[QUESTION] Long running background tasks

Hey everyone, I’m currently trying to implement an API endpoint using FastAPI which starts a long running background task using asyncio.create_task(startlongrunningtask()) and then without waiting for that task to finish, return a response to the client. The long running task starts up fine, but times out after e few seconds with the error message [CRITICAL] WORKER TIMEOUT and Booting worker with pid: 25

How can I prevent the worker from getting timed out?

Is it possible to create such long running tasks which continue even after sending a response to the client?

Thanks a lot and best regards

[dmontagu]

@xiaodiannao I’m curious to hear if switching to def fixes it.

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This isn’t really the intended use case for BackgroundTasks – I think BackgroundTasks is intended more for shorter, low-compute tasks like triggering an email send or similar.

For tasks involving multi-minute calls to subprocesses, I think long term you would benefit from putting in the effort to set up a celery worker and a task queue, and have the celery worker do the subprocess stuff. (I have a similar setup on one of my own projects, which runs multi-minute compute-intensive jobs.) That will decouple your server from the worker, and may help you avoid/preempt a lot of issues arising from long-running and/or compute-intensive jobs running alongside your server.

If you wanted to do this, the full stack fastapi postgres template has a celery worker; you should be able to adapt it to your use case.

[euri10]

happy to help but you’d need to give more info on what the tasks does, how you’re running the app, what worker stops (uvicorn one or gunicorn)

the below snippet shows a long running background task in action, it just sleeps and doesn’t use asyncio.create_task like you describe though

Connected to pydev debugger (build 192.6817.19)
email-validator not installed, email fields will be treated as str.
To install, run: pip install email-validator
2019-10-10 13:18:21,122 uvicorn      INFO     Started server process [29570]
2019-10-10 13:18:21,122 uvicorn      INFO     Waiting for application startup.
2019-10-10 13:18:21,124 uvicorn      INFO     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
2019-10-10 13:18:29,706 uvicorn      INFO     ('127.0.0.1', 46716) - "POST /backgroundasync HTTP/1.1" 200
2019-10-10 13:18:29,706 __main__     DEBUG    sleeping 30s
2019-10-10 13:18:59,707 __main__     DEBUG    slept 30s

import asyncio
import logging
from typing import Dict

import uvicorn
from fastapi import FastAPI
from pydantic import BaseModel
from starlette.background import BackgroundTasks

logging.basicConfig(format="%(asctime)s %(name)-12s %(levelname)-8s %(message)s")
logger: logging.Logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)


app = FastAPI()


class Item(BaseModel):
    amount: int


async def background_async(amount: int) -> None:
    logger.debug(f"sleeping {amount}s")
    await asyncio.sleep(amount)
    logger.debug(f"slept {amount}s")


@app.post("/backgroundasync")
async def sleepingtheback(
    item: Item, background_tasks: BackgroundTasks
) -> Dict[str, str]:
    background_tasks.add_task(background_async, item.amount)
    return {"message": f"sleeping {item.amount} in the back"}


if __name__ == "__main__":
    uvicorn.run(app=app)