.. _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.
