How can I add a background thread to flask?

Question

I'm busy writing a small game server to try out flask. The game exposes an API via REST to users. It's easy for users to perform actions and query data, however I'd like to service the "game world" outside the app.run() loop to update game entities, etc. Given that Flask is so cleanly implemented, I'd like to see if there's a Flask way to do this.

You mean something like Flask-Admin? Or if you are using an ORM (SQL-Alchemy), then you can just create a new db session to query the database even if the application is running. — reptilicus, Jan 17 '13 at 18:38
If you actually need to do a lot of computation, you might want to use the subprocess module, and simply spawn new processes to do that additional computation. — Maus, Jan 17 '13 at 22:02
That is a plan, however the sub process will be manipulating data structures, that you'd like to access and set via the exposed flask api. Will I not run into problems? — Marinus, Jan 18 '13 at 14:41
this looks pretty good http://flask.pocoo.org/docs/patterns/celery/ — cecilphillip, Jun 14 '14 at 22:39
possible duplicate of [Background Worker with Flask](http://stackoverflow.com/questions/11256002/background-worker-with-flask) — George Hilliard, Aug 13 '14 at 19:40
It looks like there's [a hackish way to do it](https://stackoverflow.com/a/9932189/240418), but I don't think this is technically supported. I also found [this answer](https://stackoverflow.com/a/11257228/240418), which talks about using flask-celery for this. — girasquid, Jan 17 '13 at 18:42
@girasquid Agreed, celery or some other task queue system is ideal for this sort of thing - you generally have less control over threads or sub-processes (since the parent process may be reaped by the server without notice). — Sean Vieira, Jan 18 '13 at 03:14

score 101 · Accepted Answer · edited Sep 26 '21 at 12:38

101

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

edited Sep 26 '21 at 12:38

Neuron - Freedom for Ukraine

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answered Apr 06 '14 at 21:27

caio

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I found this to be problematic when using flask's auto-reload functionality (a new thread got created on every reload). To fix this, I used [werkzeug.serving.is_running_from_reloader](http://werkzeug.pocoo.org/docs/0.10/serving/#werkzeug.serving.is_running_from_reloader) to only create it when the app is not running from the reloader. – raffomania Sep 06 '15 at 13:05
2

@caio it should be "with dataLock:" capital L above. – Jesse Sanford Jun 20 '16 at 19:43
This is a nice solution; helps deal with flask apps that use multiprocessing or threading modules. I like it. – Shan Valleru Jan 14 '17 at 07:08
2

This example is a little confusing because the object created called "yourThread" is not a thread. It's a timer: suggest you rename it. And, when yourTimer is executed ( in doStuff ), I don't know if yourThread is valid - ie, if you can execute cancel on a Timer that hasn't been executed. It has the efficiency issue that it's creating a new object every execution, if that might be an issue. – Brian Bulkowski Apr 10 '17 at 04:07
1

The correct statement for checking "is_running_in_background()" is like so: from werkzeug.serving import is_running_from_reloader if is_running_from_reloader() == False: startBackground() – Brian Bulkowski Apr 10 '17 at 16:25
Starting the thread from the non reloader process will prevent the thread from accessing your Flask app context (it runs in the reloader process), so depending on what you want to achieve, you have to decide between starting it from the main process (`is_running_from_reloader()` returning `False`) or the reloader process (`is_running_from_reloader()` returning `True`). BTW, on reload, the previous reloader process is killed (with all its threads) and a new one is spawned. – Tey' Aug 14 '20 at 22:41

score 11 · Answer 2 · answered Jul 19 '17 at 04:18

In addition to using pure threads or the Celery queue (note that flask-celery is no longer required), you could also have a look at flask-apscheduler:

https://github.com/viniciuschiele/flask-apscheduler

A simple example copied from https://github.com/viniciuschiele/flask-apscheduler/blob/master/examples/jobs.py:

from flask import Flask
from flask_apscheduler import APScheduler


class Config(object):
    JOBS = [
        {
            'id': 'job1',
            'func': 'jobs:job1',
            'args': (1, 2),
            'trigger': 'interval',
            'seconds': 10
        }
    ]

    SCHEDULER_API_ENABLED = True


def job1(a, b):
    print(str(a) + ' ' + str(b))

if __name__ == '__main__':
    app = Flask(__name__)
    app.config.from_object(Config())

    scheduler = APScheduler()
    # it is also possible to enable the API directly
    # scheduler.api_enabled = True
    scheduler.init_app(app)
    scheduler.start()

    app.run()

Can we start multiple jobs concurently that will not share same object storage? — alper, Sep 13 '21 at 21:23

Dmitry Ponyatov · Answer 3 · 2021-02-08T11:44:13.253

First, you should use any WebSocket or polling mechanics to notify the frontend part about changes that happened. I use Flask-SocketIO wrapper, and very happy with async messaging for my tiny apps.

Nest, you can do all logic which you need in a separate thread(s), and notify the frontend via SocketIO object (Flask holds continuous open connection with every frontend client).

As an example, I just implemented page reload on backend file modifications:

<!doctype html>
<script>
    sio = io()

    sio.on('reload',(info)=>{
        console.log(['sio','reload',info])
        document.location.reload()
    })
</script>

class App(Web, Module):

    def __init__(self, V):
        ## flask module instance
        self.flask = flask
        ## wrapped application instance
        self.app = flask.Flask(self.value)
        self.app.config['SECRET_KEY'] = config.SECRET_KEY
        ## `flask-socketio`
        self.sio = SocketIO(self.app)
        self.watchfiles()

    ## inotify reload files after change via `sio(reload)``
    def watchfiles(self):
        from watchdog.observers import Observer
        from watchdog.events import FileSystemEventHandler
        class Handler(FileSystemEventHandler):
            def __init__(self,sio):
                super().__init__()
                self.sio = sio
            def on_modified(self, event):
                print([self.on_modified,self,event])
                self.sio.emit('reload',[event.src_path,event.event_type,event.is_directory])
        self.observer = Observer()
        self.observer.schedule(Handler(self.sio),path='static',recursive=True)
        self.observer.schedule(Handler(self.sio),path='templates',recursive=True)
        self.observer.start()

How can I add a background thread to flask?

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