Background Tasks stuck other requests when i use a http middleware

[w-Bro]

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Example Code

import time

from fastapi import FastAPI, BackgroundTasks, Request, Response
from loguru import logger

app = FastAPI()


@app.middleware("http")
async def logger_request(request: Request, call_next) -> Response:
    logger.debug(f"http middleware [{request.url}]")
    response = await call_next(request)
    return response


class Worker:
    def __init__(self, arg):
        self.arg = arg
        logger.debug(f"worker[{arg}] init")

    def work(self):
        for i in range(10):
            logger.debug(f"worker[{self.arg}] work[{i}]")
            time.sleep(1)


@app.get("/")
async def root():
    return {"message": "Hello World"}


@app.get("/work/{worker_id}")
async def work(back_task: BackgroundTasks, worker_id: int):
    worker = Worker(worker_id)
    back_task.add_task(worker.work)

    return {"message": f"work[{worker_id}] created"}


@app.get("/hello/{name}")
async def say_hello(name: str):
    return {"message": f"Hello {name}"}

Description

• router "/work/{worker_id}" would create a task(a sync task, actually) on backgroud, after calling it, other new requests will stuck until the task finish
• it just happen when i use a http middleware
• i knew it is a starlette issue but i could not find a solution yet, anybody tell me how to solve this?

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.74.1

Python Version

Python 3.7.10

Additional Context

No response

[rafsaf]

You are blocking event loop by calling sleep which is synchronous. This is also true for every sync code. Your code runs in single thread in single process, async works well that when there are a lot IO operations (db calls, server2server requests), for that time event loop "awaits" and can do other job, but remember at the end of the day it is still one process and one thread, CPU bound task will block it.

Of course there are option to scale it, for example see some fresh reddit discussion, https://www.reddit.com/r/Python/comments/sxovwp/async_io_tasks_vs_threads/

If you are using uvicorn, type in uvicorn --help and read about workers

--workers INTEGER               Number of worker processes. Defaults to the
                                  $WEB_CONCURRENCY environment variable if
                                  available, or 1. Not valid with --reload.

Having more workers (processes) would give you power to have multiple CPU bound code running in parallel (max is number of processes).

Some other - maybe better setups are workers in very different host/processes/containers and some queue. You add job to be done to Redis for example and 'worker' instance collect and execute them one after another. This requires some more complex system so if your requirement is to have MAX of few background task at once, more processes should be enough for you

[w-Bro]

You are blocking event loop by calling sleep which is synchronous. This is also true for every sync code. Your code runs in single thread in single process, async works well that when there are a lot IO operations (db calls, server2server requests), for that time event loop "awaits" and can do other job, but remember at the end of the day it is still one process and one thread, CPU bound task will block it.

Of course there are option to scale it, for example see some fresh reddit discussion, https://www.reddit.com/r/Python/comments/sxovwp/async_io_tasks_vs_threads/

If you are using uvicorn, type in uvicorn --help and read about workers

--workers INTEGER               Number of worker processes. Defaults to the
                                  $WEB_CONCURRENCY environment variable if
                                  available, or 1. Not valid with --reload.

Having more workers (processes) would give you power to have multiple CPU bound code running in parallel (max is number of processes).

Some other - maybe better setups are workers in very different host/processes/containers and some queue. You add job to be done to Redis for example and 'worker' instance collect and execute them one after another. This requires some more complex system so if your requirement is to have MAX of few background task at once, more processes should be enough for you

it's on me that i forgot to say that i had already use multi workers, even 16 workers(16 cores of my AMD cpu), it still stuck.

[ryuujo1573]

Hi, I might be stuck with some similar situation, what's wrong with my code:

@app.middleware("http")
async def signature(request: Request, call_next):
    try:
        signature = request.headers.get('X-Coding-Signature')
    except AttributeError:
        return Response(status_code=status.HTTP_403_FORBIDDEN, content='Authentification failed.')
    content = await request.body()
    sha1 = hmac.new(bytes(SECRET_TOKEN, encoding="utf8"), content, 'sha1')
    sha1 = sha1.hexdigest()
    calculate_signature = 'sha1=' + sha1
    if not calculate_signature == signature:
        return Response(status_code=status.HTTP_403_FORBIDDEN, content='Authentification failed.')
    else:
        return await call_next(request)

and the route hook

@app.post('/hook')
def simple_hook(json: dict[str, Any]):

    try:
        id = json['sender']['id']
        name = json['sender']['name']
        # eventName = json['eventName']

        print("[coding.net] %s(%d): %s. " % (name, id, 'eventName'))

    except Error as e:
        print("ERROR: %s" % e)
        return {'code': -1}

    return {'code': 0,'message':'done!'}

Everything was just fine without the signature middleware above. Any hints?
Thank you for your work and your any reply

[Kludex]

More about your issue: encode/starlette#1441

[ryuujo1573]

Subscribed, thank you.