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<@ULVA73B9P> what's the best way to run child flow...
# ask-marvin
i
@Marvin what's the best way to run child flows in parallel?
m
To assist you accurately, could you please specify which version of Prefect you are using (e.g., Prefect 2.x or Prefect 3.x)? If you're unsure, I can provide guidance for both versions.
i
@Marvin 2.20
m
In Prefect 2.20, running child flows in parallel can be accomplished by triggering multiple subflows within the parent flow. Each subflow behaves like a normal flow, meaning it can run independently and concurrently with others. Here's a high-level overview of how you can achieve this: 1. Define your Subflows: Create flow functions intended to be run as subflows. 2. Use Asynchronous Execution: Python's asynchronous features can be leveraged to allow subflows to run in parallel. Prefect's concurrency settings don't block parallel execution inherently, so it's important to manage this through Python's async tools. 3. Task and Task Runner Management: When a subflow starts, it creates a new task runner. This means each subflow runs its tasks independently of others. Here's a simple example of how you could set this up:
Copy code
python
from prefect import flow

@flow
def child_flow_1(data):
    # Process data
    pass

@flow
def child_flow_2(data):
    # Process data
    pass

@flow
def parent_flow():
    # You could use asyncio.gather or a similar async method to trigger child flows in parallel
    child_flow_1(data_1)
    child_flow_2(data_2)
In the above structure, 
child_flow_1
and 
child_flow_2
can be executed in parallel if managed in an asynchronous context. However, there are no built-in Prefect configurations specific to running flows asynchronously in the same runtime process. This can be managed by either the infrastructure (multi-threading or multi-processing) or by running each flow on separate Prefect agents/executors. For more detailed guidance, you might want to experiment with Python's 
asyncio
if your flows are CPU bound, or consider using Prefect's work queues or agents with different concurrency settings if they're I/O bound. Further reading can be found in the Prefect documentation here: Prefect Flow Concepts.
i
@Marvin what I did was the following:
Copy code
await asyncio.gather(
    flow1(),
    flow2(),
    flow3()
)
but running this in prefect, the flows run sequentially. Why?
m
Using 
asyncio.gather
in this manner should typically allow the flows to run concurrently, assuming they are designed to support asynchronous execution. However, if the flows are running sequentially, it might be due to how Prefect handles flows and tasks internally. Here are a few things you might want to consider or check: 1. Async Compatibility: Ensure that your flows and tasks are compatible with asynchronous execution. This means using 
await
for long-running operations or I/O within your flows and tasks. 2. Task Runners: Prefect uses task runners to manage the execution of tasks. If these flows are bound by synchronous operations or if the task runners themselves are not set up for concurrency, this might cause sequential execution. 3. Concurrency Settings: Check if you've set any global or flow-level concurrency constraints in Prefect that could limit the parallelization. 4. Infrastructure Configuration: Ensure that the infrastructure where you're running these flows (such as Prefect agents or executors) is configured to allow concurrent runs. For instance, a local Dask or concurrent task runner could be configured for concurrency. Here's what you should try or check in your setup: - Confirm that each task within the flows (
flow1
, 
flow2
, etc.) is using 
await
where applicable and is non-blocking. - If you're running this within a Prefect Cloud or Prefect Orion environment, ensure there are no concurrency limits set that could affect execution. - Consider using Prefect's 
concurrent.futures
process or thread pools if you need a guaranteed way to run tasks concurrently that are inherently synchronous. If these points are correctly set and configured, and you continue to experience sequential execution, there might be something specific in your environment's or your code's setup impacting the concurrency. For further guidance and troubleshooting, review the Prefect documentation on handling concurrent flows, or check Prefect's community forums for others who might have experienced similar issues.
When using 
asyncio.gather
to run flows concurrently in Prefect, but experiencing sequential execution, there are several potential reasons and solutions to consider: 1. Async Context: Ensure that your flows are not blocking the event loop. If there are blocking operations inside your flows or tasks, they won't take advantage of asyncio’s concurrency. 2. Use of Async/Await: Ensure that each task and sub-operation within your flows properly uses 
await
for any asynchronous or I/O-bound operations. If tasks within your flows are not designed to be asynchronous, they will block the execution. 3. Task Runner Configuration: By default, Prefect might use task runners that don't execute tasks concurrently. You might need to configure asynchronous task runners explicitly. 4. Global Concurrency Limits: Check if there are any global or flow-specific concurrency limits set in your Prefect configuration. These can affect how flows execute, even if asyncio is set up correctly. 5. Infrastructure Setup: If running this in a managed environment (like Prefect Cloud or using a Prefect agent), ensure that your infrastructure allows for concurrent execution. This might involve using an appropriate executor or agent configuration. Here’s an example of ensuring that tasks within a flow are asynchronous:
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python
from prefect import flow, task
import asyncio

@task
async def task1():
    # perform some async operation
    await asyncio.sleep(1)

@task
async def task2():
    # perform some async operation
    await asyncio.sleep(1)

@flow
async def flow1():
    await task1()

@flow
async def flow2():
    await task2()

async def main():
    await asyncio.gather(
        flow1(),
        flow2()
    )

if __name__ == "__main__":
    asyncio.run(main())
Steps to Troubleshoot: - Verify that flows and tasks have been implemented to support async operations. - Review Prefect's concurrent and async execution configurations in your environment. - Consider using task runners or agents that support concurrency, such as Dask or concurrent.futures within the Prefect ecosystem. If these steps are followed, and flows are still running sequentially, further investigation into the Prefect configuration or environment-specific constraints may be necessary.
i
@Marvin would using "task_group" also help me out?
m
Using 
task_group
in Prefect can help organize and manage the execution of tasks, potentially facilitating parallel execution, but the documentation for Prefect 2.x doesn't specifically mention a 
task_group
feature that would directly allow or enhance parallel execution like 
asyncio.gather
. Here's what you can explore: 1. Task Concurrency: Prefect allows you to use task concurrency limits to control how many instances of a task (or tasks with a specific tag) can run simultaneously. This is particularly useful in managing and preventing resource contention when running multiple tasks concurrently. 2. Task Runners: You can configure your flows to use different task runners, like 
DaskTaskRunner
, which are designed to handle parallel execution more effectively. If your tasks within flows are properly set up for parallel execution, these task runners can execute them concurrently. 3. Infrastructure and Design: It's crucial that your infrastructure supports concurrent execution, and that tasks are designed to leverage this. If tasks are I/O bound, consider leveraging Python's async features. If you're considering concurrency and potential parallel execution, setting up a task runner that supports such a pattern (like 
DaskTaskRunner
) might be more beneficial than trying to perform this solely through code logic like 
asyncio.gather
. To explore more about how you can achieve task parallelism in Prefect, refer to Prefect's Task Runners Documentation. If you want further help on how to set up your flows and tasks for concurrency, I suggest exploring task runners and Prefect's concurrency settings further.
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