Multiprocessing with Gunicorn + FastAPI

[IWillPull]

I've been trying to make multiprocessing work with FastAPI, but it just doesn't seem to happen.

This part is what I've been struggling the most:

def app(environ, start_response):
    registry = CollectorRegistry()
    multiprocess.MultiProcessCollector(registry)
    data = generate_latest(registry)
    status = '200 OK'
    response_headers = [
        ('Content-type', CONTENT_TYPE_LATEST),
        ('Content-Length', str(len(data)))
    ]
    start_response(status, response_headers)
    return iter([data])

Had no idea where to put this, but after some experimentation it turns out, the simplest solution of creating a metrics app looks like this (including other parts from the documentation):

app = FastAPI(debug=False)

# Preparing gunicorn multiprocessing HACKS
def make_metrics_app():
    registry = CollectorRegistry()
    multiprocess.MultiProcessCollector(registry)
    return prometheus_client.make_asgi_app(registry=registry)


metrics_app = make_metrics_app()
app.mount("/metrics", metrics_app))

Just posting this to help others save hours of researching and debugging.

[csmarchbanks]

Thanks for the report! Would you be interested in adding some documentation to the README similar to how we have an example for flask, but under the multiprocess section?

[IWillPull]

@csmarchbanks If you think this is a correct way of doing things, I will add the instructions to the README

[ashirviskas]

(me from my alt acc)
Added my PR here: #812

[Canas]

Looking at the README I see this

It is a best practice to create this registry inside the context of a request

Doesn't this mean that we could add that setup in a route? (see below)
I've seen other people also use the gunicorn when_ready hook to do the setup. Not sure what would be the best way and why.

@app.get("/metrics")
def metrics(request: Request):
    registry = CollectorRegistry()
    multiprocess.MultiProcessCollector(registry)
    data = generate_latest(registry)
    res = Response(content=data)
    res.headers["Content-Type"] = CONTENT_TYPE_LATEST
    return res

[csmarchbanks]

Closing this as the information is now in the readme!