celery - Distributed Task Queue
Celery is a distributed task queue.
It was first created for Django, but is now usable from Python. It can also operate with other languages via HTTP+JSON.
This introduction is written for someone who wants to use Celery from within a Django project. For information about using it from pure Python see Can I use Celery without Django?, for calling out to other languages see Executing tasks on a remote web server.
It is used for executing tasks asynchronously, routed to one or more worker servers, running concurrently using multiprocessing.
It is designed to solve certain problems related to running websites demanding high-availability and performance.
It is perfect for filling caches, posting updates to twitter, mass
downloading data like syndication feeds or web scraping. Use-cases are
plentiful. Implementing these features asynchronously using
easy and fun, and the performance improvements can make it more than
This is a high level overview of the architecture.
The broker is an AMQP server pushing tasks to the worker servers.
A worker server is a networked machine running
celeryd. This can be one or
more machines, depending on the workload. See A look inside the worker to
see how the worker server works.
The result of the task can be stored for later retrieval (called its "tombstone").
Uses AMQP messaging (RabbitMQ, ZeroMQ, Qpid) to route tasks to the worker servers. Experimental support for STOMP (ActiveMQ) is also available.
You can run as many worker servers as you want, and still be guaranteed that the task is only executed once.
Tasks are executed concurrently using the Python 2.6
`multiprocessingmodule (also available as a back-port to older python versions)
Supports periodic tasks, which makes it a (better) replacement for cronjobs.
When a task has been executed, the return value can be stored using either a MySQL/Oracle/PostgreSQL/SQLite database, Memcached, MongoDB, Redis or Tokyo Tyrant back-end. For high-performance you can also use AMQP messages to publish results.
If the task raises an exception, the exception instance is stored, instead of the return value.
All tasks has a Universally Unique Identifier (UUID), which is the task id, used for querying task status and return values.
Tasks can be retried if they fail, with a configurable maximum number of retries.
Tasks can be configured to run at a specific time and date in the future (ETA) or you can set a countdown in seconds for when the task should be executed.
Supports task-sets, which is a task consisting of several sub-tasks. You can find out how many, or if all of the sub-tasks has been executed. Excellent for progress-bar like functionality.
maplike function that uses tasks, called
However, you rarely want to wait for these results in a web-environment. You'd rather want to use Ajax to poll the task status, which is available from a URL like
celery/<task_id>/status/. This view returns a JSON-serialized data structure containing the task status, and the return value if completed, or exception on failure.
The worker can collect statistics, like, how many tasks has been executed by type, and the time it took to process them. Very useful for monitoring and profiling.
- Pool workers are supervised, so if for some reason a worker crashes
it is automatically replaced by a new worker.
Can be configured to send e-mails to the administrators when a task fails.
API Reference Documentation
You can install
celery either via the Python Package Index (PyPI)
or from source.
To install using
$ pip install celery
To install using
$ easy_install celery
Downloading and installing from source
Download the latest version of
You can install it by doing the following,:
$ tar xvfz celery-0.0.0.tar.gz $ cd celery-0.0.0 $ python setup.py build # python setup.py install # as root
Using the development version
You can clone the repository by doing the following:
$ git clone git://github.com/ask/celery.git
Setting up RabbitMQ
To use celery we need to create a RabbitMQ user, a virtual host and allow that user access to that virtual host:
$ rabbitmqctl add_user myuser mypassword $ rabbitmqctl add_vhost myvhost
From RabbitMQ version 1.6.0 and onward you have to use the new ACL features to allow access:
$ rabbitmqctl set_permissions -p myvhost myuser "" ".*" ".*"
If you are still using version 1.5.0 or below, please use
$ rabbitmqctl map_user_vhost myuser myvhost
Configuring your Django project to use Celery
You only need three simple steps to use celery with your Django project.
Create the celery database tables:$ python manage.py syncdb
- Configure celery to use the AMQP user and virtual host we created
before, by adding the following to your
settings.py:BROKER_HOST = "localhost" BROKER_PORT = 5672 BROKER_USER = "myuser" BROKER_PASSWORD = "mypassword" BROKER_VHOST = "myvhost"
There are more options available, like how many processes you want to process
work in parallel (the
CELERY_CONCURRENCY setting), and the backend used
for storing task statuses. But for now, this should do. For all of the options
available, please consult the API Reference
Note: If you're using SQLite as the Django database back-end,
celeryd will only be able to process one task at a time, this is
because SQLite doesn't allow concurrent writes.
Running the celery worker server
To test this we'll be running the worker server in the foreground, so we can see what's going on without consulting the logfile:
$ python manage.py celeryd
However, in production you probably want to run the worker in the background, as a daemon:
$ python manage.py celeryd --detach
For a complete listing of the command line arguments available, with a short description, you can use the help command:
$ python manage.py help celeryd
Defining and executing tasks
Please note All of these tasks has to be stored in a real module, they can't
be defined in the python shell or ipython/bpython. This is because the celery
worker server needs access to the task function to be able to run it.
So while it looks like we use the python shell to define the tasks in these
examples, you can't do it this way. Put them in the
tasks module of your
Django application. The worker server will automatically load any
file for all of the applications listed in
Executing tasks using
apply_async can be done from the
python shell, but keep in mind that since arguments are pickled, you can't
use custom classes defined in the shell session.
While you can use regular functions, the recommended way is to define a task class. This way you can cleanly upgrade the task to use the more advanced features of celery later.
This is a task that basically does nothing but take some arguments, and return a value:
from celery.task import Task from celery.registry import tasks class MyTask(Task): def run(self, some_arg, **kwargs): logger = self.get_logger(**kwargs) logger.info("Did something: %s" % some_arg) return 42 tasks.register(MyTask)
As you can see the worker is sending some keyword arguments to this task, this is the default keyword arguments. A task can choose not to take these, or only list the ones it want (the worker will do the right thing). The current default keyword arguments are:
The currently used log file, can be passed on to
self.get_loggerto gain access to the workers log file via a
The current loglevel used.
The unique id of the executing task.
Name of the executing task.
How many times the current task has been retried. (an integer starting a
Now if we want to execute this task, we can use the
delay method (
celery.task.Base.Task.delay) of the task class
(this is a handy shortcut to the
apply_async method which gives
you greater control of the task execution).
>>> from myapp.tasks import MyTask >>> MyTask.delay(some_arg="foo")
At this point, the task has been sent to the message broker. The message broker will hold on to the task until a celery worker server has successfully picked it up.
Note If everything is just hanging when you execute
delay, please check
that RabbitMQ is running, and that the user/password has access to the virtual
host you configured earlier.
Right now we have to check the celery worker logfiles to know what happened with
the task. This is because we didn't keep the
AsyncResult object returned
AsyncResult lets us find the state of the task, wait for the task to
finish and get its return value (or exception if the task failed).
So, let's execute the task again, but this time we'll keep track of the task:
>>> result = MyTask.delay("hello") >>> result.ready() # returns True if the task has finished processing. False >>> result.result # task is not ready, so no return value yet. None >>> result.get() # Waits until the task is done and return the retval. 42 >>> result.result 42 >>> result.successful() # returns True if the task didn't end in failure. True
If the task raises an exception, the return value of
result.result will contain the exception instance
raised by the task.
Worker auto-discovery of tasks
celeryd has an auto-discovery feature like the Django Admin, that
automatically loads any
tasks.py module in the applications listed
settings.INSTALLED_APPS. This autodiscovery is used by the celery
worker to find registered tasks for your Django project.
Periodic tasks are tasks that are run every
Here's an example of a periodic task:
from celery.task import PeriodicTask from celery.registry import tasks from datetime import timedelta class MyPeriodicTask(PeriodicTask): run_every = timedelta(seconds=30) def run(self, **kwargs): logger = self.get_logger(**kwargs) logger.info("Running periodic task!") >>> tasks.register(MyPeriodicTask)
A look inside the worker
For discussions about the usage, development, and future of celery, please join the celery-users mailing list.
Come chat with us on IRC. The #celery channel is located at the Freenode network.
If you have any suggestions, bug reports or annoyances please report them to our issue tracker at http://github.com/ask/celery/issues/
celery happens at Github: http://github.com/ask/celery
You are highly encouraged to participate in the development
celery. If you don't like Github (for some reason) you're welcome
to send regular patches.
This software is licensed under the
New BSD License. See the
file in the top distribution directory for the full license text.