Rabbitmq -Publish_Subscribe模式- python编码实现

Posted 2020-08-02 温柔易淡

what is Exchanges ??

Let\'s quickly go over what we covered in the previous tutorials:

• A producer is a user application that sends messages.
• A queue is a buffer that stores messages.
• A consumer is a user application that receives messages.

 

The core idea in the messaging model in RabbitMQ is that the producer never sends any messages directly to a queue. Actually, quite often the producer doesn\'t even know if a message will be delivered to any queue at all.

# 个人简单翻译下： 这个核心的概念就是说在rabbitmy消息模式里面，消息生产者不会直接把消息发送到队列里面，实际上，生产者甚至都不知道信息如何被投送到所有的队列里面。

 

Instead, the producer can only send messages to an exchange. An exchange is a very simple thing. On one side it receives messages from producers and the other side it pushes them to queues. The exchange must know exactly what to do with a message it receives. Should it be appended to a particular queue? Should it be appended to many queues? Or should it get discarded. The rules for that are defined by the exchange type.

# 个人简单翻译下：  

 

如下图所示：反而，这个生产这仅仅能够发送消息到exchangs上，这个exchangs 是 个非常简单的东西，它一边接受来自生产者的消息，一边把消息推送

给队列，这个exchangs 必须确切的知道如何处理接受到的消息，它收到了消息，应该把它添加到特别的队列里面吗？应该把它添加到许多队列吗？或者把它丢弃，这些规则都被exchange的类型定义了。

 

 

how many exchanges types ??

There are a few exchange types available: direct, topic, headers and fanout(扇出). We\'ll focus on the last one -- the fanout. Let\'s create an exchange of that type, and call it logs:

channel.exchange_declare(exchange=\'logs\',type=\'fanout\')

　　

what is fanout(one of exchanges type)??

The fanout exchange is very simple. As you can probably guess from the name, it just broadcasts all the messages it receives to all the queues it knows. And that\'s exactly what we need for our logger.

#简单的翻译下就是说：　扇出这个交换器非常简单，你都能够从这个名字里面猜出来，它就是把他接受到的信息广播到它所知道的队列里，这恰好是我们ｌｏｇｇｅｒ需要的

 

 

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Listing exchanges

To list the exchanges on the server you can run the ever useful rabbitmqctl:

$ sudo rabbitmqctl list_exchanges
Listing exchanges ...
logs      fanout
amq.direct      direct
amq.topic       topic
amq.fanout      fanout
amq.headers     headers
...done.

 

 

In this list there are some amq.* exchanges and the default (unnamed) exchange. These are created by default, but it is unlikely you\'ll need to use them at the moment.

 

 

Nameless exchange

In previous parts of the tutorial we knew nothing about exchanges, but still were able to send messages to queues. That was possible because we were using a default exchange, which we identify by the empty string ("").

#简单的翻译下：　在前面的教程中我们没有指定ｅｘｃｈａｎｇｅ也能够发送消息到队列里面，那很有可能我们使用了默认的ｅｘｃｈａｎｇｅ

Recall how we published a message before:

channel.basic_publish(exchange=\'\',routing_key=\'hello\',body=message)

The exchange parameter（参数） is the the name of the exchange. The empty string denotes the default or nameless exchange: messages are routed to the queue with the name specified byrouting_key, if it exists.

#　这个ｅｘｃｈａｎｇｅ的参数是ｅｘｃｈａｎｇｅ的名字，这里没写为空的话代表着使用默认的ｅｘｃｈａｎｇｅ或者不可能命名的ｅｘｃｈａｎｇｅ。消息能够被路由到队列是因为使用　ｒｏｕｔｉｎｇ＿ｋｅｙ这个特殊的字段。

 

此时我们修改下代码

 

Now, we can publish to our named exchange instead:

channel.basic_publish(exchange=\'logs\',routing_key=\'\',body=message)

　　

Temporary queues

As you may remember previously we were using queues which had a specified name (rememberhello and task_queue?). Being able to name a queue was crucial for us -- we needed to point the workers to the same queue. Giving a queue a name is important when you want to share the queue between producers and consumers.

# 简单的翻译下：你可能还记得我们前面在使用队列的时候指定了特殊的管道名字，这样命名是为了对我们有用，我们需要把那些运行的程序去指向同一个队列，命名一个队列非常重要当我们在生产者和消费者共享队列的时候

 

But that\'s not the case for our logger. We want to hear about all log messages, not just a subset of them. We\'re also interested only in currently flowing messages not in the old ones. To solve that we need two things.

Firstly, whenever we connect to Rabbit we need a fresh, empty queue. To do it we could create a queue with a random name, or, even better - let the server choose a random queue name for us. We can do this by not supplying the queue parameter to queue_declare:

result=channel.queue_declare()

　　

At this point result.method.queue contains a random queue name. For example it may look likeamq.gen-JzTY20BRgKO-HjmUJj0wLg.

Secondly, once we disconnect the consumer the queue should be deleted. There\'s an exclusive flag for that:

result=channel.queue_declare(exclusive=True)

　　

# translate : 上面说的这样指定命名队列的情况不适用于我们的logger，我们想要监听所有的日志信息，不是监听一个子集，我们同样对只在当前信息流的信息感兴趣，而不是对旧信息感兴趣，为了解决这个问题，我们需要做两件事，第一，无论什么时候我们链接rabbit 我们都能够得到新鲜的消息，空队列，为了做到它，我们需要一个随机命名的队列，或者更好的是，让服务自己选择一个随机队列名字给我们，我们能够做到这点，通过提供这个队列参数queue_declare，在这点上，result.method.queue 包含了随机的队列名字，例如它可能看起来想amq.gen-JzTY20BRgKO-HjmUJj0wLg...第二，一旦我们和消费者市区了连接在，这个队列就应该删除，这里有一个独有的标志：

 

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Bindings

 

We\'ve already created a fanout exchange and a queue. Now we need to tell the exchange to send messages to our queue. That relationship between exchange and a queue is called a binding

#　ｔｒａｎｓｌａｔｅ：我们已经创建了扇出的交换器和一个队列，现在我们需要告诉这个交换器发送消息到我们队列，这时，交换器和队列他们之间两者的关系成为捆版

channel.queue_bind(exchange=\'logs\',queue=result.method.queue)

　　

From now on the logs exchange will append messages to our queue.

Listing bindings

You can list existing bindings using, you guessed it, rabbitmqctl list_bindings.

 

 

 

Putting it all together

 

The producer program, which emits log messages, doesn\'t look much different from the previous tutorial. The most important change is that we now want to publish messages to our logs exchange instead of the nameless one. We need to supply a routing_key when sending, but its value is ignored for fanout exchanges. Here goes the code for emit_log.py script:

# translate： 这个生产这程序，能够发送日志消息，看起来不不同于前面的教程，这个最重要的改变是我们现在想发送广播信息到我们的日志交换器而不是未命名的交换器，我们需要提供一个routeing_key当我们发送的时候，但是它的值是可以忽略的对于扇出交换器，这里能够得到这个代码：

github地址：https://github.com/rabbitmq/rabbitmq-tutorials/blob/master/python/emit_log.py

 

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

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

channel.exchange_declare(exchange=\'logs\',
                         type=\'fanout\')

message=\' \'.join(sys.argv[1:]) or"info: Hello World!"channel.basic_publish(exchange=\'logs\',
                      routing_key=\'\',
                      body=message)
print(" [x] Sent %r"%message)
connection.close()

 

As you see, after establishing the connection we declared the exchange. This step is neccesary as publishing to a non-existing exchange is forbidden.

The messages will be lost if no queue is bound to the exchange yet, but that\'s okay for us; if no consumer is listening yet we can safely discard the message.The code for receive_logs.py:

# translate： 正如你所看到的，在建立连接后我们可以声明这个交换器，这一步的话广播到不存在的交换器是会被拒绝的，

这个信息会被丢失如果没有队列去限制这个交换器，但是这对我们来说是OK，如果没有消费者去监听的话，我们就可以安全的丢弃这个消息，这个代码如下：

The code for receive_logs.py:

#!/usr/bin/env pythonimport pika

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

channel.exchange_declare(exchange=\'logs\',
                         type=\'fanout\')

result=channel.queue_declare(exclusive=True)
queue_name=result.method.queuechannel.queue_bind(exchange=\'logs\',
                   queue=queue_name)

print(\' [*] Waiting for logs. To exit press CTRL+C\')

defcallback(ch, method, properties, body):
    print(" [x] %r"%body)

channel.basic_consume(callback,
                      queue=queue_name,
                      no_ack=True)

channel.start_consuming()

 代码地址：https://github.com/rabbitmq/rabbitmq-tutorials/blob/master/python/receive_logs.py

 We\'re done. If you want to save logs to a file, just open a console and type:

$ python receive_logs.py > logs_from_rabbit.log

　If you wish to see the logs on your screen, spawn a new terminal and run:

$ python receive_logs.py

　　And of course, to emit logs type:

$ python emit_log.py

　　Using rabbitmqctl list_bindings you can verify that the code actually creates bindings and queues as we want. With two receive_logs.py programs running you should see something like:

$ sudo rabbitmqctl list_bindings
Listing bindings ...
logs    exchange        amq.gen-JzTY20BRgKO-HjmUJj0wLg  queue           []
logs    exchange        amq.gen-vso0PVvyiRIL2WoV3i48Yg  queue           []
...done.

　　

The interpretation of the result is straightforward: data from exchange logs goes to two queues with server-assigned names. And that\'s exactly what we intended.

To find out how to listen for a subset of messages, let\'s move on to tutorial 4