.. _concurrency-eventlet:

===========================
 Concurrency with Eventlet
===========================

.. _eventlet-introduction:

Introduction
============

The `Eventlet`_ homepage describes it as;
A concurrent networking library for Python that allows you to
change how you run your code, not how you write it.

    * It uses `epoll(4)`_ or `libevent`_ for
      `highly scalable non-blocking I/O`_.
    * `Coroutines`_ ensure that the developer uses a blocking style of
      programming that is similar to threading, but provide the benefits of
      non-blocking I/O.
    * The event dispatch is implicit, which means you can easily use Eventlet
      from the Python interpreter, or as a small part of a larger application.

Celery supports Eventlet as an alternative execution pool implementation.
It is in some cases superior to multiprocessing, but you need to ensure
your tasks do not perform blocking calls, as this will halt all
other operations in the worker until the blocking call returns.

The multiprocessing pool can take use of multiple processes, but how many is
often limited to a few processes per CPU.  With Eventlet you can efficiently
spawn hundreds, or thousands of green threads.  In an informal test with a
feed hub system the Eventlet pool could fetch and process hundreds of feeds
every second, while the multiprocessing pool spent 14 seconds processing 100
feeds.  Note that is one of the applications evented I/O is especially good
at (asynchronous HTTP requests).  You may want a a mix of both Eventlet and
multiprocessing workers, and route tasks according to compatibility or
what works best.

Enabling Eventlet
=================

You can enable the Eventlet pool by using the ``-P`` option to
:program:`celeryd`::

    $ celeryd -P eventlet -c 1000

.. _eventlet-examples:

Examples
========

See the `Eventlet examples`_ directory in the Celery distribution for
some examples taking use of Eventlet support.

.. _`Eventlet`: http://eventlet.net
.. _`epoll(4)`: http://linux.die.net/man/4/epoll
.. _`libevent`: http://monkey.org/~provos/libevent/
.. _`highly scalable non-blocking I/O`:
    http://en.wikipedia.org/wiki/Asynchronous_I/O#Select.28.2Fpoll.29_loops
.. _`Coroutines`: http://en.wikipedia.org/wiki/Coroutine
.. _`Eventlet examples`:
    https://github.com/ask/celery/tree/master/examples/eventlet

