Question

After running tasks via celery's periodic task scheduler, beat, why do I have so many unconsumed queues remaining in RabbitMQ?

Setup

Django web app running on Heroku
Tasks scheduled via celery beat
Tasks run via celery worker
Message broker is RabbitMQ from ClouldAMQP

Procfile

web: gunicorn --workers=2 --worker-class=gevent --bind=0.0.0.0:$PORT project_name.wsgi:application
scheduler: python manage.py celery worker --loglevel=ERROR -B -E --maxtasksperchild=1000
worker: python manage.py celery worker -E --maxtasksperchild=1000 --loglevel=ERROR

settings.py

CELERYBEAT_SCHEDULE = {
    'do_some_task': {
        'task': 'project_name.apps.appname.tasks.some_task',
        'schedule': datetime.timedelta(seconds=60 * 15),
        'args': ''
    },
}

tasks.py

@celery.task
def some_task()
    # Get some data from external resources
    # Save that data to the database
    # No return value specified

Result

Every time the task runs, I get (via the RabbitMQ web interface):

An additional message in the "Ready" state under my "Queued Messages"
An additional queue with a single message in the "ready" state
- This queue has no listed consumers

It ended up being my setting for CELERY_RESULT_BACKEND.

Previously, it was:

CELERY_RESULT_BACKEND = 'amqp'

I no longer had unconsumed messages / queues in RabbitMQ after I changed it to:

CELERY_RESULT_BACKEND = 'database'

What was happening, it would appear, is that after a task was executed, celery was sending info about that task back via rabbitmq, but, there was nothing setup to consume these responses messages, hence a bunch of unread ones ending up in the queue.

NOTE: This means that celery would be adding database entries recording the outcomes of tasks. To keep my database from getting loaded up with useless messages, I added:

# Delete result records ("tombstones") from database after 4 hours
# http://docs.celeryproject.org/en/latest/configuration.html#celery-task-result-expires
CELERY_TASK_RESULT_EXPIRES = 14400

Relevant parts from Settings.py

########## CELERY CONFIGURATION
import djcelery
# https://github.com/celery/django-celery/
djcelery.setup_loader()

INSTALLED_APPS = INSTALLED_APPS + (
    'djcelery',
)

# Compress all the messages using gzip
# http://celery.readthedocs.org/en/latest/userguide/calling.html#compression
CELERY_MESSAGE_COMPRESSION = 'gzip'

# See: http://docs.celeryproject.org/en/latest/configuration.html#broker-transport
BROKER_TRANSPORT = 'amqplib'

# Set this number to the amount of allowed concurrent connections on your AMQP
# provider, divided by the amount of active workers you have.
#
# For example, if you have the 'Little Lemur' CloudAMQP plan (their free tier),
# they allow 3 concurrent connections. So if you run a single worker, you'd
# want this number to be 3. If you had 3 workers running, you'd lower this
# number to 1, since 3 workers each maintaining one open connection = 3
# connections total.
#
# See: http://docs.celeryproject.org/en/latest/configuration.html#broker-pool-limit
BROKER_POOL_LIMIT = 3

# See: http://docs.celeryproject.org/en/latest/configuration.html#broker-connection-max-retries
BROKER_CONNECTION_MAX_RETRIES = 0

# See: http://docs.celeryproject.org/en/latest/configuration.html#broker-url
BROKER_URL = os.environ.get('CLOUDAMQP_URL')

# Previously, had this set to 'amqp', this resulted in many read / unconsumed
# queues and messages in RabbitMQ
# See: http://docs.celeryproject.org/en/latest/configuration.html#celery-result-backend
CELERY_RESULT_BACKEND = 'database'

# Delete result records ("tombstones") from database after 4 hours
# http://docs.celeryproject.org/en/latest/configuration.html#celery-task-result-expires
CELERY_TASK_RESULT_EXPIRES = 14400
########## END CELERY CONFIGURATION

Looks like you are getting back responses from your consumed tasks.

You can avoid that by doing: