.. _guide-beat: ================ Periodic Tasks ================ .. contents:: :local: Introduction ============ :program:`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 :setting:`CELERYBEAT_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. .. _beat-timezones: Time Zones ========== The periodic task schedules uses the UTC time zone by default, but you can change the time zone used using the :setting:`CELERY_TIMEZONE` setting. If you use a time zone other than UTC it's recommended to install the :mod:`pytz` library as this can improve the accuracy and keep your timezone specifications up to date: .. code-block:: bash $ pip install -U pytz An example time zone could be `Europe/London`: .. code-block:: python CELERY_TIMEZONE = 'Europe/London' .. admonition:: Changing the time zone The default scheduler (storing the schedule in the :file:`celerybeat-schedule` file) will automatically detect that the timezone has changed, and so will reset the schedule itself, but other schedulers may not be so smart (e.g. the Django database scheduler) and in that case you will have to reset the schedule manually. .. _beat-entries: Entries ======= To schedule a task periodically you have to add an entry to the :setting:`CELERYBEAT_SCHEDULE` setting. Example: Run the `tasks.add` task every 30 seconds. .. code-block:: python from datetime import timedelta CELERYBEAT_SCHEDULE = { 'runs-every-30-seconds': { 'task': 'tasks.add', 'schedule': timedelta(seconds=30), 'args': (16, 16) }, } CELERY_TIMEZONE = 'UTC' Using a :class:`~datetime.timedelta` for the schedule means the task will be executed 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`_. .. _beat-entry-fields: 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 :class:`~datetime.timedelta`, or a :class:`~celery.schedules.crontab`. You can also define your own custom schedule types, by extending the interface of :class:`~celery.schedules.schedule`. * `args` Positional arguments (:class:`list` or :class:`tuple`). * `kwargs` Keyword arguments (:class:`dict`). * `options` Execution options (:class:`dict`). This can be any argument supported by :meth:`~celery.task.base.Task.apply_async`, e.g. `exchange`, `routing_key`, `expires`, and so on. * `relative` By default :class:`~datetime.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 :program:`celery beat` was started. .. _beat-crontab: 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 :class:`~celery.schedules.crontab` schedule type: .. code-block:: python from celery.schedules import crontab CELERYBEAT_SCHEDULE = { # Executes every Monday morning at 7:30 A.M '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: | | | 3am, 6am, 9am, noon, 3pm, 6pm, 9pm. | +-----------------------------------------+--------------------------------------------+ | ``crontab(minute=0,`` | Same as previous. | | ``hour=[0,3,6,9,12,15,18,21])`` | | +-----------------------------------------+--------------------------------------------+ | ``crontab(minute='*/15')`` | Execute every 15 minutes. | +-----------------------------------------+--------------------------------------------+ | ``crontab(day_of_week='sunday')`` | Execute every minute (!) at Sundays. | +-----------------------------------------+--------------------------------------------+ | ``crontab(minute='*',`` | Same as previous. | | ``hour='*',`` | | | ``day_of_week='sun')`` | | +-----------------------------------------+--------------------------------------------+ | ``crontab(minute='*/10',`` | Execute every ten minutes, but only | | ``hour='3,17,22',`` | between 3-4 am, 5-6 pm and 10-11 pm on | | ``day_of_week='thu,fri')`` | 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(day_of_month='2')`` | Execute on the second day of every month. | | | | +-----------------------------------------+--------------------------------------------+ | ``crontab(day_of_month='2-30/3')`` | Execute on every even numbered day. | | | | +-----------------------------------------+--------------------------------------------+ | ``crontab(day_of_month='1-7,15-21')`` | Execute on the first and third weeks of | | | the month. | +-----------------------------------------+--------------------------------------------+ | ``crontab(day_of_month='11',`` | Execute on 11th of May every year. | | ``month_of_year='5')`` | | +-----------------------------------------+--------------------------------------------+ | ``crontab(month_of_year='*/3')`` | Execute on the first month of every | | | quarter. | +-----------------------------------------+--------------------------------------------+ See :class:`celery.schedules.crontab` for more documentation. .. _beat-timezones: Timezones ========= By default the current local timezone is used, but you can also set a specific timezone by enabling the :setting:`CELERY_ENABLE_UTC` setting and configuring the :setting:`CELERY_TIMEZONE` setting: .. code-block:: python CELERY_ENABLE_UTC = True CELERY_TIMEZONE = 'Europe/London' .. admonition:: Django Users For Django users the timezone specified in the ``TIME_ZONE`` setting will be used, but *not if the :setting:`CELERY_ENABLE_UTC` setting is enabled*. Celery is also compatible with the new ``USE_TZ`` setting introduced in Django 1.4. .. note:: The `pytz`_ library is recommended when setting a default timezone. If :mod:`pytz` is not installed it will fallback to the mod:`dateutil` library, which depends on a system timezone file being available for the timezone selected. Timezone definitions change frequently, so for the best results an up to date :mod:`pytz` installation should be used. .. _`pytz`: http://pypi.python.org/pypi/pytz/ .. _beat-starting: Starting the Scheduler ====================== To start the :program:`celery beat` service: .. code-block:: bash $ celery beat You can also start embed `beat` inside the worker by enabling workers `-B` option, this is convenient if you only intend to use one worker node: .. code-block:: bash $ celery 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: .. code-block:: bash $ celery beat -s /home/celery/var/run/celerybeat-schedule .. note:: To daemonize beat see :ref:`daemonizing`. .. _beat-custom-schedulers: Using custom scheduler classes ------------------------------ Custom scheduler classes can be specified on the command line (the `-S` argument). The default scheduler is :class:`celery.beat.PersistentScheduler`, which is simply keeping track of the last run times in a local database file (a :mod:`shelve`). `django-celery` also ships with a scheduler that stores the schedule in the Django database: .. code-block:: bash $ celery beat -S djcelery.schedulers.DatabaseScheduler Using `django-celery`'s scheduler you can add, modify and remove periodic tasks from the Django Admin.