RabbitMQ--work queues（二）

封装一个task到一个message，并发送到queue。consumer会去除task并执行这个task。

这里我们简化了操作，发送消息到队列中，consumer取出消息计算里面'.'号有几个就sleep几秒。

task.py

#!/usr/bin/env python
import pika
import sys

connection = pika.BlockingConnection(pika.ConnectionParameters(
        host='localhost'))
channel = connection.channel()

channel.queue_declare(queue='task_queue', durable=True)

message = ' '.join(sys.argv[1:]) or "Hello World!"
channel.basic_publish(exchange='',
                      routing_key='task_queue',
                      body=message,
                      properties=pika.BasicProperties(
                         delivery_mode = 2, # make message persistent
                      ))
print(" [x] Sent %r" % message)
connection.close()

work.py

#!/usr/bin/env python
import pika
import time

connection = pika.BlockingConnection(pika.ConnectionParameters(
        host='localhost'))
channel = connection.channel()

channel.queue_declare(queue='task_queue', durable=True)
print(' [*] Waiting for messages. To exit press CTRL+C')

def callback(ch, method, properties, body):
    print(" [x] Received %r" % body)
    time.sleep(body.count(b'.'))
    print(" [x] Done")
    ch.basic_ack(delivery_tag = method.delivery_tag)

channel.basic_qos(prefetch_count=1)
channel.basic_consume(callback,
                      queue='task_queue')

channel.start_consuming()

代码解释

channel.queue_declare(queue='task_queue', durable=True)
告诉rabbitmq永不丢失queue，即使rabbitmq server挂掉。

def callback(ch, method, properties, body):
    print " [x] Received %r" % (body,)
    time.sleep( body.count('.') )
    print " [x] Done"
    ch.basic_ack(delivery_tag = method.delivery_tag)

channel.basic_consume(callback,
                      queue='hello')

在先前例子中，如果consumer突然死掉，回丢失掉正在处理的信息。如何避免呢？如果consumer死掉，怎么将这条信息发送的其他consumer呢？就是使用上面代码

channel.basic_qos(prefetch_count=1)

This tells RabbitMQ not to give more than one message to a worker at a time. 
Or, in other words, don't dispatch a new message to a worker until it has processed and acknowledged the previous one.
Instead, it will dispatch it to the next worker that is not still busy. 
直到消费完这个消息再给consumer派送新的消息，如果没消费完，将消息发送给另一个consumer.