[Python] FastAPI Global Timeout Middleware Not Working for Sync Routes

I am trying to enforce a global request timeout in my FastAPI application using a custom TimeoutMiddleware. The middleware is supposed to cancel any request that takes longer than the specified timeout.

Here is my TimeoutMiddleware implementation:

from starlette.middleware.base import BaseHTTPMiddleware
from fastapi import FastAPI, Request
from starlette.responses import JSONResponse
import asyncio

class TimeoutMiddleware(BaseHTTPMiddleware):
def __init__(self, app, timeout: int = 5):
super().__init__(app)
self.timeout = timeout

async def dispatch(self, request: Request, call_next):
try:
response = await asyncio.wait_for(call_next(request), timeout=self.timeout)
return response
except asyncio.TimeoutError:
return JSONResponse(status_code=408, content={"detail": "Request Timeout"})

def create_app(routers, timeout: int = 2) -> FastAPI:
app = FastAPI()
app.add_middleware(TimeoutMiddleware, timeout=timeout)
for router, prefix in routers:
app.include_router(router, prefix=prefix)
return app



Problem: The middleware works correctly for async routes but does not work for sync routes.

Example where timeout works:

import asyncio
from fastapi import APIRouter

aparat_router = APIRouter()

@aparat_router.api_route("/video", methods=["GET", "POST"])
async def aparat_messages():
await asyncio.sleep(5) # This correctly triggers the timeout
return {"message": "Delayed Response"}



Example where timeout does NOT work:

import time
from fastapi import APIRouter

aparat_router = APIRouter()

@aparat_router.api_route("/video", methods=["GET", "POST"])
def aparat_messages():
time.sleep(5) # This does NOT trigger the timeout
return {"message": "Delayed Response"}



Expected Behavior: Both sync and async routes should respect the timeout and return a 408 Request Timeout if execution exceeds the limit. Actual Behavior: The timeout only works for async routes. Sync routes block the event loop and do not trigger the timeout. Question: How can I ensure that my global timeout middleware also works for synchronous routes? Is there a better approach to handling request timeouts in FastAPI that supports both sync and async routes?

fastapi==0.104.0
uvicorn==0.23.0

Continue reading...