How can I add a background thread to flask?

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Your additional threads must be initiated from the same app that is called by the WSGI server.

The example below creates a background thread that executes every 5 seconds and manipulates data structures that are also available to Flask routed functions.

import threading
import atexit
from flask import Flask

POOL_TIME = 5 #Seconds
    
# variables that are accessible from anywhere
commonDataStruct = {}
# lock to control access to variable
dataLock = threading.Lock()
# thread handler
yourThread = threading.Thread()

def create_app():
    app = Flask(__name__)

    def interrupt():
        global yourThread
        yourThread.cancel()

    def doStuff():
        global commonDataStruct
        global yourThread
        with dataLock:
            pass
            # Do your stuff with commonDataStruct Here

        # Set the next thread to happen
        yourThread = threading.Timer(POOL_TIME, doStuff, ())
        yourThread.start()   

    def doStuffStart():
        # Do initialisation stuff here
        global yourThread
        # Create your thread
        yourThread = threading.Timer(POOL_TIME, doStuff, ())
        yourThread.start()

    # Initiate
    doStuffStart()
    # When you kill Flask (SIGTERM), clear the trigger for the next thread
    atexit.register(interrupt)
    return app

app = create_app()

Call it from Gunicorn with something like this:

gunicorn -b 0.0.0.0:5000 --log-config log.conf --pid=app.pid myfile:app

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