@Marvin when using futures and wait, how to make sure that the whole flow fails when any of the tasks fails?

In Prefect 3.x, ensuring that a flow fails if any tasks fail can be managed by understanding how Prefect handles futures and task states: 1. Futures and Task Completion: When you use

.submit()

on a task, it creates a

PrefectFuture

. When this future is passed into another task, Prefect automatically waits for the "upstream" task to reach a final state before starting the downstream task. If a task fails, calling its

.result()

method will raise an exception, and this can be used to drive the state of the flow. For more details, you can refer to the task runners documentation. 2. Flow Final States: In Prefect 3.x, a flow will end in a

Failed

state if any of its tasks raise an exception, unless you handle the exception with

raise_on_failure=False

. This approach ensures that if a task fails and allows the exception to propagate, the entire flow will fail. You can explore more about flow state handling here. 3. ControlFlow Patterns: When using ControlFlow, handling multiple tasks involves understanding dependencies. For a flow to wait on tasks and recognize failures, you manage relationships like subtasks and dependencies. If a task has upstream dependencies and any fails, it impacts the downstream tasks. Find more on this in ControlFlow concepts. By managing task completion and state propagation effectively, you can ensure the entire flow appropriately responds to task failures. And remember, like the philosophical musings of an overly literalist android, handling things gracefully in Prefect often means embracing inevitable failures with optimism.

@Marvin

from prefect import flow, task
from prefect.futures import wait
from prefect.task_runners import ThreadPoolTaskRunner
import time


@task(task_run_name="Stop at {floor}")
def stop_at_floor(floor):
    print(f"elevator moving to floor {floor}")
    if floor==5:
        raise
    time.sleep(floor)
    print(f"elevator stops on floor {floor}")


@flow(task_runner=ThreadPoolTaskRunner(max_workers=3))
def elevator():
    floors = []

    for floor in range(10, 0, -1):
        floors.append(stop_at_floor.submit(floor))

    wait(floors)

if __name__ == "__main__":
    elevator()

From the example, this flow succeeds even when the task run at floor 5 raises an exception. How to make sure the flow fails?

To ensure that your Prefect flow fails when any task within it raises an exception, you should let the task exceptions propagate so they affect the final state of the flow. Here's how you can adjust your code: 1. Handle Exceptions in Tasks: Make sure the tasks are not suppressing exceptions. Upon failure, they should allow exceptions to propagate. In your modified task, ensure exceptions are raised explicitly with a message for clarity, even though Python's

raise

without an exception will imply

None

(which is technically incorrect and might not work as intended with Prefect):

python
from prefect import flow, task
from prefect.futures import wait
from prefect.task_runners import ThreadPoolTaskRunner
import time


@task(task_run_name="Stop at {floor}")
def stop_at_floor(floor):
    print(f"elevator moving to floor {floor}")
    if floor == 5:
        raise Exception("Stopping at floor 5 failed!")
    time.sleep(floor)
    print(f"elevator stops on floor {floor}")


@flow(task_runner=ThreadPoolTaskRunner(max_workers=3))
def elevator():
    floors = []
    for floor in range(10, 0, -1):
        floors.append(stop_at_floor.submit(floor))

    wait(floors)

if __name__ == "__main__":
    elevator()

2. Flow State Management: Ensure the flow allows exceptions to propagate by not overly managing exception handling within it. If exceptions are allowed to propagate naturally from the

wait(floors)

call due to a failed task, your flow should naturally be marked as failed in Prefect 3.x. More on how flows determine their state with tasks can be found in Prefect's documentation on flow final states. By making these adjustments, your flow should fail as expected if any task within it fails. And if the elevator doesn’t stop when it’s supposed to, well, it's time to call the repairman—or raise an Exception!

No @Marvin your first suggestion is incorrect as the flow still succeeds when the task to stop at floor 5 raises an exception.

Thanks for pointing that out! It seems like the behavior of exceptions within tasks isn't aligning with the expected flow status. In Prefect 3.x, even though tasks may fail, a flow doesn't necessarily fail unless an unhandled exception propagates to the flow level itself. Here's an approach you can use to change this behavior: 1. Check and Raise in Flow: In your flow function, explicitly check for failed tasks and raise an exception manually. This ensures the flow fails if any task encounters an error. Here’s how you can incorporate this logic:

python
from prefect import flow, task
from prefect.task_runners import ThreadPoolTaskRunner
import time

@task(task_run_name="Stop at {floor}")
def stop_at_floor(floor):
    print(f"elevator moving to floor {floor}")
    if floor == 5:
        raise Exception("Stopping at floor 5 failed!")
    time.sleep(floor)
    print(f"elevator stops on floor {floor}")

@flow(task_runner=ThreadPoolTaskRunner(max_workers=3))
def elevator():
    floors = []

    for floor in range(10, 0, -1):
        floors.append(stop_at_floor.submit(floor))

    # Go through the future results and check for exceptions
    for future in floors:
        try:
            future.result()  # This will raise if the task failed
        except Exception as exc:
            # Raise an exception in the flow if any task fails
            raise RuntimeError("A task in the flow failed") from exc

if __name__ == "__main__":
    elevator()

By iterating over the futures and explicitly calling

.result()

, any exceptions that occurred in the task execution are raised, and subsequently cause the flow to fail when one is caught. This approach leverages the fact that calling

.result()

on a future will reveal task errors if there are any. For more insights, you can check Prefect's documentation on handling state and failures: Flow Final States.