Hypercorn runs with duplicated process

[stevstrong]

Hi all,

I am not sure whether this is really hypercorn issue, but could not imagine what else can be so please bear with me.

I am running a server with hypercorn on Ubuntu 20.04.

The problem is that it is runs with a duplicated process in background.

root     2278497  0.8  0.1  41872 33568 pts/7    S    10:03   0:00 /usr/bin/python3 /usr/local/bin/hypercorn -c config.toml main:app --reload
root     2278499  0.0  0.0  17304 11332 pts/7    S    10:03   0:00 /usr/bin/python3 -c from multiprocessing.resource_tracker import main;main(4)
root     2278500  0.7  0.1  41648 34148 pts/7    S    10:03   0:00 /usr/bin/python3 -c from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=5, pipe_handle=7) --multiprocessing-fork

The main process is 2278497, but there are duplicated processes 2278499 and 2278450.
This causes unwanted effects by executing twice the same tasks.

How can I avoid that?

EDIT
A minimal example:

# test_main.py
from fastapi import FastAPI

app = FastAPI()

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

print("main module loaded.")

I then type:

sudo hypercorn test_main:app

and the stdout is:

main module loaded.
main module loaded.
[2022-11-02 15:08:45 +0100] [2364437] [INFO] Running on http://127.0.0.1:8000 (CTRL + C to quit)

If I use uvicorn the message is printed only one time, as expected:

$ sudo uvicorn test_main:app

main module loaded.
INFO:     Started server process [2364692]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

[stevstrong]

I just provided a minimal example to reproduce the problem in the first post.

Anybody any idea?
Maybe it has to do with multiple python versions installed?

[synodriver]

I guess that is because hypercorn fork before import, while uvicorn import first.

[stevstrong]

Am I doing something wrong?
I just followed the getting started docs...

[pgjones]

This is expected - is it causing a problem? (Hypercorn runs with a minimum of 2 processes).

[stevstrong]

Well, yes, it causing a problem, as it executes two times the same procedure in parallel, which should be executed only once...

[stevstrong]

Is there any option which forces hypercorn to run only with one process?

[pgjones]

What does it actually execute twice though? I think it will log twice, but only one app worker will run - the other worker manages the reloading.

[stevstrong]

In the real application the main module includes some other modules, and some of the other modules start a separate thread (kind of background service).
This thread should have only one instance.
But if hypercorn includes twice the same module, then I will have twice the mentioned thread, which is unwanted.

[pgjones]

Ah, I see. I would consider it problematic for a module to create side effects (start a thread) when imported. Instead I suggest you start the thread actively and place that call within a startup function. If you are using Quart you can do something like this,

@app.before_serving
async def startup():
    start_background_service()

Edit: I see you are using FastAPI, in which case you can use a startup event https://fastapi.tiangolo.com/advanced/events/

[stevstrong]

OK, it looks like a reasonable solution, thank you very much for your support.