[QUESTION] about threads issue with fastapi.

Hi, I have a question about the threads issue with fastapi.

When I run the example from tutorial uvicorn main:app --reload --port 4681 --host 0.0.0.0 with the following main.py

from fastapi import FastAPI
app = FastAPI()
@app.get("/")
def read_root():
    return {"Hello": "World"}
@app.get("/items/{item_id}")
def read_item(item_id: int, q: str = None):
    return {"item_id": item_id, "q": q}

and it show up the following information

INFO: Started server process [18983]
INFO: Waiting for application startup.
INFO: Uvicorn running on http://0.0.0.0:4681 (Press CTRL+C to quit)

then I use ps -o nlwp 18983 to see how many threads this process (18983) is using.

However, everytime when I send a request to this service, the number of threads increase without being closed. To be more specific, when I send 1000 requests, this process ended up with 1000 threads running.

This is problematic because I tried to serve another more complicated applications, allocating arbitrary number of threads would finally get my machine out of resources.

Is there any thing I could have done wrong? Thanks in advance!

[emarbo]

Hello, I have configured the number of FastAPI threads setting the default thread pool executor as follows:

# main.py
from concurrent.futures import ThreadPoolExecutor
import asyncio

loop = asyncio.get_running_loop()
loop.set_default_executor(ThreadPoolExecutor(max_workers=5))

I’ve ended using this solution because Starlette run_in_threadpool uses the default executor when calls the loop.run_in_executor. I suppose it would be better if the executor could be configured explicitly in the Starlette or FastAPI configurations.

@lzhbrian do you think this could work for your project?

I am using the FastAPI Project Template with this environment:

Python 3.7.2
fastapi==0.42.0
starlette==0.12.9
uvicorn==0.9.0

[seahrh]

Taking @emarbo’s code example and stuffing it in a startup event worked for me.

main.py

@app.on_event("startup")
def set_default_executor():
    from concurrent.futures import ThreadPoolExecutor
    import asyncio

    loop = asyncio.get_running_loop()
    loop.set_default_executor(
        ThreadPoolExecutor(max_workers=5)
    )

fastapi==0.68.0 gunicorn==20.1.0 uvicorn[standard]==0.14.0