How does a Celery worker consuming from multiple queues decide which to consume from first?

I am using Celery to perform asynchronous background tasks, with Redis as the backend. I'm interested in the behaviour of a Celery worker in the following situation:

I am running a worker as a daemon using celeryd. This worker has been assigned two queues to consume through the -Q option:

celeryd -E -Q queue1,queue2

How does the worker decide where to fetch the next task to consume from? Does it randomly consume a task from either queue1 or queue2? Will it prioritise fetching from queue1 because it is first in the list of arguments passed to -Q?

Solution

From my testing, it processes multiple queues round-robin style.

If I use this test code:

from celery import task
import time


@task
def my_task(item_id):
    time.sleep(0.5)
    print('Processing item "%s"...' % item_id)


def add_items_to_queue(queue_name, items_count):
    for i in xrange(0, items_count):
        my_task.apply_async(('%s-%d' % (queue_name, i),), queue=queue_name)


add_items_to_queue('queue1', 10)
add_items_to_queue('queue2', 10)
add_items_to_queue('queue3', 5)

And start the queue with (using django-celery):

`manage.py celery worker -Q queue1,queue2,queue3`

It outputs:

Processing item "queue1-0"...
Processing item "queue3-0"...
Processing item "queue2-0"...
Processing item "queue1-1"...
Processing item "queue3-1"...
Processing item "queue2-1"...
Processing item "queue1-2"...
Processing item "queue3-2"...
Processing item "queue2-2"...
Processing item "queue1-3"...
Processing item "queue3-3"...
Processing item "queue2-3"...
Processing item "queue1-4"...
Processing item "queue3-4"...
Processing item "queue2-4"...
Processing item "queue1-5"...
Processing item "queue2-5"...
Processing item "queue1-6"...
Processing item "queue2-6"...
Processing item "queue1-7"...
Processing item "queue2-7"...
Processing item "queue1-8"...
Processing item "queue2-8"...
Processing item "queue1-9"...
Processing item "queue2-9"...

So it pulls one item from each queue before going on to the next queue1 item even though ALL of the queue1 tasks were published before the queue2 & 3 tasks.

Note: As @WarLord pointed out, this exact behavior will only work when CELERYD_PREFETCH_MULTIPLIER is set to 1. If it's greater than 1, then that means items will be fetched from the queue in batches. So if you have 4 processes with the PREFETCH_MULTIPLIER set to 4, that means there will be 16 items pulled from the queue right off the bat, so you won't get the exact output as above, but it will still roughly follow round-robin.