This document is for Celery's development version, which can be significantly different from previous releases. Get the stable docs here: 3.1.

Periodic Tasks

Introduction

celery beat is a scheduler. It kicks off tasks at regular intervals, which are then executed by the worker nodes available in the cluster.

By default the entries are taken from the beat_schedule setting, but custom stores can also be used, like storing the entries in an SQL database.

You have to ensure only a single scheduler is running for a schedule at a time, otherwise you would end up with duplicate tasks. Using a centralized approach means the schedule does not have to be synchronized, and the service can operate without using locks.

Time Zones

The periodic task schedules uses the UTC time zone by default, but you can change the time zone used using the timezone setting.

An example time zone could be Europe/London:

timezone = 'Europe/London'

This setting must be added to your app, either by configuration it directly using (app.conf.timezone = 'Europe/London'), or by adding it to your configuration module if you have set one up using app.config_from_object. See Configuration for more information about configuration options.

The default scheduler (storing the schedule in the celerybeat-schedule file) will automatically detect that the time zone has changed, and so will reset the schedule itself, but other schedulers may not be so smart (e.g. the Django database scheduler, see below) and in that case you will have to reset the schedule manually.

Django Users

Celery recommends and is compatible with the new USE_TZ setting introduced in Django 1.4.

For Django users the time zone specified in the TIME_ZONE setting will be used, or you can specify a custom time zone for Celery alone by using the timezone setting.

The database scheduler will not reset when timezone related settings change, so you must do this manually:

$ python manage.py shell
>>> from djcelery.models import PeriodicTask
>>> PeriodicTask.objects.update(last_run_at=None)

Entries

To call a task periodically you have to add an entry to the beat schedule list.

from celery import Celery
from celery.schedules import crontab

app = Celery()

@app.on_after_configure.connect
def setup_periodic_tasks(sender, **kwargs):
    # Calls test('hello') every 10 seconds.
    sender.add_periodic_task(10.0, test.s('hello'), name='add every 10')

    # Calls test('world') every 30 seconds
    sender.add_periodic_task(30.0, test.s('world'), expires=10)

    # Executes every Monday morning at 7:30 A.M
    sender.add_periodic_task(
        crontab(hour=7, minute=30, day_of_week=1),
        test.s('Happy Mondays!'),
    )

@app.task
def test(arg):
    print(arg)

Setting these up from within the on_after_configure handler means that we will not evaluate the app at module level when using test.s().

The add_periodic_task() function will add the entry to the beat_schedule setting behind the scenes, which also can be used to set up periodic tasks manually:

Example: Run the tasks.add task every 30 seconds.

app.conf.beat_schedule = {
    'add-every-30-seconds': {
        'task': 'tasks.add',
        'schedule': 30.0,
        'args': (16, 16)
    },
}
app.conf.timezone = 'UTC'

Note

If you are wondering where these settings should go then please see Configuration. You can either set these options on your app directly or you can keep a separate module for configuration.

If you want to use a single item tuple for args, don’t forget that the constructor is a comma and not a pair of parentheses.

Using a timedelta for the schedule means the task will be sent in 30 second intervals (the first task will be sent 30 seconds after celery beat starts, and then every 30 seconds after the last run).

A Crontab like schedule also exists, see the section on Crontab schedules.

Like with cron, the tasks may overlap if the first task does not complete before the next. If that is a concern you should use a locking strategy to ensure only one instance can run at a time (see for example Ensuring a task is only executed one at a time).

Available Fields

  • task

    The name of the task to execute.

  • schedule

    The frequency of execution.

    This can be the number of seconds as an integer, a timedelta, or a crontab. You can also define your own custom schedule types, by extending the interface of schedule.

  • args

    Positional arguments (list or tuple).

  • kwargs

    Keyword arguments (dict).

  • options

    Execution options (dict).

    This can be any argument supported by apply_async(), e.g. exchange, routing_key, expires, and so on.

  • relative

    By default timedelta schedules are scheduled “by the clock”. This means the frequency is rounded to the nearest second, minute, hour or day depending on the period of the timedelta.

    If relative is true the frequency is not rounded and will be relative to the time when celery beat was started.

Crontab schedules

If you want 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.schedules import crontab

app.conf.beat_schedule = {
    # Executes every Monday morning at 7:30 A.M
    'add-every-monday-morning': {
        'task': 'tasks.add',
        'schedule': crontab(hour=7, minute=30, day_of_week=1),
        'args': (16, 16),
    },
}

The syntax of these Crontab expressions are very flexible. Some examples:

Example Meaning
crontab() Execute every minute.
crontab(minute=0, hour=0) Execute daily at midnight.
crontab(minute=0, hour='*/3') Execute every three hours: 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 divisible by three. This means: at every hour except: 1am, 5am, 7am, 11am, 1pm, 5pm, 7pm, 11pm
crontab(minute=0, hour='*/5') Execute hour divisible by 5. This means that it is triggered at 3pm, not 5pm (since 3pm equals the 24-hour clock value of “15”, which is divisible by 5).
crontab(minute=0, hour='*/3,8-17') Execute every hour divisible by 3, and every hour during office hours (8am-5pm).
crontab(0, 0, day_of_month='2') Execute on the second day of every month.
crontab(0, 0,
day_of_month='2-30/3')
Execute on every even numbered day.
crontab(0, 0,
day_of_month='1-7,15-21')
Execute on the first and third weeks of the month.
crontab(0, 0, day_of_month='11',
month_of_year='5')
Execute on the eleventh of May every year.
crontab(0, 0,
month_of_year='*/3')
Execute on the first month of every quarter.

See celery.schedules.crontab for more documentation.

Solar schedules

If you have a task that should be executed according to sunrise, sunset, dawn or dusk, you can use the solar schedule type:

from celery.schedules import solar

app.conf.beat_schedule = {
    # Executes at sunset in Melbourne
    'add-at-melbourne-sunset': {
        'task': 'tasks.add',
        'schedule': solar('sunset', -37.81753, 144.96715),
        'args': (16, 16),
    },
}

The arguments are simply: solar(event, latitude, longitude)

Be sure to use the correct sign for latitude and longitude:

Sign Argument Meaning
+ latitude North
- latitude South
+ longitude East
- longitude West

Possible event types are:

Event Meaning
dawn_astronomical Execute at the moment after which the sky is no longer completely dark. This is when the sun is 18 degrees below the horizon.
dawn_nautical Execute when there is enough sunlight for the horizon and some objects to be distinguishable; formally, when the sun is 12 degrees below the horizon.
dawn_civil Execute when there is enough light for objects to be distinguishable so that outdoor activities can commence; formally, when the Sun is 6 degrees below the horizon.
sunrise Execute when the upper edge of the sun appears over the eastern horizon in the morning.
solar_noon Execute when the sun is highest above the horizon on that day.
sunset Execute when the trailing edge of the sun disappears over the western horizon in the evening.
dusk_civil Execute at the end of civil twilight, when objects are still distinguishable and some stars and planets are visible. Formally, when the sun is 6 degrees below the horizon.
dusk_nautical Execute when the sun is 12 degrees below the horizon. Objects are no longer distinguishable, and the horizon is no longer visible to the naked eye.
dusk_astronomical Execute at the moment after which the sky becomes completely dark; formally, when the sun is 18 degrees below the horizon.

All solar events are calculated using UTC, and are therefore unaffected by your timezone setting.

In polar regions, the sun may not rise or set every day. The scheduler is able to handle these cases, i.e. a sunrise event won’t run on a day when the sun doesn’t rise. The one exception is solar_noon, which is formally defined as the moment the sun transits the celestial meridian, and will occur every day even if the sun is below the horizon.

Twilight is defined as the period between dawn and sunrise, and between sunset and dusk. You can schedule an event according to “twilight” depending on your definition of twilight (civil, nautical or astronomical), and whether you want the event to take place at the beginning or end of twilight, using the appropriate event from the list above.

See celery.schedules.solar for more documentation.

Starting the Scheduler

To start the celery beat service:

$ celery -A proj beat

You can also start embed beat inside the worker by enabling workers -B option, this is convenient if you’ll never run more than one worker node, but it’s not commonly used and for that reason is not recommended for production use:

$ celery -A proj worker -B

Beat needs to store the last run times of the tasks in a local database file (named celerybeat-schedule by default), so it needs access to write in the current directory, or alternatively you can specify a custom location for this file:

$ celery -A proj beat -s /home/celery/var/run/celerybeat-schedule

Note

To daemonize beat see Daemonization.

Using custom scheduler classes

Custom scheduler classes can be specified on the command-line (the -S argument).

The default scheduler is celery.beat.PersistentScheduler, which is simply keeping track of the last run times in a local database file (a shelve).

django-celery also ships with a scheduler that stores the schedule in the Django database:

$ celery -A proj beat -S djcelery.schedulers.DatabaseScheduler

Using django-celery‘s scheduler you can add, modify and remove periodic tasks from the Django Admin.