Getting celery beat error 'SchedulingError: Couldn't apply scheduled task'

Hi, Thank you for writing a useful project for celery.

I am attempting to use this with scheduled tasks that run every minute that are configured in settings.py using celery beat config like below:

CELERYBEAT_SCHEDULE = {
    'check_products': {
      'task': 'com.ixlayer.tasks.check_products', 'schedule': timedelta(minutes=1),
    },
}

However sometimes I get an exception in the process that runs celery beat.

Traceback (most recent call last):
  File "/Users/kowsari/workspace/ixlayer-be/venv/lib/python2.7/site-packages/celery/beat.py", line 222, in apply_entry
    result = self.apply_async(entry, producer=producer, advance=False)
  File "/Users/kowsari/workspace/ixlayer-be/venv/lib/python2.7/site-packages/celery/beat.py", line 328, in apply_async
    entry, exc=exc)), sys.exc_info()[2])
  File "/Users/kowsari/workspace/ixlayer-be/venv/lib/python2.7/site-packages/celery/beat.py", line 320, in apply_async
    **entry.options)
  File "/Users/kowsari/workspace/ixlayer-be/venv/lib/python2.7/site-packages/celery_once/tasks.py", line 99, in apply_async
    raise e
SchedulingError: Couldn't apply scheduled task check_products:

I am using this configuration.

app.conf.ONCE = {
  'backend': 'celery_once.backends.Redis',
  'settings': {
    'url': settings.BROKER_URL,
    'default_timeout': 60 * 60
  }
}

Is this celerybeat getting the AlreadyQueued exception? Or is this something else? Help is much appreciated.

Thanks

[xuhcc]

I had a very similar problem with periodic task, and solution was to add graceful=True to its options:

@shared_task(base=QueueOnce, once={'graceful': True})
def my_task():
    ...

In my case AlreadyQueued exception was invisible due to logging config, so everything looked like a worker not picking up tasks.

[cameronmaske]

As a workaround, for now, I’d suggest using a pure celery task to trigger celery once task, i.e.

CELERYBEAT_SCHEDULE = {
    'example': {
      'task': 'tasks.scheduled_task', 'schedule': timedelta(minutes=1),
    },
}

@task()
def scheduled_task():
	scheduled_once_task.delay()

@task(base=QueueOnce)
def scheduled_once_task():
	....

I don’t have the bandwidth currently to fix this, but a PR (while it may be tricky, please include a test case for a scheduled task to aid the review) is welcome.