I have 4 tasks: t1, t2, t3, t4 How to start t3 and t4 after t1 and t2 successfully finished?

It sounds like the usual Prefect core approach will do: set t3 and t4 to each have both t1 and t2 as upstream tasks.

Hi @Ruslan - @Amanda Wee is correct! To expand a little further, you can set task ordering using the imperative API like this:

from prefect import Task, Flow
from prefect.tasks.notifications import SlackTask

class GetValue(Task):
    def run(self):
        return "Test SlackTask!"

flow = Flow("slack-test")

value = GetValue()
slack_task = SlackTask()

slack_task.set_upstream(value, key="message", flow=flow)

or with the functional API like this (both with a data dependency and without shown):

from prefect import task, Flow
from prefect.tasks.notifications import SlackTask

@task
def get_value():
    return "Test SlackTask!"

slack_task = SlackTask()

with Flow("slack-test") as flow:
    value = get_value()
    slack_task(message=value) # data dependencies will automatically create upstream/downstream relationships
    slack_task(upstream_tasks=[value]) # if there isn't a data dependency, but you still need to run tasks in a specific order, you can use the `upstream_tasks` kwarg, which takes a list of task references

You can read more about dependencies here 🙂

@nicholas thank you for reply! but I don’t see answer on question there, I saw this doc, it is hard to understand and too poor examples there. In addition your examples are about 2 tasks too

Sorry you feel that way @Ruslan - you're welcome to PR the docs (or open a ticket) for any examples you feel could be improved. You should be able to do the same for 4 tasks as you would for 2 (or any number).

with Flow("some flow") as flow:
  task_1 = t1()
  task_2 = t2()
  t3(upstream_tasks=[task_1, task_2])
  t4(upstream_tasks=[task_1, task_2])

yeah thank you, like this! but what if I need to start t3 if t1 succeded and t2 finished (failed or succeded)? (not any_successful)

You'll probably want to use the

all_finished

trigger for that.

no t1 have to be only succeded

What I'd do is introduce a dummy task t5 that runs if t2 finishes, and then t3 depends on both t1 and t5 succeeding.

yeah thats it, but what is the best way to make dummy? def dummy(): None or how?

You would need the

@prefect.task(trigger=prefect.triggers.all_finished)

decorator too, and probably just

return

in the function.

thank you very much Amanda!