This document describes an older version of Celery (2.2). For the latest stable version please go here.
celerybeat 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.
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 = {
"runs-every-30-seconds": {
"task": "tasks.add",
"schedule": timedelta(seconds=30),
"args": (16, 16)
},
}
Using a timedelta for the schedule means the task will be executed 30 seconds after celerybeat starts, and then every 30 seconds after the last run. A crontab like schedule also exists, see the section on Crontab schedules.
task
The name of the task to execute.
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 celerybeat was started.
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
"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. |
|
Same as previous. |
crontab(minute="*/15") | Execute every 15 minutes. |
crontab(day_of_week="sunday") | Execute every minute (!) at Sundays. |
|
Same as previous. |
|
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). |
To start the celerybeat service:
$ celerybeat
You can also start celerybeat with celeryd by using the -B option, this is convenient if you only intend to use one worker node:
$ celeryd -B
Celerybeat 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:
$ celerybeat -s /home/celery/var/run/celerybeat-schedule
Note
To daemonize celerybeat see Running celeryd as a daemon.
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:
$ celerybeat -S djcelery.schedulers.DatabaseScheduler
Using django-celery‘s scheduler you can add, modify and remove periodic tasks from the Django Admin.