Periodic Tasks

• Introduction
• Time Zones
• Entries
  • Available Fields
• Crontab schedules
• Starting the Scheduler
  • Using custom scheduler classes

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 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.

Time Zones

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

An example time zone could be Europe/London:

CELERY_TIMEZONE = 'Europe/London'

This setting must be added to your app, either by configuration it directly using (app.conf.CELERY_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 CELERY_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 schedule a task periodically you have to add an entry to the CELERYBEAT_SCHEDULE setting.

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

from datetime import timedelta
CELERYBEAT_SCHEDULE = {
    'add-every-30-seconds': {
        'task': 'tasks.add',
        'schedule': timedelta(seconds=30),
        'args': (16, 16)
    },
}
CELERY_TIMEZONE = 'UTC'

注解

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.

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
CELERYBEAT_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:

See celery.schedules.crontab for more documentation.

Starting the Scheduler

To start the celery beat service:

$ celery beat

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

$ 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:

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

注解

To daemonize beat see Running the worker as a daemon.

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.