How to stop a loop on shutdown in FastAPI?

Question

I have a route / which started an endless loop (technically until the websocket is disconnected but in this simplified example it is truly endless). How do I stop this loop on shutdown:

from fastapi import FastAPI

import asyncio

app = FastAPI()
running = True

@app.on_event("shutdown")
def shutdown_event():
    global running
    running = False

@app.get("/")
async def index():
    while running:
        await asyncio.sleep(0.1)

According to the docs @app.on_event("shutdown") should be called during the shutdown, but is suspect it is called similar to the lifetime event which is called after everything is finished which is a deadlock in this situation.

To test:

1. i run it as uvicorn module.filename:app --host 0.0.0.0
2. curl http://ip:port/
3. then stop the server (pressing CTRL+C)

and you see that it hangs forever since running is never set to false because shutdown_event is not called. (Yes you can force shutdown by pressing CTRL+C)

Answer 1

import signal
import asyncio
from fastapi import FastAPI

app = FastAPI()
running = True

def stop_server(*args):
    global running
    running = False

@app.on_event("startup")
def startup_event():
    signal.signal(signal.SIGINT, stop_server)

@app.get("/")
async def index():
    while running:
        await asyncio.sleep(0.1)

Source: https://github.com/tiangolo/fastapi/discussions/9373#discussioncomment-5573492

Setting up and catching the SIGINT signal allows to catch the first CNTR+C. This will set running to False which ends the loop in index(). Terminating the running request allowing the server to shutdown.