How can I add a background thread to flask?
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
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()
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()