kramer65 kramer65 - 10 months ago 141
Python Question

How to run recurring task in the Python Flask framework?

I'm building a website which provides some information to the visitors. This information is aggregated in the background by polling a couple external APIs every 5 seconds. The way I have it working now is that I use APScheduler jobs. I initially preferred APScheduler because it makes the whole system more easy to port (since I don't need to set cron jobs on the new machine). I start the polling functions as follows:

from apscheduler.scheduler import Scheduler

@app.before_first_request
def initialize():
apsched = Scheduler()
apsched.start()

apsched.add_interval_job(checkFirstAPI, seconds=5)
apsched.add_interval_job(checkSecondAPI, seconds=5)
apsched.add_interval_job(checkThirdAPI, seconds=5)


This kinda works, but there's some trouble with it:


  1. For starters, this means that the interval-jobs are running outside of the Flask context. So far this hasn't been much of a problem, but when calling an endpoint fails I want the system to send me an email (saying "hey calling API X failed"). Because it doesn't run within the Flask context however, it complaints that flask-mail cannot be executed (
    RuntimeError('working outside of application context')
    ).

  2. Secondly, I wonder how this is going to behave when I don't use the Flask built-in debug server anymore, but a production server with lets say 4 workers. Will it start every job four times then?



All in all I feel that there should be a better way of running these recurring tasks, but I'm unsure how. Does anybody out there have an interesting solution to this problem? All tips are welcome!

[EDIT]
I've just been reading about Celery with its schedules. Although I don't really see how Celery is different from APScheduler and whether it could thus solve my two points, I wonder if anyone reading this thinks that I should investigate more in Celery?

[CONCLUSION]
About two years later I'm reading this, and I thought I could let you guys know what I ended up with. I figured that @BluePeppers was right in saying that I shouldn't be tied so closely to the Flask ecosystem. So I opted for regular cron-jobs running every minute which are set using Ansible. Although this makes it a bit more complex (I needed to learn Ansible and convert some code so that running it every minute would be enough) I think this is more robust.
I'm currently using the awesome pythonr-rq for queueing a-sync jobs (checking APIs and sending emails). I just found out about rq-scheduler. I haven't tested it yet, but it seems to do precisely what I needed in the first place. So maybe this is a tip for future readers of this question.

For the rest, I just wish all of you a beautiful day!

Answer

(1)

You can use the app.app_context() context manager to set the application context. I imagine usage would go something like this:

from apscheduler.scheduler import Scheduler

def checkSecondApi():
    with app.app_context():
        # Do whatever you were doing to check the second API

@app.before_first_request
def initialize():
    apsched = Scheduler()
    apsched.start()

    apsched.add_interval_job(checkFirstAPI, seconds=5)
    apsched.add_interval_job(checkSecondAPI, seconds=5)
    apsched.add_interval_job(checkThirdAPI, seconds=5)

Alternatively, you could use a decorator

def with_application_context(app):
    def inner(func):
        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            with app.app_context():
                return func(*args, **kwargs)
        return wrapper
    return inner

@with_application_context(app)
def checkFirstAPI():
    # Check the first API as before

(2)

Yes it will still work. The sole (significant) difference is that your application will not be communicating directly with the world; it will be going through a reverse proxy or something via fastcgi/uwsgi/whatever. The only concern is that if you have multiple instances of the app starting, then multiple schedulers will be created. To manage this, I would suggest you move your backend tasks out of the Flask application, and use a tool designed for running tasks regularly (i.e. Celery). The downside to this is that you won't be able to use things like Flask-Mail, but imo, it's not too good to be so closely tied to the Flask ecosystem; what are you gaining by using Flask-Mail over a standard, non Flask, mail library?

Also, breaking up your application makes it much easier to scale up individual components as the capacity is required, compared to having one monolithic web application.