Pika + RabbitMQ: setting basic_qos to prefetch=1 still appears to consume all messages in the queue

Question

I've got a python worker client that spins up a 10 workers which each hook onto a RabbitMQ queue. A bit like this:

#!/usr/bin/python
worker_count=10

def mqworker(queue, configurer):
    connection = pika.BlockingConnection(pika.ConnectionParameters(host='mqhost'))
    channel = connection.channel()
    channel.queue_declare(queue=qname, durable=True)
    channel.basic_consume(callback,queue=qname,no_ack=False)
    channel.basic_qos(prefetch_count=1)
    channel.start_consuming()


def callback(ch, method, properties, body):
    doSomeWork();
    ch.basic_ack(delivery_tag = method.delivery_tag)

if __name__ == '__main__':
    for i in range(worker_count):
        worker = multiprocessing.Process(target=mqworker)
        worker.start()

The issue I have is that despite setting basic_qos on the channel, the first worker to start accepts all the messages off the queue, whilst the others sit there idle. I can see this in the rabbitmq interface, that even when I set worker_count to be 1 and dump 50 messages on the queue, all 50 go into the 'unacknowledged' bucket, whereas I'd expect 1 to become unacknowledged and the other 49 to be ready.

Why isn't this working?

Answer 1

I appear to have solved this by moving where basic_qos is called.

Placing it just after channel = connection.channel() appears to alter the behaviour to what I'd expect.