Consume multiple queues in python / pika

问题:

I am trying to create a consumer that would subscribe to multiple queues, and then process messages as they arrive.

The problem is that when there is some data already present in the first queue, it consumes the first queue and never goes to consume the second queue. However, when the first queue is empty, it does go to the next queue, and then consumes both queues simultaneously.

I had first implemented threading but want to steer clear of it, when pika library does it for me without much complexity. Below is my code:

import pika

mq_connection = pika.BlockingConnection(pika.ConnectionParameters('x.x.x.x'))
mq_channel = mq_connection.channel()
mq_channel.basic_qos(prefetch_count=1)


def callback(ch, method, properties, body):
    print body
    mq_channel.basic_ack(delivery_tag=method.delivery_tag)

mq_channel.basic_consume(callback, queue='queue1', consumer_tag="ctag1.0")
mq_channel.basic_consume(callback, queue='queue2', consumer_tag="ctag2.0")
mq_channel.start_consuming()

回答1:

One possible solution is to use non blocking connection and consume messages.

import pika


def callback(channel, method, properties, body):
    print(body)
    channel.basic_ack(delivery_tag=method.delivery_tag)


def on_open(connection):
    connection.channel(on_channel_open)


def on_channel_open(channel):
    channel.basic_consume(callback, queue='queue1')
    channel.basic_consume(callback, queue='queue2')


parameters = pika.URLParameters('amqp://guest:guest@localhost:5672/%2F')
connection = pika.SelectConnection(parameters=parameters,
                                   on_open_callback=on_open)

try:
    connection.ioloop.start()
except KeyboardInterrupt:
    connection.close()

This will connect to multiple queues and will consume messages accordingly.

回答2:

The issue is most likely that the first call has issued a Basic.Consume and has already received messages from a pre-populated queue before the second call is issued. You might want to try setting the QoS prefetch count to 1, which will limit RabbitMQ from sending you more than one message at a time.