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 a day.

Celery is written in Python, but the protocol can be implemented
in any language. It can also operate with other languages using
web hooks.

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