Why do CELERY_ROUTES have both a “queue” and a “routing_key”?

Question

My understanding of AMQP is that messages only have the following components:

1. The message body
2. The routing key
3. The exchange

Queues

Answer 1

The point of having the queue declared in there, is for celery to create those queues and set up the configuration with RabbitMQ.

For a lower level AMQP client, you need to first declare the queue, then the exchange, and then finally bind the exchange to the queue. Later when posting messages, you just post to the exchange.

It seems like celery use this structure to do it automatically.

Answer 2

Is true that on Celery there is a bit of confusion when you go to Queues, one thing you must keep in mind is that queue parameter refers to a Celery Kombu Queue Object and not directly to a AMQP queue, you can understand this by reading this extract from the docs. Of course the fact that celery creates the queue and exchange with the same name is the origin of confusion of the usage of queue parameter. Always in the docs you can read this paragraph:

If you have another queue but on another exchange you want to add, just specify a custom exchange and exchange type:

CELERY_QUEUES = (
    Queue('feed_tasks',    routing_key='feed.#'),
    Queue('regular_tasks', routing_key='task.#'),
    Queue('image_tasks',   exchange=Exchange('mediatasks', type='direct'),
                       routing_key='image.compress'),
)

So in this way you could bind 2 different queue on the same exchange. After to route the task using only the exchange and the key you could use Routers class

class MyRouter(object):

    def route_for_task(self, task, args=None, kwargs=None):
        if task == 'myapp.tasks.compress_video':
            return {'exchange': 'video',
                    'exchange_type': 'topic',
                    'routing_key': 'video.compress'}
        return None

More here http://celery.readthedocs.org/en/latest/userguide/routing.html#routers