@Marvin how to limit subflows parallel execution in a flow

To limit the parallel execution of subflows in a Prefect flow, you need to use task execution controls that are available in Python's asynchronous programming. Subflows will block execution of the parent flow until completion unless you run them asynchronously. You can achieve parallelism with some restrictions by using AnyIO task groups or Python's

asyncio.gather

, but you would need to manage the concurrency manually to limit it. Here is an excerpt from the Prefect documentation:

Subflows will block execution of the parent flow until completion. However, asynchronous subflows can be run in parallel by using AnyIO task groups or

asyncio.gather

.

With this in mind, to limit the number of concurrent subflows, you can create a semaphore within an AnyIO task group or a similar synchronization primitive that limits concurrency when using

asyncio.gather

. Unfortunately, the Prefect documentation doesn't provide an explicit built-in feature to limit subflow concurrency directly, so managing it manually through these async tools is necessary. Here is a brief example using

asyncio.Semaphore

to limit concurrency in an asynchronous context:

python
import asyncio

async def subflow(semaphore, *args, **kwargs):
    async with semaphore:
        # Subflow logic here
        ...

async def main():
    semaphore = asyncio.Semaphore(5)  # Adjust number to the desired concurrency limit
    await asyncio.gather(
        subflow(semaphore),
        subflow(semaphore),
        # More subflows
    )

if __name__ == '__main__':
    asyncio.run(main())

In this example, the semaphore is set with a concurrency limit of 5. You would replace the

subflow

coroutine with your actual subflow logic, and the semaphore will ensure no more than 5 subflows run concurrently.