[BUG] BackgroundTasks not working properly

Describe the bug

The request remains open/active until the background task finishes running, making the purpose of BackgroundTasks kind of useless. On the snippet below the request takes about 10 seconds to complete. I followed the documentation guide but it didnt seem to work.

To Reproduce

async def background_task(text: str, seconds: int):
    import time

    time.sleep(seconds)
    print(text)

@app.get("/run", status_code=status.HTTP_200_OK)
async def test(background_tasks: BackgroundTasks, text: str = "Test", secs: int = 10):
    background_tasks.add_task(background_task, text, secs)
    return {"text": text, "secs": secs}

Screenshot

Expected behavior

The request to finish right away and the task to run in background.

Environment

• OS: Linux Manjaro 19
• FastAPI Version 0.54.1
• Python version 3.8.2

[daviskirk]

I’m not 100% sure about this but here’s a few guesses:

1. Have you tried to run it with await asyncio.sleep(seconds) instead of the time.sleep function? time.sleep is synchronous as far as I know and since background tasks that are defined as “async” just use the event loop as their “background engine”, using a synchronous method in that background task will still globally block the process.

2. Alternatively, you can try removing the “async” from def background_task. In that case the task should run in a thread pool instead which would then also not block.

[daviskirk]

@soares7vinicius just fyi this: https://github.com/encode/starlette/issues/436 explains your problem es well. The tldr:

• async function background task => runs in asyncio loop (everything that doesn’t use “await” will block)
• normal function background task => runs in a threadpool (and will thus not block in this case)