I have a flow

Question

Heya This might be stupid question sweat smile I have a flow with multiple tasks in something like airflow I d need to explicitly define the dependencies of tasks in a dag flow i e ```task 1 gt gt task 2 task 2 gt gt task 3 task 2 gt gt task 4 ``` Now prefect determines this implicitly by the parameter direction from task to task i e ```result 1 = task 1 result 2 = task 2 result 1 result 3 = task 3 result 2 result 4 = task 4 result 2 ``` Which is fine no problems there but what do we do when there is task 5 that does not require any result from the tasks it is dependent on For example task 5 absolutely must run if task 4 is done but has no other relationship with it like ```result 1 = task 1 result 2 = task 2 result 1 result 3 = task 3 result 2 result 4 = task 4 result 2 result 5 = task 5 ``` With sequential task runner default this is OK but if I were to use concurrent one I want to have the tasks 3 and 4 run at the same time but 5 must not run before task 4 is done

jpuris · Answer

Is the best practice to simply default using `wait for` arg added by ` task` decorator For example ```result 5 = task 5 wait for= result 4 ``` But what do we do then when task 4 returns nothing i e ``` task def task 4 pass flow def my flow result 1 = task 1 result 2 = task 2 result 1 result 3 = task 3 result 2 result 4 = task 4 result 2 It does not return anything result 5 = task 5 wait for=result 4 ```

Christopher Boyd · Answer

yes

Christopher Boyd · Answer

wait for is the dependency key here but you are correct if it s the sequential task runner it will still run in sequence

Christopher Boyd · Answer

you would need concurrent task runner or even dask concurrent will run semi parallel until it s blocked so if you have task4 with a dependency on task3 and task5 with no wait for task5 still won t run because it will be blocked waiting for task4 to return before it gets a chance

jpuris · Answer

I see Thank you < Christopher Boyd> Lastly when task 4 function has no return would this not cause issues for result 4 variable init ``` task def task 4 some param pass result 2 = None result 4 = task 4 result 2 It does not return anything ``` I d then think it is obligatory to return something by every task function ever i e ``` task def task 4 some param print I did nothing and returning None return None result 2 = None result 4 = task 4 result 2 ``` Which then works OK

jpuris · Answer

Actually all is OK ```def returns nothing pass nothing = returns nothing print nothing ``` outputs ```None``` +1