How to run a permanent background process within the python-flask framework?

I am about to code a simple browser-based game in which players can connect to it via browser using python and flask. Now, I want this game still to be running when no player is connected to the backend, as things are going on in the game. That can be handled by a separate process running and accessing a database, and updating entries on the database when some event happens.

Is there then a simple way to incorporate that into a flask appication I am currently running as

import sys
import json
from flask import Flask, request, render_template

app = Flask(__name__)

from mf import gameplay

game = gameplay.Game()

@app.route('/')
def index():
	return render_template('mainmap.html')

@app.route('/api', methods=['POST'])
def api():
	data_json = request.data.decode()
	data = json.loads(data_json)
	return game(data_json)

if __name__ == '__main__':
	app.run('0.0.0.0', 5000, threaded=True)

And is there a way to "notify" the background process when a player is (re)connecting to the game, so that the background process "knows" a new player has connected, and the background process performs some specific actions?

You can use a task queue or a message broker system to handle asynchronous processing.

Examples include: Dramatiq, Huey, Redis Queue, APScheduler, QueueWorker, Celery, and so on.

An example out of many is Celery

pip install celery

You can create a file that has your background tasks; let us call it background_tasks.py

from celery import Celery

# Create a Celery instance. This can be done better. I am writing quick code
celery = Celery('background_tasks', broker='pyamqp://guest@localhost//')

@celery.task
def process_game_event(event_data):
    # Here you can apply your game events, perform specific actions, etc

Then in your app.py, you can incorporate the background process:

...
from background_tasks import process_game_event
...

@app.route('/api', methods=['POST'])
def api():
    data_json = request.data.decode()
    data = json.loads(data_json)
    process_game_event.delay(data_json) # Process events asynchronously using Celery
    return game(data_json)

Once again, yiou can use any other alternatives. This is only an example out of many, and your logic will be different based on your usecases.