Single FastAPI on_startup() hook with multiple Gunicorn workers (run only once for all processes combined)

main.py

# run with gunicorn:
# gunicorn --preload --workers 4 --worker-class=uvicorn.workers.UvicornWorker app.main:app
# ('preload' is the important bit here)
# alternatively, set GUNICORN_CMD_ARGS='--preload'

import ctypes
import multiprocessing as mp

from fastapi import FastAPI

app = FastAPI(
    title='Demo App',
    lifespan='on'
)

workers_started: mp.Value = mp.Value(ctypes.c_ushort)


async def on_started():
    '''Called when the first worker process is up and running'''
    pass


@app.on_event('startup')
async def on_startup():
    '''Called for every new Gunicorn worker'''
    with workers_started.get_lock():
        workers_started.value += 1
        if workers_started.value == 1:
            await on_started()