Periodic Tasks

Introduction

The celerybeat service enables you to schedule tasks to run at intervals.

Periodic tasks are defined as special task classes. Here’s an example of a periodic task:

from celery.decorators import periodic_task
from datetime import timedelta

@periodic_task(run_every=timedelta(seconds=30))
def every_30_seconds():
    print("Running periodic task!")

Crontab-like schedules

If you want a little more control over when the task is executed, for example, a particular time of day or day of the week, you can use the crontab schedule type:

from celery.task.schedules import crontab
from celery.decorators import periodic_task

@periodic_task(run_every=crontab(hour=7, minute=30, day_of_week=1))
def every_monday_morning():
    print("Execute every Monday at 7:30AM.")

The syntax of these crontab expressions is very flexible. Some examples:

Example	Meaning
crontab()	Execute every minute.
crontab(minute=0, hour=0)	Execute daily at midnight.
crontab(minute=0,	Execute every three hours—at midnight, 3am, 6am, 9am, noon, 3pm, 6pm, 9pm.
crontab(minute=0, hour=[0,3,6,9,12,15,18,21])	Same as previous.
crontab(minute=”*/15”)	Execute every 15 minutes.
crontab(day_of_week=”sunday”)	Execute every minute (!) at sundays.
crontab(minute=”*”, hour=”*”, day_of_week=”sun”)	Same as previous.
crontab(minute=”*/10”, hour=”3,17,22”, day_of_week=”thu,fri”)	Execute every ten minutes, but only between 3-4 am, 5-6 pm and 10-11 pm on thursdays or fridays.
crontab(minute=0, hour=”*/2,*/3”)	Execute every even hour, and every hour divisable by three. This means: at every hour except: 1am, 5am, 7am, 11am, 1pm, 5pm, 7pm, 11pm
crontab(minute=0, hour=”*/5”)	Execute hour divisable by 5. This means that it is triggered at 3pm, not 5pm (since 3pm equals the 24-hour clock value of “15”, which is divisable by 5).
crontab(minute=0, hour=”*/3,8-17”)	Execute every hour divisable by 3, and every hour during office hours (8am-5pm).

Starting celerybeat

If you want to use periodic tasks you need to start the celerybeat service. You have to make sure only one instance of this server is running at any time, or else you will end up with multiple executions of the same task.

To start the celerybeat service:

$ celerybeat

You can also start celerybeat with celeryd by using the -B option, this is convenient if you only have one server:

$ celeryd -B