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Celery is an open source asynchronous task queue/job queue based on distributed message passing. It is focused on real-time operation, but supports scheduling as well.

The execution units, called tasks, are executed concurrently on one or more worker nodes using multiprocessing, Eventlet or gevent. Tasks can execute asynchronously (in the background) or synchronously (wait until ready).

Celery is used in production systems to process millions of tasks every hour.

Celery is written in Python, but the protocol can be implemented in any language. It can also operate with other languages using webhooks. There's also RCelery for the Ruby programming language, and a PHP client.

The recommended message broker is RabbitMQ, but support for Redis, MongoDB, Beanstalk, Amazon SQS, CouchDB and databases (using SQLAlchemy or the Django ORM) is also available.

Celery is easy to integrate with web frameworks, some of which even have integration packages:

Django	django-celery
Pyramid	pyramid_celery
Pylons	celery-pylons
Flask	flask-celery
web2py	web2py-celery
Tornado	tornado-celery

This is a high level overview of the architecture.

The broker delivers tasks to the worker nodes. A worker node is a networked machine running celeryd. This can be one or more machines depending on the workload.

The result of the task can be stored for later retrieval (called its "tombstone").

You probably want to see some code by now, so here's an example task adding two numbers:

from celery import task

@task
def add(x, y):
    return x + y

You can execute the task in the background, or wait for it to finish:

>>> result = add.delay(4, 4)
>>> result.wait() # wait for and return the result
8

Simple!

Messaging	Supported brokers include RabbitMQ, Redis, MongoDB, Beanstalk, SQL databases, Amazon SQS and more.
Fault-tolerant	Excellent configurable error recovery when using RabbitMQ, ensures your tasks are never lost.
Distributed	Runs on one or more machines. Supports broker clustering and HA when used in combination with RabbitMQ. You can set up new workers without central configuration (e.g. use your grandma's laptop to help if the queue is temporarily congested).
Concurrency	Concurrency is achieved by using multiprocessing, Eventlet, gevent or a mix of these.
Scheduling	Supports recurring tasks like cron, or specifying an exact date or countdown for when after the task should be executed.
Latency	Low latency means you are able to execute tasks while the user is waiting.
Return Values	Task return values can be saved to the selected result store backend. You can wait for the result, retrieve it later, or ignore it.
Result Stores	Database, MongoDB, Redis, Memcached, Cassandra, or AMQP (message notification).
Webhooks	Your tasks can also be HTTP callbacks, enabling cross-language communication.
Rate limiting	Supports rate limiting by using the token bucket algorithm, which accounts for bursts of traffic. Rate limits can be set for each task type, or globally for all.
Routing	Using AMQP's flexible routing model you can route tasks to different workers, or select different message topologies, by configuration or even at runtime.
Remote-control	Worker nodes can be controlled from remote by using broadcast messaging. A range of built-in commands exist in addition to the ability to easily define your own. (AMQP/Redis only)
Monitoring	You can capture everything happening with the workers in real-time by subscribing to events. A real-time web monitor is in development.
Serialization	Supports Pickle, JSON, YAML, or easily defined custom schemes. One task invocation can have a different scheme than another.
Tracebacks	Errors and tracebacks are stored and can be investigated after the fact.
UUID	Every task has an UUID (Universally Unique Identifier), which is the task id used to query task status and return value.
Retries	Tasks can be retried if they fail, with configurable maximum number of retries, and delays between each retry.
Task Sets	A Task set is a task consisting of several sub-tasks. You can find out how many, or if all of the sub-tasks has been executed, and even retrieve the results in order. Progress bars, anyone?
Made for Web	You can query status and results via URLs, enabling the ability to poll task status using Ajax.
Error Emails	Can be configured to send emails to the administrators when tasks fails.

The latest documentation with user guides, tutorials and API reference is hosted at Github.

You can install Celery either via the Python Package Index (PyPI) or from source.

To install using pip,:

$ pip install -U Celery

To install using easy_install,:

$ easy_install -U Celery

Celery also defines a group of bundles that can be used to install Celery and the dependencies for a given feature.

The following bundles are available:

Download the latest version of Celery from http://pypi.python.org/pypi/celery/

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

You can clone the repository by doing the following:

$ git clone https://github.com/ask/celery
$ cd celery
$ python setup.py develop

The development version will usually also depend on the development version of kombu, the messaging framework Celery uses to send and receive messages, so you should also install that from git:

$ git clone https://github.com/ask/kombu
$ cd kombu
$ python setup.py develop

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/

http://wiki.github.com/ask/celery/

Development of celery happens at Github: http://github.com/ask/celery

You are highly encouraged to participate in the development of celery. If you don't like Github (for some reason) you're welcome to send regular patches.

Be sure to also read the Contributing to Celery section in the documentation.

This software is licensed under the New BSD License. See the LICENSE file in the top distribution directory for the full license text.