Django - run a function every x seconds

Question

I'm working on a Django app. I have an API endpoint, which if requested, must carry out a function that must be repeated a few times (until a certain condition is true). How I'm dealing with it right now is -

def shut_down(request):
  # Do some stuff
  while True:
    result = some_fn()
    if result:
      break
    time.sleep(2)

  return True

While I know that this is a terrible approach and that I shouldn't be blocking for 2 seconds, I can't figure out how to get around it.
This works, after say a wait of 4 seconds. But I'd like something that keeps the loop running in the background, and stop once some_fn returns True. (Also, it is certain that some_fn will return True)

EDIT -
Reading Oz123's response gave me an idea which seems to work. Here's what I did -

def shut_down(params):
    # Do some stuff
    # Offload the blocking job to a new thread

    t = threading.Thread(target=some_fn, args=(id, ), kwargs={})
    t.setDaemon(True)
    t.start()

    return True

def some_fn(id):
    while True:
        # Do the job, get result in res
        # If the job is done, return. Or sleep the thread for 2 seconds before trying again.

        if res:
            return
        else:
            time.sleep(2)

This does the job for me. It's simple but I don't know how efficient multithreading is in conjunction with Django.
If anyone can point out pitfalls of this, criticism is appreciated.

Answer 1

For many small projects celery is overkill. For those projects you can use schedule, it's very easy to use.

With this library you can make any function execute a task periodically:

import schedule
import time

def job():
    print("I'm working...")

schedule.every(10).minutes.do(job)
schedule.every().hour.do(job)
schedule.every().day.at("10:30").do(job)
schedule.every().monday.do(job)
schedule.every().wednesday.at("13:15").do(job)

while True:
    schedule.run_pending()
    time.sleep(1) 

The example runs in a blocking manner, but if you look in the FAQ, you will find that you can also run tasks in a parallel thread, such that you are not blocking, and remove the task once not needed anymore:

import threading    
import time 

from schedule import Scheduler

def run_continuously(self, interval=1):
    """Continuously run, while executing pending jobs at each elapsed
    time interval.
    @return cease_continuous_run: threading.Event which can be set to
    cease continuous run.
    Please note that it is *intended behavior that run_continuously()
    does not run missed jobs*. For example, if you've registered a job
    that should run every minute and you set a continuous run interval
    of one hour then your job won't be run 60 times at each interval but
    only once.
    """

    cease_continuous_run = threading.Event()

    class ScheduleThread(threading.Thread):

        @classmethod
        def run(cls):
            while not cease_continuous_run.is_set():
                self.run_pending()
                time.sleep(interval)

    continuous_thread = ScheduleThread()
    continuous_thread.setDaemon(True)
    continuous_thread.start()
    return cease_continuous_run


Scheduler.run_continuously = run_continuously

Here is an example for usage in a class method:

    def foo(self):
        ...
        if some_condition():
           return schedule.CancelJob  # a job can dequeue it

    # can be put in __enter__ or __init__
    self._job_stop = self.scheduler.run_continuously()

    logger.debug("doing foo"...)
    self.foo() # call foo
    self.scheduler.every(5).seconds.do(
        self.foo) # schedule foo for running every 5 seconds
    
    ...
    # later on foo is not needed any more:
    self._job_stop.set()
    
    ...
    
    def __exit__(self, exec_type, exc_value, traceback):
        # if the jobs are not stop, you can stop them
        self._job_stop.set()
    

Answer 2

This answer expands on Oz123's answer a little bit.

In order to get things working, I created a file called mainapp/jobs.py to contain my scheduled jobs. Then, in my apps.py module, I put from . import jobs in the ready method. Here's my entire apps.py file:

from django.apps import AppConfig
import os

class MainappConfig(AppConfig):
    name = 'mainapp'

    def ready(self):
        from . import jobs

        if os.environ.get('RUN_MAIN', None) != 'true':
            jobs.start_scheduler()

(The RUN_MAIN check is because python manage.py runserver runs the ready method twice—once in each of two processes—but we only want to run it once.)

Now, here's what I put in my jobs.py file. First, the imports. You'll need to import Scheduler, threading and time as below. The F and UserHolding imports are just for what my job does; you won't import these.

from django.db.models import F
from schedule import Scheduler
import threading
import time

from .models import UserHolding

Next, write the function you want to schedule. The following is purely an example; your function won't look anything like this.

def give_admin_gold():
    admin_gold_holding = (UserHolding.objects
        .filter(inventory__user__username='admin', commodity__name='gold'))

    admin_gold_holding.update(amount=F('amount') + 1)

Next, monkey-patch the schedule module by adding a run_continuously method to its Scheduler class. Do this by using the below code, which is copied verbatim from Oz123's answer.

def run_continuously(self, interval=1):
    """Continuously run, while executing pending jobs at each elapsed
    time interval.
    @return cease_continuous_run: threading.Event which can be set to
    cease continuous run.
    Please note that it is *intended behavior that run_continuously()
    does not run missed jobs*. For example, if you've registered a job
    that should run every minute and you set a continuous run interval
    of one hour then your job won't be run 60 times at each interval but
    only once.
    """

    cease_continuous_run = threading.Event()

    class ScheduleThread(threading.Thread):

        @classmethod
        def run(cls):
            while not cease_continuous_run.is_set():
                self.run_pending()
                time.sleep(interval)

    continuous_thread = ScheduleThread()
    continuous_thread.setDaemon(True)
    continuous_thread.start()
    return cease_continuous_run

Scheduler.run_continuously = run_continuously

Finally, define a function to create a Scheduler object, wire up your job, and call the scheduler's run_continuously method.

def start_scheduler():
    scheduler = Scheduler()
    scheduler.every().second.do(give_admin_gold)
    scheduler.run_continuously()

Answer 3

I recommend you use Celery's task management. You can refer this to set up this app (package if you're from javaScript background).

Once set, you can alter the code to:

@app.task
def check_shut_down():
    if not some_fun():
        # add task that'll run again after 2 secs
        check_shut_down.delay((), countdown=3)
    else:
        # task completed; do something to notify yourself
        return True

Answer 4

I can't comment on oz123's (https://stackoverflow.com/a/44897678/1108505) and Tanner Swett's (https://stackoverflow.com/a/60244694/5378866) excellent post, but as a final note I wanted to add that if you use Gunicorn and you have X number of workers, the section:

from django.apps import AppConfig
import os

class MainappConfig(AppConfig):
    name = 'mainapp'

    def ready(self):
        from . import jobs

        if os.environ.get('RUN_MAIN', None) != 'true':
            jobs.start_scheduler()

will be executed that same number of times, launching X schedulers at the same time.

If we only want it to run only one instance (for example if you're going to create objects in the database), we would have to add in our gunicorn.conf.py file something like this:

def on_starting(server):
    from app_project import jobs
    jobs.start_scheduler()

And finally in the gunicorn call add the argument --preload

Answer 5

Here is my solution, with sources noted. This function will allow you to create a scheduler that you can start with your app, then add and subtract jobs at will. The check_interval variable allows you to trade-off between system resources and job execution timing.

from schedule import Scheduler
import threading
import warnings
import time


class RepeatTimer(threading.Timer):
    """Add repeated run of target to timer functionality. Source: https://stackoverflow.com/a/48741004/16466191"""
    running: bool = False

    def __init__(self, *args, **kwargs):
        threading.Timer.__init__(self, *args, **kwargs)

    def start(self) -> None:
        """Protect from running start method multiple times"""
        if not self.running:
            super(RepeatTimer, self).start()
            self.running = True
        else:
            warnings.warn('Timer is already running, cannot be started again.')

    def cancel(self) -> None:
        """Protect from running stop method multiple times"""
        if self.running:
            super(RepeatTimer, self).cancel()
            self.running = False
        else:
            warnings.warn('Timer is already canceled, cannot be canceled again.')

    def run(self):
        """Replace run method of timer to run continuously"""
        while not self.finished.wait(self.interval):
            self.function(*self.args, **self.kwargs)


class ThreadedScheduler(Scheduler, RepeatTimer):
    """Non-blocking scheduler. Advice taken from: https://stackoverflow.com/a/50465583/16466191"""
    def __init__(
            self,
            run_pending_interval: float,
    ):
        """Initialize parent classes"""
        Scheduler.__init__(self)
        super(RepeatTimer, self).__init__(
            interval=run_pending_interval,
            function=self.run_pending,
        )


def print_work(what_to_say: str):
    print(what_to_say)


if __name__ == '__main__':
    my_schedule = ThreadedScheduler(run_pending_interval=1)
    job1 = my_schedule.every(1).seconds.do(print_work, what_to_say='Did_job1')
    job2 = my_schedule.every(2).seconds.do(print_work, what_to_say='Did_job2')
    my_schedule.cancel()
    my_schedule.start()
    time.sleep(7)
    my_schedule.cancel_job(job1)
    my_schedule.start()
    time.sleep(7)
    my_schedule.cancel()