Django & Celery — Routing problems - django

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()

Answer 1

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(), )