Using BackgroundTasks blocks StreamingResponse?

[rohan-mehta]

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Example Code

import asyncio
import logging

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
from starlette.background import BackgroundTask

app = FastAPI()


async def stream_data():
    for i in range(10):  # Simulating a stream of data
        yield f"data: {i}\n"
        await asyncio.sleep(0.1)
        logging.info(f"Sent data chunk: {i}")
    logging.info("Streaming finished")


async def post_streaming_cleanup():
    logging.info("Cleanup started")
    await asyncio.sleep(100)  
    logging.info("Cleanup finished")


@app.get("/stream")
async def get_stream():
    response = StreamingResponse(
        stream_data(), background=BackgroundTask(post_streaming_cleanup)
    )
    return response

Description

If you call the endpoint above (e.g. via curl http://localhost:8080/stream), it just hangs after the response stream completes.
This means that the connection is closed not when the StreamingResponse finishes, but after the BackgroundTask finishes. Same thing happens even if I use the BackgroundTasks dependency instead.

Any way to make the task NOT block the response from completion?

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.104.1

Pydantic Version

2.5.2

Python Version

Python 3.11.6

Additional Context

No response

[binbjz]

Version info

# Name                    Version                  Build      Channel
python                    3.11.7                   hb885b13_0
fastapi                   0.109.0                  pypi_0      pypi
pydantic                  2.5.3                    pypi_0      pypi
pydantic-core             2.14.6                   pypi_0      pypi

ProductName:		macOS
ProductVersion:		14.2.1
BuildVersion:		23C71

Please try the following code snippet.

import time
import asyncio
import logging
from fastapi import FastAPI, BackgroundTasks
from fastapi.responses import StreamingResponse

app = FastAPI()

logging.basicConfig(level=logging.INFO)


async def stream_data():
    for i in range(10):
        yield f"data: {i}\n"
        await asyncio.sleep(0.1)
        logging.info(f"Sent data chunk: {i}")
    logging.info("Streaming finished")


def background_cleanup():
    logging.info("Cleanup started")
    time.sleep(19)
    logging.info("Cleanup finished")


async def start_background_task():
    loop = asyncio.get_event_loop()
    await loop.run_in_executor(None, background_cleanup)


@app.get("/stream")
async def get_stream(background_tasks: BackgroundTasks):
    response = StreamingResponse(stream_data())
    background_tasks.add_task(start_background_task)

    return response

Or update the following two functions to use async？

async def background_cleanup():
    logging.info("Cleanup started")
    await asyncio.sleep(19)
    logging.info("Cleanup finished")


async def start_background_task():
    task = asyncio.create_task(background_cleanup())
    await task

$ uvicorn async_data_streamer:app --reload
INFO:     Will watch for changes in these directories: ['/private/tmp']
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [77874] using WatchFiles
INFO:     Started server process [77876]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:55875 - "GET /stream HTTP/1.1" 200 OK
INFO:root:Sent data chunk: 0
INFO:root:Sent data chunk: 1
INFO:root:Sent data chunk: 2
INFO:root:Sent data chunk: 3
INFO:root:Sent data chunk: 4
INFO:root:Sent data chunk: 5
INFO:root:Sent data chunk: 6
INFO:root:Sent data chunk: 7
INFO:root:Sent data chunk: 8
INFO:root:Sent data chunk: 9
INFO:root:Streaming finished
INFO:root:Cleanup started
INFO:root:Cleanup finished
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO:     Finished server process [77876]
INFO:     Stopping reloader process [77874]

$ curl -v http://127.0.0.1:8000/stream
*   Trying 127.0.0.1:8000...
* Connected to 127.0.0.1 (127.0.0.1) port 8000
> GET /stream HTTP/1.1
> Host: 127.0.0.1:8000
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 200 OK
< date: Wed, 24 Jan 2024 12:07:40 GMT
< server: uvicorn
< transfer-encoding: chunked
<
data: 0
data: 1
data: 2
data: 3
data: 4
data: 5
data: 6
data: 7
data: 8
data: 9
* Connection #0 to host 127.0.0.1 left intact

[rohan-mehta]

@binbjz thanks so much, this works perfectly!