How to run recurring task in the Python Flask framework?

(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.