FastAPI Background task in middleware

Question

I'm trying to add background tasks to my middleware, but haven't found any official way to do it in their docs, this is what I have tried so far:

async def task1():
    logging.info("Waiting ...")
    time.sleep(5)
    logging.info("Waited.")

@app.middleware("http")
async def add_process_time_header(request, call_next, bt=Depends(BackgroundTasks)):
    a = bt.dependency()
    a.add_task(task1)
    a()

    return await call_next(request)

This is blocking my requests, also shouldn't I be able to call async functions without await? I want to completely ignore the results of the background tasks, I dont need results from bg tasks

Answer 1

You can attach a BackgroundTask instance to a response to instruct Starlette to perform a background task after the response is returned.

Code taken directly from this github issue thread and worked for me perfectly:

from starlette.background import BackgroundTask
from somewhere import your_background_task

@app.middleware("http")
async def middleware(request: Request, call_next):

    # middleware code

    response.background = BackgroundTask(your_background_task, arg)
    return response

Answer 2

I found some way to solve your problem, i think you can find better way,but for now you can use like this:

import logging
import time
import asyncio
from fastapi import FastAPI, Request

app = FastAPI()
@app.get("/")
async def call_api():
    await asyncio.sleep(5)
    return True

async def task1():
    logging.info("Waiting ...")
    await asyncio.sleep(5)
    logging.info("Waited.")

@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.time()

    response , res__task1 = await asyncio.gather(*[call_next(request),task1()])
    process_time = time.time() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    print(process_time)

    return response

A total of 5 seconds are spent on all tasks