How does a Celery worker consuming from multiple queues decide which to consume from first?

Question

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:

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Answer 1

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 kogan3.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