Getting httpx.PoolTimeout when issuing huge number of requests (10000) in asynchronously that take 100 ms each

[marcinbarczynski]

To reproduce:

demo-fastapi.py:

from fastapi import FastAPI
import asyncio

app = FastAPI()


@app.get("/")
async def root():
    await asyncio.sleep(0.1)  # no PoolTimeout without this line 
    return {}

httpx-pool-timeout.py:

import asyncio
import httpx
import uvloop
import sys


httpx_session = None


async def send_async_request():
    resp = await httpx_session.get(sys.argv[1])
    if resp.status_code != 200:
        raise RuntimeError(resp)


async def main_async():
    global httpx_session
    httpx_session = httpx.AsyncClient()
    tasks = []
    for _ in range(10_000):
        tasks.append(asyncio.create_task(send_async_request()))
    await asyncio.gather(*tasks)
    await httpx_session.aclose()


if __name__ == "__main__":
    uvloop.install()
    asyncio.run(main_async())

Run:

$ uvicorn demo-fastapi:app --no-access-log --loop=uvloop

and then

$ python httpx-pool-timeout.py http://localhost:8000

fails with:

Traceback (most recent call last):
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_synchronization.py", line 38, in wait
    await self._event.wait()
  File "/path/to/virtualenv/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 1842, in wait
    if await self._event.wait():
  File "/path/to/python3.10/lib/python3.10/asyncio/locks.py", line 214, in wait
    await fut
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_exceptions.py", line 8, in map_exceptions
    yield
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_synchronization.py", line 37, in wait
    with anyio.fail_after(timeout):
  File "/path/to/virtualenv/lib/python3.10/site-packages/anyio/_core/_tasks.py", line 118, in __exit__
    raise TimeoutError
TimeoutError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_transports/default.py", line 60, in map_httpcore_exceptions
    yield
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_transports/default.py", line 353, in handle_async_request
    resp = await self._pool.handle_async_request(req)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 234, in handle_async_request
    raise exc
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 227, in handle_async_request
    connection = await status.wait_for_connection(timeout=timeout)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 34, in wait_for_connection
    await self._connection_acquired.wait(timeout=timeout)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_synchronization.py", line 36, in wait
    with map_exceptions(exc_map):
  File "/path/to/python3.10/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpcore/_exceptions.py", line 12, in map_exceptions
    raise to_exc(exc)
httpcore.PoolTimeout

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/media/q/Encrypted/Starfish/httpx-pool-timeout.py", line 28, in <module>
    asyncio.run(main_async())
  File "/path/to/python3.10/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "uvloop/loop.pyx", line 1517, in uvloop.loop.Loop.run_until_complete
  File "/media/q/Encrypted/Starfish/httpx-pool-timeout.py", line 22, in main_async
    await asyncio.gather(*tasks)
  File "/media/q/Encrypted/Starfish/httpx-pool-timeout.py", line 11, in send_async_request
    resp = await httpx_session.get(sys.argv[1])
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1751, in get
    return await self.request(
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1527, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1614, in send
    response = await self._send_handling_auth(
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1642, in _send_handling_auth
    response = await self._send_handling_redirects(
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1679, in _send_handling_redirects
    response = await self._send_single_request(request)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_client.py", line 1716, in _send_single_request
    response = await transport.handle_async_request(request)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_transports/default.py", line 352, in handle_async_request
    with map_httpcore_exceptions():
  File "/path/to/python3.10/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/path/to/virtualenv/lib/python3.10/site-packages/httpx/_transports/default.py", line 77, in map_httpcore_exceptions
    raise mapped_exc(message) from exc
httpx.PoolTimeout

I am using Python 3.10 and httpx 0.23.0.
The problem doesn't happen if the requests are faster (i.e. no asyncio.sleep in the server code) or there is a lower volume of requests (e.g. 1000).

[marcinbarczynski]

It turned out that the default pool timeout is only 5 seconds: https://github.com/encode/httpx/blob/master/httpx/_config.py#L361

[tomchristie]

Indeed. https://www.python-httpx.org/advanced/#timeout-configuration