How to limit or set size for the default ThreadPoolExecutor which be used to run blocking background task in FastAPI?

[Nohysiwe]

I noticed that blocking background task was running in the default ThreadPoolExecutor for the current thread's asyncio.Eventloop, so how to limit or set size of the default ThreadPoolExecutor. ：)

[insomnes]

@Nohysiwe
You can try to set default ThreadPoolExecutor in your main file

from concurrent.futures import ThreadPoolExecutor
import asyncio

loop = asyncio.get_running_loop()
loop.set_default_executor(ThreadPoolExecutor(max_workers=4))

[Nohysiwe]

@insomnes
Thank you.

[Nohysiwe]

@insomnes
I think that using loop = asyncio.get_event_loop() is better than loop = asyncio.get_running_loop() in here.

[Nohysiwe]

And I find that I can use ProcessPoolExecutor same as ThreadPoolExecutor . But it's not a good idea when the app run with multi worker.

[Nohysiwe]

@insomnes
I find that it's ineffective to set default ThreadPoolExecutor in main file. Because the loop used by uvicorn is from asyncio.new_event_loop(). It run after import my main file in the code of uvicorn.

# in uvicorn/loops/asyncio.py/asyncio_setup()
def asyncio_setup():
    if (
        sys.version_info.major >= 3
        and sys.version_info.minor >= 8
        and platform.system() == "Windows"
    ):
        selector = selectors.SelectSelector()
        loop = asyncio.SelectorEventLoop(selector)
        asyncio.set_event_loop(loop)
    else:
        loop = asyncio.new_event_loop()
        asyncio.set_event_loop(loop)

[Kludex]

I'm not sure why are you doing this, but...

You can add the @insomnes logic inside of this: https://fastapi.tiangolo.com/advanced/events/?h=startup#startup-event

[Nohysiwe]

@Kludex
Thank you for your help :)

[tiangolo]

Thanks for the help here @insomnes and @Kludex ! 👏 🙇

Thanks for reporting back and closing the issue @Nohysiwe 👍

Sorry for the long delay! 🙈 I wanted to personally address each issue/PR and they piled up through time, but now I'm checking each one in order.

[Kludex]

The answer for this question is a bit different at the moment, see encode/starlette#1724 (comment) for the answer.