Django & Celery — Routing problems

Question

I'm using Django and Celery and I'm trying to setup routing to multiple queues. When I specify a task's routing_key and exchange (either in the task decorator or using apply_async()), the task isn't added to the broker (which is Kombu connecting to my MySQL database).

If I specify the queue name in the task decorator (which will mean the routing key is ignored), the task works fine. It appears to be a problem with the routing/exchange setup.

Any idea what the problem could be?

Here's the setup:

settings.py

INSTALLED_APPS = (
    ...
    'kombu.transport.django',
    'djcelery',
)
BROKER_BACKEND = 'django'
CELERY_DEFAULT_QUEUE = 'default'
CELERY_DEFAULT_EXCHANGE = "tasks"
CELERY_DEFAULT_EXCHANGE_TYPE = "topic"
CELERY_DEFAULT_ROUTING_KEY = "task.default"
CELERY_QUEUES = {
    'default': {
        'binding_key':'task.#',
    },
    'i_tasks': {
        'binding_key':'important_task.#',
    },
}

tasks.py

from celery.task import task

@task(routing_key='important_task.update')
def my_important_task():
    try:
        ...
    except Exception as exc:
        my_important_task.retry(exc=exc)

Initiate task:

from tasks import my_important_task
my_important_task.delay()

Solution

You are using the Django ORM as a broker, which means declarations are only stored in memory (see the, inarguably hard to find, transport comparison table at http://readthedocs.org/docs/kombu/en/latest/introduction.html#transport-comparison)

So when you apply this task with routing_key important_task.update it will not be able to route it, because it hasn't declared the queue yet.

It will work if you do this:

@task(queue="i_tasks", routing_key="important_tasks.update")
def important_task():
    print("IMPORTANT")

---

But it would be much simpler for you to use the automatic routing feature, since there's nothing here that shows you need to use a 'topic' exchange, to use automatic routing simply remove the settings:

• CELERY_DEFAULT_QUEUE,
• CELERY_DEFAULT_EXCHANGE,
• CELERY_DEFAULT_EXCHANGE_TYPE
• CELERY_DEFAULT_ROUTING_KEY
• CELERY_QUEUES

And declare your task like this:

@task(queue="important")
def important_task():
    return "IMPORTANT"

and then to start a worker consuming from that queue:

$ python manage.py celeryd -l info -Q important

or to consume from both the default (celery) queue and the important queue:

$ python manage.py celeryd -l info -Q celery,important

---

Another good practice is to not hardcode the queue names into the task and use CELERY_ROUTES instead:

@task
def important_task():
    return "DEFAULT"

then in your settings:

CELERY_ROUTES = {"myapp.tasks.important_task": {"queue": "important"}}

---

If you still insist on using topic exchanges then you could add this router to automatically declare all queues the first time a task is sent:

class PredeclareRouter(object):
    setup = False

    def route_for_task(self, *args, **kwargs):
        if self.setup:
            return
        self.setup = True
        from celery import current_app, VERSION as celery_version
        # will not connect anywhere when using the Django transport
        # because declarations happen in memory.
        with current_app.broker_connection() as conn:
            queues = current_app.amqp.queues
            channel = conn.default_channel
            if celery_version >= (2, 6):
                for queue in queues.itervalues():
                    queue(channel).declare()
            else:
                from kombu.common import entry_to_queue
                for name, opts in queues.iteritems():
                    entry_to_queue(name, **opts)(channel).declare()
CELERY_ROUTES = (PredeclareRouter(), )