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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?

Jonathan Evans
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3 Answers3

20

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.

Troy
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    In 2019, This is not the correct answer, see correct and updated order from @crazyshezy The order now is based on arrival to queues. – JAR.JAR.beans Sep 03 '19 at 13:55
8

NOTE: This answer has deprecated: latest version of Celery works very differently then what it was in 2013...

A worker consuming several queues consumes task, FIFO order is maintained across multiple queues too.

Example:

Queue1 : (t1, t2, t5, t7)
Queue2 : (t0,t3,t4,t6)

Assuming 0-7 represents the order of the tasks published

order of Consumption is t0, t1, t2, t3, t4, t5, t6, t7

Crazyshezy
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    Is there any way to modify this behavior? Perhaps to give priority to one queue over another: t1, t2, t5, t7, t0, t3, t4, t6. – BlueBomber Jul 23 '13 at 14:57
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    Not with RabbitMQ, could be possible with the other message transports e.g. with Redis but that would involve subclassing. Note that the order you describe is actually starvation, if Queue1 is constantly busy the messages in Queue2 would never be processed. – asksol Jul 31 '14 at 17:38
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    @Crazyshezy - Are you saying that if I put 500 items into Queue1 and THEN put 1 item into Queue 2, that Queue 2 item won't get processed until the 500 items are done since it got there after them?? That seems to defeat the purpose of having multiple queues if it treats them as if they were just one queue... – Troy Oct 07 '15 at 20:33
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    From my testing, it processes the queues round-robin style, so, in your example, the Consumption order would be t1, t0, t2, t3, t5, t4, t7, t6. Though, the order can vary, some, if you have PREFETCH_MULTIPLIER set to other-than 1 and the tasks take differing amounts of time to complete. – Troy Oct 08 '15 at 01:17
  • According to this https://github.com/celery/celery/issues/2192#issuecomment-51799123 it's determined by the _broker_. So Rabbit MQ seems to deliver in FIFO style, other brokers may do round robin I guess. – Anentropic Mar 23 '18 at 11:28
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    2019 testing with RabbitMQ seems to suggest FIFO, which is indeed strange and seems to really defeat the purpose of multiple queues, really strange. – JAR.JAR.beans Sep 03 '19 at 14:16
  • Interesting @JAR.JAR.beans, I was testing using the `pyamqp` broker library, interfacing with RabbitMQ, and I get an approximate round-robin behaviour. – Josh Smeaton May 05 '20 at 12:03
3

Using the pyamqp broker library pointing at a rabbitmq server, tasks get processed in a round-robin style. See proof below.

It appears that the order tasks are processed in is determined by the broker library, not the actual backend (rabbitmq vs redis is not the issue).

Software versions:

$ pip freeze | egrep "celery|kombu|amqp"
amqp==2.5.2
celery==4.4.2
kombu==4.6.8

from time import sleep

@app.task
def sleepy(name):
    print(f"Processing: {name}")
    sleep(0.5)

Then in another shell, queue the tasks:

from time import sleep

def queue_them():
    for x in range(50):
        sleepy.apply_async(args=(f"Q1-T{x}",), queue="Q1")
    sleep(0.1)
    for x in range(20):
        sleepy.apply_async(args=(f"Q2-T{x}",), queue="Q2")
    sleep(0.1)
    sleepy.apply_async(args=("Q3-T0",), queue="Q3")
    for x in range(30):
        sleepy.apply_async(args=(f"Q2MOAR-T{x}",), queue="Q2")

# setup - get celery to setup the queues and exchanges
sleepy.apply_async(args=("nothing",), queue="Q1")
sleepy.apply_async(args=("nothing",), queue="Q2")
sleepy.apply_async(args=("nothing",), queue="Q3")

# run the test
queue_them()

And in another shell, run celery:

$ celery worker -A myapp.celery --pool=prefork --concurrency=2 -Ofair --queues=Q1,Q3,Q2

[2020-05-05 21:59:11,547] WARNING [celery.redirected:235] Processing: Q1-T1
[2020-05-05 21:59:11,547] WARNING [celery.redirected:235] Processing: Q1-T0
[2020-05-05 21:59:12,052] WARNING [celery.redirected:235] Processing: Q1-T2
[2020-05-05 21:59:12,053] WARNING [celery.redirected:235] Processing: Q1-T3
[2020-05-05 21:59:12,556] WARNING [celery.redirected:235] Processing: Q1-T5
[2020-05-05 21:59:12,556] WARNING [celery.redirected:235] Processing: Q1-T4
[2020-05-05 21:59:13,062] WARNING [celery.redirected:235] Processing: Q1-T6
[2020-05-05 21:59:13,063] WARNING [celery.redirected:235] Processing: Q1-T7
[2020-05-05 21:59:13,565] WARNING [celery.redirected:235] Processing: Q1-T9
[2020-05-05 21:59:13,565] WARNING [celery.redirected:235] Processing: Q1-T8
[2020-05-05 21:59:14,069] WARNING [celery.redirected:235] Processing: Q1-T10
[2020-05-05 21:59:14,069] WARNING [celery.redirected:235] Processing: Q3-T0
[2020-05-05 21:59:14,571] WARNING [celery.redirected:235] Processing: Q2-T0
[2020-05-05 21:59:14,572] WARNING [celery.redirected:235] Processing: Q2-T1
[2020-05-05 21:59:15,078] WARNING [celery.redirected:235] Processing: Q1-T11
[2020-05-05 21:59:15,078] WARNING [celery.redirected:235] Processing: Q2-T2
[2020-05-05 21:59:15,581] WARNING [celery.redirected:235] Processing: Q2-T3
[2020-05-05 21:59:15,581] WARNING [celery.redirected:235] Processing: Q1-T12
[2020-05-05 21:59:16,084] WARNING [celery.redirected:235] Processing: Q1-T13
[2020-05-05 21:59:16,084] WARNING [celery.redirected:235] Processing: Q2-T4
[2020-05-05 21:59:16,586] WARNING [celery.redirected:235] Processing: Q1-T14
[2020-05-05 21:59:16,586] WARNING [celery.redirected:235] Processing: Q2-T5
[2020-05-05 21:59:17,089] WARNING [celery.redirected:235] Processing: Q1-T15
[2020-05-05 21:59:17,089] WARNING [celery.redirected:235] Processing: Q2-T6
[2020-05-05 21:59:17,591] WARNING [celery.redirected:235] Processing: Q1-T16
[2020-05-05 21:59:17,592] WARNING [celery.redirected:235] Processing: Q2-T7
[2020-05-05 21:59:18,094] WARNING [celery.redirected:235] Processing: Q1-T17
[2020-05-05 21:59:18,094] WARNING [celery.redirected:235] Processing: Q2-T8
[2020-05-05 21:59:18,597] WARNING [celery.redirected:235] Processing: Q1-T18
[2020-05-05 21:59:18,597] WARNING [celery.redirected:235] Processing: Q2-T9
[2020-05-05 21:59:19,102] WARNING [celery.redirected:235] Processing: Q1-T19
[2020-05-05 21:59:19,102] WARNING [celery.redirected:235] Processing: Q1-T20
[2020-05-05 21:59:19,607] WARNING [celery.redirected:235] Processing: Q1-T21
[2020-05-05 21:59:19,607] WARNING [celery.redirected:235] Processing: Q1-T22
[2020-05-05 21:59:20,110] WARNING [celery.redirected:235] Processing: Q1-T23
[2020-05-05 21:59:20,110] WARNING [celery.redirected:235] Processing: Q2-T10
[2020-05-05 21:59:20,614] WARNING [celery.redirected:235] Processing: Q1-T24
[2020-05-05 21:59:20,614] WARNING [celery.redirected:235] Processing: Q2-T11
[2020-05-05 21:59:21,118] WARNING [celery.redirected:235] Processing: Q1-T25
[2020-05-05 21:59:21,118] WARNING [celery.redirected:235] Processing: Q1-T26
[2020-05-05 21:59:21,622] WARNING [celery.redirected:235] Processing: Q2-T12
[2020-05-05 21:59:21,622] WARNING [celery.redirected:235] Processing: Q1-T27
[2020-05-05 21:59:22,124] WARNING [celery.redirected:235] Processing: Q1-T28
[2020-05-05 21:59:22,124] WARNING [celery.redirected:235] Processing: Q2-T13
[2020-05-05 21:59:22,627] WARNING [celery.redirected:235] Processing: Q2-T14
[2020-05-05 21:59:22,627] WARNING [celery.redirected:235] Processing: Q1-T29
[2020-05-05 21:59:23,129] WARNING [celery.redirected:235] Processing: Q1-T31
[2020-05-05 21:59:23,129] WARNING [celery.redirected:235] Processing: Q1-T30
[2020-05-05 21:59:23,631] WARNING [celery.redirected:235] Processing: Q2-T15
[2020-05-05 21:59:23,632] WARNING [celery.redirected:235] Processing: Q1-T32
[2020-05-05 21:59:24,134] WARNING [celery.redirected:235] Processing: Q1-T33
[2020-05-05 21:59:24,134] WARNING [celery.redirected:235] Processing: Q2-T16
[2020-05-05 21:59:24,636] WARNING [celery.redirected:235] Processing: Q2-T17
[2020-05-05 21:59:24,636] WARNING [celery.redirected:235] Processing: Q2-T18
[2020-05-05 21:59:25,138] WARNING [celery.redirected:235] Processing: Q2-T19
[2020-05-05 21:59:25,139] WARNING [celery.redirected:235] Processing: Q1-T34
[2020-05-05 21:59:25,641] WARNING [celery.redirected:235] Processing: Q1-T35
[2020-05-05 21:59:25,642] WARNING [celery.redirected:235] Processing: Q2MOAR-T0
[2020-05-05 21:59:26,144] WARNING [celery.redirected:235] Processing: Q1-T36
[2020-05-05 21:59:26,144] WARNING [celery.redirected:235] Processing: Q1-T37
[2020-05-05 21:59:26,649] WARNING [celery.redirected:235] Processing: Q2MOAR-T1
[2020-05-05 21:59:26,649] WARNING [celery.redirected:235] Processing: Q1-T38
[2020-05-05 21:59:27,153] WARNING [celery.redirected:235] Processing: Q2MOAR-T2
[2020-05-05 21:59:27,154] WARNING [celery.redirected:235] Processing: Q1-T39
[2020-05-05 21:59:27,656] WARNING [celery.redirected:235] Processing: Q2MOAR-T3
[2020-05-05 21:59:27,656] WARNING [celery.redirected:235] Processing: Q2MOAR-T4
[2020-05-05 21:59:28,159] WARNING [celery.redirected:235] Processing: Q2MOAR-T5
[2020-05-05 21:59:28,160] WARNING [celery.redirected:235] Processing: Q1-T40
[2020-05-05 21:59:28,664] WARNING [celery.redirected:235] Processing: Q2MOAR-T6
[2020-05-05 21:59:28,664] WARNING [celery.redirected:235] Processing: Q1-T41
[2020-05-05 21:59:29,167] WARNING [celery.redirected:235] Processing: Q2MOAR-T7
[2020-05-05 21:59:29,167] WARNING [celery.redirected:235] Processing: Q1-T42

And similar results when celery is run with a concurrency of 1:

[2020-05-05 22:01:33,879] WARNING [celery.redirected:235] Processing: Q1-T0
[2020-05-05 22:01:34,385] WARNING [celery.redirected:235] Processing: Q1-T1
[2020-05-05 22:01:34,888] WARNING [celery.redirected:235] Processing: Q1-T2
[2020-05-05 22:01:35,391] WARNING [celery.redirected:235] Processing: Q1-T3
[2020-05-05 22:01:35,894] WARNING [celery.redirected:235] Processing: Q1-T4
[2020-05-05 22:01:36,397] WARNING [celery.redirected:235] Processing: Q1-T5
[2020-05-05 22:01:36,899] WARNING [celery.redirected:235] Processing: Q3-T0
[2020-05-05 22:01:37,404] WARNING [celery.redirected:235] Processing: Q2-T0
[2020-05-05 22:01:37,907] WARNING [celery.redirected:235] Processing: Q2-T1
[2020-05-05 22:01:38,411] WARNING [celery.redirected:235] Processing: Q1-T6
[2020-05-05 22:01:38,913] WARNING [celery.redirected:235] Processing: Q2-T2
[2020-05-05 22:01:39,417] WARNING [celery.redirected:235] Processing: Q2-T3
[2020-05-05 22:01:39,919] WARNING [celery.redirected:235] Processing: Q2-T4
[2020-05-05 22:01:40,422] WARNING [celery.redirected:235] Processing: Q1-T7
[2020-05-05 22:01:40,925] WARNING [celery.redirected:235] Processing: Q2-T5
[2020-05-05 22:01:41,429] WARNING [celery.redirected:235] Processing: Q1-T8
Josh Smeaton
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