Retrieve list of tasks in a queue in Celery

Question

How can I retrieve a list of tasks in a queue that are yet to be processed?

Answer 1

I think the only way to get the tasks that are waiting is to keep a list of tasks you started and let the task remove itself from the list when it's started.

With rabbitmqctl and list_queues you can get an overview of how many tasks are waiting, but not the tasks itself: http://www.rabbitmq.com/man/rabbitmqctl.1.man.html

If what you want includes the task being processed, but are not finished yet, you can keep a list of you tasks and check their states:

from tasks import add
result = add.delay(4, 4)

result.ready() # True if finished

Or you let Celery store the results with CELERY_RESULT_BACKEND and check which of your tasks are not in there.

Answer 2

If you control the code of the tasks then you can work around the problem by letting a task trigger a trivial retry the first time it executes, then checking inspect().reserved(). The retry registers the task with the result backend, and celery can see that. The task must accept self or context as first parameter so we can access the retry count.

@task(bind=True)
def mytask(self):
    if self.request.retries == 0:
        raise self.retry(exc=MyTrivialError(), countdown=1)
    ...

This solution is broker agnostic, ie. you don't have to worry about whether you are using RabbitMQ or Redis to store the tasks.

EDIT: after testing I've found this to be only a partial solution. The size of reserved is limited to the prefetch setting for the worker.

Answer 3

EDIT: See other answers for getting a list of tasks in the queue.

You should look here: Celery Guide - Inspecting Workers

Basically this:

>>> from celery.task.control import inspect

# Inspect all nodes.
>>> i = inspect()

# Show the items that have an ETA or are scheduled for later processing
>>> i.scheduled()

# Show tasks that are currently active.
>>> i.active()

# Show tasks that have been claimed by workers
>>> i.reserved()

Depending on what you want

Answer 4

if you are using rabbitMQ, use this in terminal:

sudo rabbitmqctl list_queues

it will print list of queues with number of pending tasks. for example:

Listing queues ...
0b27d8c59fba4974893ec22d478a7093    0
0e0a2da9828a48bc86fe993b210d984f    0
[e-mail] 0
11926b79e30a4f0a9d95df61b6f402f7    0
15c036ad25884b82839495fb29bd6395    1
[e-mail]    0
celery  166
celeryev.795ec5bb-a919-46a8-80c6-5d91d2fcf2aa   0
celeryev.faa4da32-a225-4f6c-be3b-d8814856d1b6   0

the number in right column is number of tasks in the queue. in above, celery queue has 166 pending task.

Answer 5

To retrieve tasks from backend, use this

from amqplib import client_0_8 as amqp
conn = amqp.Connection(host="localhost:5672 ", userid="guest",
                       password="guest", virtual_host="/", insist=False)
chan = conn.channel()
name, jobs, consumers = chan.queue_declare(queue="queue_name", passive=True)

Answer 6

If you don't use prioritized tasks, this is actually pretty simple if you're using Redis. To get the task counts:

redis-cli -h HOST -p PORT -n DATABASE_NUMBER llen QUEUE_NAME

But, prioritized tasks use a different key in redis, so the full picture is slightly more complicated. The full picture is that you need to query redis for every priority of task. In python (and from the Flower project), this looks like:

PRIORITY_SEP = '\x06\x16'
DEFAULT_PRIORITY_STEPS = [0, 3, 6, 9]


def make_queue_name_for_pri(queue, pri):
    """Make a queue name for redis

    Celery uses PRIORITY_SEP to separate different priorities of tasks into
    different queues in Redis. Each queue-priority combination becomes a key in
    redis with names like:

     - batch1\x06\x163 <-- P3 queue named batch1

    There's more information about this in Github, but it doesn't look like it 
    will change any time soon:

      - https://github.com/celery/kombu/issues/422

    In that ticket the code below, from the Flower project, is referenced:

      - https://github.com/mher/flower/blob/master/flower/utils/broker.py#L135

    :param queue: The name of the queue to make a name for.
    :param pri: The priority to make a name with.
    :return: A name for the queue-priority pair.
    """
    if pri not in DEFAULT_PRIORITY_STEPS:
        raise ValueError('Priority not in priority steps')
    return '{0}{1}{2}'.format(*((queue, PRIORITY_SEP, pri) if pri else
                                (queue, '', '')))


def get_queue_length(queue_name='celery'):
    """Get the number of tasks in a celery queue.

    :param queue_name: The name of the queue you want to inspect.
    :return: the number of items in the queue.
    """
    priority_names = [make_queue_name_for_pri(queue_name, pri) for pri in
                      DEFAULT_PRIORITY_STEPS]
    r = redis.StrictRedis(
        host=settings.REDIS_HOST,
        port=settings.REDIS_PORT,
        db=settings.REDIS_DATABASES['CELERY'],
    )
    return sum([r.llen(x) for x in priority_names])

If you want to get an actual task, you can use something like:

redis-cli -h HOST -p PORT -n DATABASE_NUMBER lrange QUEUE_NAME 0 -1

From there you'll have to deserialize the returned list. In my case I was able to accomplish this with something like:

r = redis.StrictRedis(
    host=settings.REDIS_HOST,
    port=settings.REDIS_PORT,
    db=settings.REDIS_DATABASES['CELERY'],
)
l = r.lrange('celery', 0, -1)
pickle.loads(base64.decodestring(json.loads(l[0])['body']))

Just be warned that deserialization can take a moment, and you'll need to adjust the commands above to work with various priorities.