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				|  |  | -=================================
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				|  |  | - celery - Distributed Task Queue
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				|  |  | -=================================
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				|  |  | -
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				|  |  | -:Version: 0.8.0
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				|  |  | -
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				|  |  | -Introduction
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				|  |  | -============
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				|  |  | -
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				|  |  | -Celery is a distributed task queue.
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				|  |  | -
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				|  |  | -It was first created for Django, but is now usable from Python.
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				|  |  | -It can also operate with other languages via HTTP+JSON.
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				|  |  | -
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				|  |  | -This introduction is written for someone who wants to use
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				|  |  | -Celery from within a Django project. For information about using it from
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				|  |  | -pure Python see `Can I use Celery without Django?`_, for calling out to other
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				|  |  | -languages see `Executing tasks on a remote web server`_.
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				|  |  | -
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				|  |  | -.. _`Can I use Celery without Django?`: http://bit.ly/WPa6n
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				|  |  | -
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				|  |  | -.. _`Executing tasks on a remote web server`: http://bit.ly/CgXSc
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				|  |  | -
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				|  |  | -It is used for executing tasks *asynchronously*, routed to one or more
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				|  |  | -worker servers, running concurrently using multiprocessing.
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				|  |  | -
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				|  |  | -It is designed to solve certain problems related to running websites
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				|  |  | -demanding high-availability and performance.
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				|  |  | -
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				|  |  | -It is perfect for filling caches, posting updates to twitter, mass
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				|  |  | -downloading data like syndication feeds or web scraping. Use-cases are
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				|  |  | -plentiful. Implementing these features asynchronously using ``celery`` is
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				|  |  | -easy and fun, and the performance improvements can make it more than
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				|  |  | -worthwhile.
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				|  |  | -
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				|  |  | -Overview
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				|  |  | -========
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				|  |  | -
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				|  |  | -This is a high level overview of the architecture.
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				|  |  | -
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				|  |  | -.. image:: http://cloud.github.com/downloads/ask/celery/Celery-Overview-v4.jpg
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				|  |  | -
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				|  |  | -The broker is an AMQP server pushing tasks to the worker servers.
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				|  |  | -A worker server is a networked machine running ``celeryd``. This can be one or
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				|  |  | -more machines, depending on the workload. See `A look inside the worker`_ to
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				|  |  | -see how the worker server works.
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				|  |  | -
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				|  |  | -The result of the task can be stored for later retrieval (called its
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				|  |  | -"tombstone").
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				|  |  | -
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				|  |  | -Features
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				|  |  | -========
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				|  |  | -
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				|  |  | -    * Uses AMQP messaging (RabbitMQ, ZeroMQ, Qpid) to route tasks to the
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				|  |  | -      worker servers. Experimental support for STOMP (ActiveMQ) is also 
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				|  |  | -      available.
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				|  |  | -
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				|  |  | -    * You can run as many worker servers as you want, and still
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				|  |  | -      be *guaranteed that the task is only executed once.*
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				|  |  | -
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				|  |  | -    * Tasks are executed *concurrently* using the Python 2.6
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				|  |  | -      ``multiprocessing`` module (also available as a back-port
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				|  |  | -      to older python versions)
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				|  |  | -
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				|  |  | -    * Supports *periodic tasks*, which makes it a (better) replacement
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				|  |  | -      for cronjobs.
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				|  |  | -
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				|  |  | -    * When a task has been executed, the return value can be stored using
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				|  |  | -      either a MySQL/Oracle/PostgreSQL/SQLite database, Memcached,
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				|  |  | -      `MongoDB`_, `Redis`_ or `Tokyo Tyrant`_ back-end. For high-performance
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				|  |  | -      you can also use AMQP messages to publish results.
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				|  |  | -
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				|  |  | -    * If the task raises an exception, the exception instance is stored,
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				|  |  | -      instead of the return value.
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				|  |  | -
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				|  |  | -    * All tasks has a Universally Unique Identifier (UUID), which is the
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				|  |  | -      task id, used for querying task status and return values.
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				|  |  | -
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				|  |  | -    * Tasks can be retried if they fail, with a configurable maximum number
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				|  |  | -      of retries.
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				|  |  | -
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				|  |  | -    * Tasks can be configured to run at a specific time and date in the
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				|  |  | -      future (ETA) or you can set a countdown in seconds for when the
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				|  |  | -      task should be executed.
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				|  |  | -
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				|  |  | -    * Supports *task-sets*, which is a task consisting of several sub-tasks.
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				|  |  | -      You can find out how many, or if all of the sub-tasks has been executed.
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				|  |  | -      Excellent for progress-bar like functionality.
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				|  |  | -
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				|  |  | -    * Has a ``map`` like function that uses tasks, called ``dmap``.
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				|  |  | -
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				|  |  | -    * However, you rarely want to wait for these results in a web-environment.
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				|  |  | -      You'd rather want to use Ajax to poll the task status, which is
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				|  |  | -      available from a URL like ``celery/<task_id>/status/``. This view
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				|  |  | -      returns a JSON-serialized data structure containing the task status,
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				|  |  | -      and the return value if completed, or exception on failure.
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				|  |  | -
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				|  |  | -    * The worker can collect statistics, like, how many tasks has been
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				|  |  | -      executed by type, and the time it took to process them. Very useful
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				|  |  | -      for monitoring and profiling.
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				|  |  | -
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				|  |  | -    * Pool workers are supervised, so if for some reason a worker crashes
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				|  |  | -        it is automatically replaced by a new worker.
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				|  |  | -
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				|  |  | -    * Can be configured to send e-mails to the administrators when a task
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				|  |  | -      fails.
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				|  |  | -
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				|  |  | -.. _`MongoDB`: http://www.mongodb.org/
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				|  |  | -.. _`Redis`: http://code.google.com/p/redis/
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				|  |  | -.. _`Tokyo Tyrant`: http://tokyocabinet.sourceforge.net/
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				|  |  | -
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				|  |  | -API Reference Documentation
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				|  |  | -===========================
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				|  |  | -
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				|  |  | -The `API Reference`_ is hosted at Github
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				|  |  | -(http://ask.github.com/celery)
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				|  |  | -
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				|  |  | -.. _`API Reference`: http://ask.github.com/celery/
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				|  |  | -
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				|  |  | -Installation
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				|  |  | -=============
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				|  |  | -
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				|  |  | -You can install ``celery`` either via the Python Package Index (PyPI)
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				|  |  | -or from source.
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				|  |  | -
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				|  |  | -To install using ``pip``,::
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				|  |  | -
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				|  |  | -    $ pip install celery
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				|  |  | -
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				|  |  | -To install using ``easy_install``,::
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				|  |  | -
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				|  |  | -    $ easy_install celery
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				|  |  | -
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				|  |  | -Downloading and installing from source
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				|  |  | ---------------------------------------
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				|  |  | -
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				|  |  | -Download the latest version of ``celery`` from
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				|  |  | -http://pypi.python.org/pypi/celery/
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				|  |  | -
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				|  |  | -You can install it by doing the following,::
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				|  |  | -
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				|  |  | -    $ tar xvfz celery-0.0.0.tar.gz
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				|  |  | -    $ cd celery-0.0.0
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				|  |  | -    $ python setup.py build
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				|  |  | -    # python setup.py install # as root
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				|  |  | -
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				|  |  | -Using the development version
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				|  |  | -------------------------------
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				|  |  | -
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				|  |  | -You can clone the repository by doing the following::
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				|  |  | -
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				|  |  | -    $ git clone git://github.com/ask/celery.git
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				|  |  | -
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				|  |  | -
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				|  |  | -Usage
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				|  |  | -=====
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				|  |  | -
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				|  |  | -Installing RabbitMQ
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				|  |  | --------------------
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				|  |  | -
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				|  |  | -See `Installing RabbitMQ`_ over at RabbitMQ's website. For Mac OS X
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				|  |  | -see `Installing RabbitMQ on OS X`_.
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				|  |  | -
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				|  |  | -.. _`Installing RabbitMQ`: http://www.rabbitmq.com/install.html
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				|  |  | -.. _`Installing RabbitMQ on OS X`:
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				|  |  | -    http://playtype.net/past/2008/10/9/installing_rabbitmq_on_osx/
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				|  |  | -
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				|  |  | -
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				|  |  | -Setting up RabbitMQ
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				|  |  | --------------------
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				|  |  | -
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				|  |  | -To use celery we need to create a RabbitMQ user, a virtual host and
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				|  |  | -allow that user access to that virtual host::
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				|  |  | -
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				|  |  | -    $ rabbitmqctl add_user myuser mypassword
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				|  |  | -
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				|  |  | -    $ rabbitmqctl add_vhost myvhost
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				|  |  | -
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				|  |  | -From RabbitMQ version 1.6.0 and onward you have to use the new ACL features
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				|  |  | -to allow access::
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				|  |  | -
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				|  |  | -    $ rabbitmqctl set_permissions -p myvhost myuser "" ".*" ".*"
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				|  |  | -
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				|  |  | -See the RabbitMQ `Admin Guide`_ for more information about `access control`_.
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				|  |  | -
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				|  |  | -.. _`Admin Guide`: http://www.rabbitmq.com/admin-guide.html
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				|  |  | -
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				|  |  | -.. _`access control`: http://www.rabbitmq.com/admin-guide.html#access-control
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				|  |  | -
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				|  |  | -
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				|  |  | -If you are still using version 1.5.0 or below, please use ``map_user_vhost``::
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				|  |  | -
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				|  |  | -    $ rabbitmqctl map_user_vhost myuser myvhost
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				|  |  | -
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				|  |  | -
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				|  |  | -Configuring your Django project to use Celery
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				|  |  | ----------------------------------------------
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				|  |  | -
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				|  |  | -You only need three simple steps to use celery with your Django project.
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				|  |  | -
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				|  |  | -    1. Add ``celery`` to ``INSTALLED_APPS``.
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				|  |  | -
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				|  |  | -    2. Create the celery database tables::
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				|  |  | -
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				|  |  | -            $ python manage.py syncdb
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				|  |  | -
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				|  |  | -    3. Configure celery to use the AMQP user and virtual host we created
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				|  |  | -        before, by adding the following to your ``settings.py``::
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				|  |  | -
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				|  |  | -            AMQP_SERVER = "localhost"
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				|  |  | -            AMQP_PORT = 5672
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				|  |  | -            AMQP_USER = "myuser"
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				|  |  | -            AMQP_PASSWORD = "mypassword"
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				|  |  | -            AMQP_VHOST = "myvhost"
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				|  |  | -
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				|  |  | -
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				|  |  | -That's it.
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				|  |  | -
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				|  |  | -There are more options available, like how many processes you want to process
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				|  |  | -work in parallel (the ``CELERY_CONCURRENCY`` setting), and the backend used
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				|  |  | -for storing task statuses. But for now, this should do. For all of the options
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				|  |  | -available, please consult the `API Reference`_
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				|  |  | -
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				|  |  | -**Note**: If you're using SQLite as the Django database back-end,
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				|  |  | -``celeryd`` will only be able to process one task at a time, this is
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				|  |  | -because SQLite doesn't allow concurrent writes.
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				|  |  | -
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				|  |  | -Running the celery worker server
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				|  |  | ---------------------------------
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				|  |  | -
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				|  |  | -To test this we'll be running the worker server in the foreground, so we can
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				|  |  | -see what's going on without consulting the logfile::
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				|  |  | -
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				|  |  | -    $ python manage.py celeryd
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				|  |  | -
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				|  |  | -
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				|  |  | -However, in production you probably want to run the worker in the
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				|  |  | -background, as a daemon:: 
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				|  |  | -
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				|  |  | -    $ python manage.py celeryd --detach
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				|  |  | -
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				|  |  | -
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				|  |  | -For a complete listing of the command line arguments available, with a short
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				|  |  | -description, you can use the help command::
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				|  |  | -
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				|  |  | -    $ python manage.py help celeryd
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				|  |  | -
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				|  |  | -
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				|  |  | -Defining and executing tasks
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				|  |  | -----------------------------
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				|  |  | -
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				|  |  | -**Please note** All of these tasks has to be stored in a real module, they can't
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				|  |  | -be defined in the python shell or ipython/bpython. This is because the celery
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				|  |  | -worker server needs access to the task function to be able to run it.
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				|  |  | -So while it looks like we use the python shell to define the tasks in these
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				|  |  | -examples, you can't do it this way. Put them in the ``tasks`` module of your
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				|  |  | -Django application. The worker server will automatically load any ``tasks.py``
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				|  |  | -file for all of the applications listed in ``settings.INSTALLED_APPS``.
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				|  |  | -Executing tasks using ``delay`` and ``apply_async`` can be done from the
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				|  |  | -python shell, but keep in mind that since arguments are pickled, you can't
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				|  |  | -use custom classes defined in the shell session.
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				|  |  | -
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				|  |  | -While you can use regular functions, the recommended way is to define
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				|  |  | -a task class. This way you can cleanly upgrade the task to use the more
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				|  |  | -advanced features of celery later.
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				|  |  | -
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				|  |  | -This is a task that basically does nothing but take some arguments,
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				|  |  | -and return a value:
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				|  |  | -
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				|  |  | -    >>> from celery.task import Task
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				|  |  | -    >>> from celery.registry import tasks
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				|  |  | -    >>> class MyTask(Task):
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				|  |  | -    ...     def run(self, some_arg, **kwargs):
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				|  |  | -    ...         logger = self.get_logger(**kwargs)
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				|  |  | -    ...         logger.info("Did something: %s" % some_arg)
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				|  |  | -    ...         return 42
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				|  |  | -    >>> tasks.register(MyTask)
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				|  |  | -
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				|  |  | -As you can see the worker is sending some keyword arguments to this task,
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				|  |  | -this is the default keyword arguments. A task can choose not to take these,
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				|  |  | -or only list the ones it want (the worker will do the right thing).
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				|  |  | -The current default keyword arguments are:
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				|  |  | -
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				|  |  | -    * logfile
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				|  |  | -
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				|  |  | -        The currently used log file, can be passed on to ``self.get_logger``
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				|  |  | -        to gain access to the workers log file via a ``logger.Logging``
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				|  |  | -        instance.
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				|  |  | -
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				|  |  | -    * loglevel
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				|  |  | -
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				|  |  | -        The current loglevel used.
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				|  |  | -
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				|  |  | -    * task_id
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				|  |  | -
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				|  |  | -        The unique id of the executing task.
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				|  |  | -
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				|  |  | -    * task_name
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				|  |  | -
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				|  |  | -        Name of the executing task.
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				|  |  | -
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				|  |  | -    * task_retries
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				|  |  | -
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				|  |  | -        How many times the current task has been retried.
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				|  |  | -        (an integer starting a ``0``).
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				|  |  | -
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				|  |  | -Now if we want to execute this task, we can use the ``delay`` method of the
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				|  |  | -task class (this is a handy shortcut to the ``apply_async`` method which gives
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				|  |  | -you greater control of the task execution).
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				|  |  | -
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				|  |  | -    >>> from myapp.tasks import MyTask
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				|  |  | -    >>> MyTask.delay(some_arg="foo")
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				|  |  | -
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				|  |  | -At this point, the task has been sent to the message broker. The message
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				|  |  | -broker will hold on to the task until a celery worker server has successfully
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				|  |  | -picked it up.
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				|  |  | -
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				|  |  | -*Note* If everything is just hanging when you execute ``delay``, please check
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				|  |  | -that RabbitMQ is running, and that the user/password has access to the virtual
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				|  |  | -host you configured earlier.
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				|  |  | -
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				|  |  | -Right now we have to check the celery worker logfiles to know what happened with
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				|  |  | -the task. This is because we didn't keep the ``AsyncResult`` object returned
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				|  |  | -by ``delay``.
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				|  |  | -
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				|  |  | -The ``AsyncResult`` lets us find the state of the task, wait for the task to
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				|  |  | -finish and get its return value (or exception if the task failed).
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				|  |  | -
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				|  |  | -So, let's execute the task again, but this time we'll keep track of the task:
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				|  |  | -
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				|  |  | -    >>> result = MyTask.delay("hello")
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				|  |  | -    >>> result.ready() # returns True if the task has finished processing.
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				|  |  | -    False
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				|  |  | -    >>> result.result # task is not ready, so no return value yet.
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				|  |  | -    None
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				|  |  | -    >>> result.get()   # Waits until the task is done and return the retval.
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				|  |  | -    42
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				|  |  | -    >>> result.result
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				|  |  | -    42
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				|  |  | -    >>> result.successful() # returns True if the task didn't end in failure.
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				|  |  | -    True
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				|  |  | -
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				|  |  | -
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				|  |  | -If the task raises an exception, the return value of ``result.successful()``
 | 
	
		
			
				|  |  | -will be ``False``, and ``result.result`` will contain the exception instance
 | 
	
		
			
				|  |  | -raised by the task.
 | 
	
		
			
				|  |  | -
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				|  |  | -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
 | 
	
		
			
				|  |  | -in ``settings.INSTALLED_APPS``. This autodiscovery is used by the celery
 | 
	
		
			
				|  |  | -worker to find registered tasks for your Django project.
 | 
	
		
			
				|  |  | -
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				|  |  | -Periodic Tasks
 | 
	
		
			
				|  |  | ----------------
 | 
	
		
			
				|  |  | -
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				|  |  | -Periodic tasks are tasks that are run every ``n`` seconds. 
 | 
	
		
			
				|  |  | -Here's an example of a periodic task:
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -    >>> from celery.task import PeriodicTask
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				|  |  | -    >>> from celery.registry import tasks
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				|  |  | -    >>> 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)
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -**Note:** Periodic tasks does not support arguments, as this doesn't
 | 
	
		
			
				|  |  | -really make sense.
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -A look inside the worker
 | 
	
		
			
				|  |  | -========================
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -.. image:: http://cloud.github.com/downloads/ask/celery/InsideTheWorker-v2.jpg
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -Getting Help
 | 
	
		
			
				|  |  | -============
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -Mailing list
 | 
	
		
			
				|  |  | -------------
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -For discussions about the usage, development, and future of celery,
 | 
	
		
			
				|  |  | -please join the `celery-users`_ mailing list. 
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -.. _`celery-users`: http://groups.google.com/group/celery-users/
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -IRC
 | 
	
		
			
				|  |  | ----
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -Come chat with us on IRC. The `#celery`_ channel is located at the `Freenode`_
 | 
	
		
			
				|  |  | -network.
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -.. _`#celery`: irc://irc.freenode.net/celery
 | 
	
		
			
				|  |  | -.. _`Freenode`: http://freenode.net
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -Bug tracker
 | 
	
		
			
				|  |  | -===========
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -If you have any suggestions, bug reports or annoyances please report them
 | 
	
		
			
				|  |  | -to our issue tracker at http://github.com/ask/celery/issues/
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -Contributing
 | 
	
		
			
				|  |  | -============
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -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.
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -License
 | 
	
		
			
				|  |  | -=======
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -This software is licensed under the ``New BSD License``. See the ``LICENSE``
 | 
	
		
			
				|  |  | -file in the top distribution directory for the full license text.
 | 
	
		
			
				|  |  | -
 | 
	
		
			
				|  |  | -.. # vim: syntax=rst expandtab tabstop=4 shiftwidth=4 shiftround
 |