How to properly close a background child process in a shutdown event?

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Example

Here’s a self-contained, minimal, reproducible, example with my use case:

from multiprocessing import Process, Value

from fastapi import FastAPI

app = FastAPI()

terminate_flag = Value('h', 0)


def work_hard(terminate_flag: Value):
    while terminate_flag.value == 0:
        # CPU-bound calculations
        pass


@app.get('/hello-world')
def hello_world():
    return {
        'message': 'hello world'
    }


process = Process(target=work_hard, args=(terminate_flag,))


@app.on_event('startup')
def startup_event():
    process.start()


@app.on_event('shutdown')
def shutdown_event():
    terminate_flag.value = 1
    process.join()

Description

I have some CPU-bound calculations known as work_hard, which contains a flagged loop to control its flow.

The function is designed to be executed in a child process that starts within FastAPI startup event, and joins within FastAPI shutdown event.

However, it seems that the FastAPI shutdown event does not get executed while uvicorn is shutting down by the user pressing Ctrl + C. It stuck at the log “Finished server process” and never really terminates.

I did try to look up for the standard practice of using multiprocessing module in FastAPI, but there are no concrete examples about it. Concurrency and async/await section mentioned that “To see how to achieve this parallelism in production see the section about Deployment.”, yet Deployment section has no such examples as well.

Thank you in advance!

Environment

• OS: Ubuntu 18.04.4 TLS
• FastAPI Version : 0.61.1
• Python version: 3.6.8

Additional context

cliff@cliff-pc:~/repo/fastapi-multiprocessing$ uvicorn app.main:app
INFO:     Started server process [24032]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
^CINFO:     Shutting down
INFO:     Finished server process [24032]

[raphaelauv]

Also to respond to your question :

FastAPI shutdown event does not get executed while uvicorn is shutting down by the user pressing Ctrl + C.

No it does

from fastapi import FastAPI
import uvicorn

async def on_shutdown():
    print("HERE")

if __name__ == '__main__': 
    web_app = FastAPI(docs_url="/", on_shutdown=[on_shutdown])
    uvicorn.run(web_app, host="0.0.0.0", port=8000)

give

INFO:     Started server process [10579]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
HERE
INFO:     Application shutdown complete.
INFO:     Finished server process [10579]

Probably you run the code inside a docker and do not use tini

[tiangolo]

Thanks for the help and discussion here everyone and thanks @clifflau1120 for coming back to close the issue. ☕

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.