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<@ULVA73B9P> How to run a nest flow waiting for ot...

# ask-marvin

Duan

01/10/2025, 2:19 AM

@Marvin How to run a nest flow waiting for other tasks? And how to run tasks after a nest flow complete?

Marvin

01/10/2025, 2:19 AM

To manage nested flows in Prefect 3.x, you can utilize the following strategies: 1. Running a Nested Flow After Other Tasks: You can define a nested flow within a parent flow and control its execution order by using task dependencies. Prefect's task orchestration allows you to set dependencies between tasks, ensuring that the nested flow only runs after the specified tasks have completed. 2. Running Tasks After a Nested Flow Completes: Similarly, you can define tasks to run after a nested flow by setting them to depend on the completion of the nested flow. This way, Prefect will handle the execution order based on these dependencies. Here's a simple example of how you might structure this in an imperative style, which you prefer:

Copy code

python
from prefect import flow, task

@task
def task_a():
    # Task logic here
    pass

@task
def task_b():
    # Task logic here
    pass

@flow
def nested_flow():
    # Nested flow logic here
    task_b()

@flow
def parent_flow():
    a = task_a()
    # Ensure nested_flow runs after task_a
    nf = nested_flow(wait_for=[a])
    # Specify tasks to run after nested_flow completes
    final_task(wait_for=[nf])

@task
def final_task():
    # Final task logic here
    pass

if __name__ == "__main__":
    parent_flow()

### Key Points: - Dependencies: Use the

wait_for

parameter to specify task dependencies. - Observability: Nested flows have first-class observability, allowing you to track their status within the Prefect UI or Prefect Cloud. - Parameters: Nested flows support parameterization, allowing you to pass different parameters for different use cases. For more details, you can refer to the Prefect documentation on writing flows. If you have more specific requirements or face issues, feel free to ask!

Duan

01/10/2025, 2:21 AM

But flow doesn't have a wait_for parameter?

Duan

01/10/2025, 2:22 AM

@Marvin But flow doesn't have a wait_for parameter?

Marvin

01/10/2025, 2:22 AM

You're correct that Prefect's

flow

decorator does not have a

wait_for

parameter. Instead, task dependencies are managed at the task level. Here's how you can handle dependencies between flows and tasks in Prefect 3.x: 1. Using Task Dependencies: You can use the

wait_for

parameter in tasks to specify dependencies. While flows themselves do not have a

wait_for

parameter, you can structure your tasks within a flow to ensure that they execute in the desired order. 2. Running a Nested Flow and Waiting for Completion: You can use the

run_deployment

function to trigger a flow from another flow and wait for its completion. This function allows you to run another flow as a "deployment" and can be used to manage dependencies between flows. 3. Using
wait_for_flow_run
: There's an asynchronous function

wait_for_flow_run

that can be used to wait for a specific flow run to complete. This can be useful if you need to manually manage flow completion before proceeding with subsequent tasks. Here's an example of how you might structure this:

Copy code

python
from prefect import flow, task
from prefect.flow_runs import wait_for_flow_run
from prefect.deployments import run_deployment

@task
def task_a():
    # Task logic here
    pass

@task
def task_b():
    # Task logic here
    pass

@flow
def nested_flow():
    task_b()

@flow
def parent_flow():
    a = task_a()
    # Run the nested flow as a deployment
    flow_run_id = run_deployment("nested_flow")
    # Wait for the nested flow to complete
    wait_for_flow_run(flow_run_id)
    # Continue with the final task after the nested flow completes
    final_task()

@task
def final_task():
    # Final task logic here
    pass

if __name__ == "__main__":
    parent_flow()

### Key Functions: - run_deployment
: Used to start another flow as a deployment. - wait_for_flow_run
: Used to wait for a flow run to complete before proceeding. For more information, you can view the Prefect documentation on flow runs and writing flows. If you have further questions or specific scenarios, feel free to ask!

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