Django celery: scheduling recurring task but with fixed start date

Question

At the moment djcelery allows me to schedule a recurring task via the PeriodicTask model. For example a task that runs on an interval like every minute, or an interval specified by a crontab like every 1st of the month at noon. What I'd really like to do however is schedule a task for a fixed date that then repeats on an interval. For example first run on 3 March 2016 at 2:00 and then every hour thereafter.

Is there a way to achieve this within django and celery(with or without djcelery)? Thanks

Answer 1

As it is stated in the docs, you may implement your own custom scheduler. You should override the is_due method, which decides whether it is time to run the task.

Below is a proof-of-concept (I haven't checked it for errors). Note, that the __reduce__ method is also overridden so that the new parameter gets serialised as well.

import celery.schedules.schedule

class myschedule(celery.schedules.schedule):

    def __init__(self, *args, **kwargs):
        super(myschedule, self).__init__(*args, **kwargs)
        self.start_date = kwargs.get('start_date', None)

    def is_due(self, last_run_at):
        if self.start_date is not None and self.now() < self.start_date:
            return (False, 20)  # try again in 20 seconds
        return super(myschedule, self).is_due(last_run_at)

    def __reduce__(self):
        return self.__class__, (self.run_every, self.relative, self.nowfun, self.start_date)

And then you use it in the config:

CELERYBEAT_SCHEDULE = {
    'add-every-30-seconds': {
        'task': 'tasks.add',
        'schedule': myschedule(timedelta(seconds=30), start_date=start_date),
        'args': (16, 16)
    },
}