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Concurrency with Eventlet

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 prefork, 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 prefork 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 prefork 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 mix of both Eventlet and prefork 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 celery worker:

$ celery worker -P eventlet -c 1000

Examples

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