RabbitMQ学习之：（六）Direct Exchange （转贴+我的评论）

From: http://lostechies.com/derekgreer/2012/04/02/rabbitmq-for-windows-direct-exchanges/

RabbitMQ for Windows: Direct Exchanges

Posted by Derek Greer on April 2, 2012

This is the fifth installment to the series: RabbitMQ for Windows.  In the last installment, we took a look at the four exchange types provided by RabbitMQ: Direct, Fanout, Topic, and Headers.  In this installment we’ll walk through an example which uses a direct exchange type directly and we’ll take a look at the push API.

In the Hello World example from the second installment of the series, we used a direct exchange type implicitly by taking advantage of the automatic  binding of queues to the default exchange using the queue name as the routing key.  The example we’ll work through this time will be similar, but we’ll declare and bind to the exchange explicitly.

This time our example will be a distributed logging application.  We’ll create a Producer console application which publishes a logging message for some noteworthy action and a Consumer console application which displays the message to the console.

PunCha：作者之前的5个帖子，很简单，看的我是行云流水~，可是看到这个帖子，让我一头雾水从而研究了2个小时，这才有了RabbitMQ学习篇一。我并不觉得这篇文章的例子是个好例子！作者的意图很明显，因为DirectExchange太简单了，没有深度，所以他参差了一个BasicConsumer函数（Block调用），但是却把简单问题复杂化了。而且混淆了一些概念，我这里作一下说明：

1. Exchange只用于Producer，RoutingKey用来绑定Exchange和Queue，这个一般在Producer这端做的，但是作者却放到了Consumer这端。可以是可以，但是作者应该加一个说明。

2. 作者这个用法说明了一个问题，假如Producer把消息发到没有绑定Queue的Exchange消息会丢失！

3. 作者让我对RMQ有了深刻的了解，嘿嘿。

Beginning with our Producer app, we’ll start by establishing a connection using the default settings, create the connection, and create a channel:

using RabbitMQ.Client;

namespace Producer
{
  class Program
  {
    static void Main(string[] args)
    {
      var connectionFactory = new ConnectionFactory();
      IConnection connection = connectionFactory.CreateConnection();
      IModel channel = connection.CreateModel();
    }
  }
}

Next, we need to declare the exchange we’ll be publishing our message to.  We need to give our exchange a name in order to reference it later, so let’s use “direct-exchange-example”:

channel.ExchangeDeclare("direct-exchange-example", ExchangeType.Direct);

The second parameter indicates the exchange type.  For the official RabbitMQ .Net client, this is just a simple string containing one of the values: direct, fanout, topic, or headers.  The type RabbitMQ.Client.ExchangeType defines each of the exchange types as a constant for convenience. [PunCha：这里为什么不用一个枚举型？没搞懂为什么要这样设计]

Next, let’s call some method which might produce a value worthy of interest.  We’ll call the method DoSomethingInteresting() and have it return a string value:

string value = DoSomethingInteresting();

For the return value, the implementation of DoSomethingInteresting() can just return the string value of a new Guid:

static string DoSomethingInteresting() 
{ 
  return Guid.NewGuid().ToString(); 
}

Next, let’s use the returned value to create a log message containing a severity level of Information:

string logMessage = string.Format("{0}: {1}", TraceEventType.Information, value);

Next, we need to convert our log message to a byte array and publish the message to our new exchange:

byte[] message = Encoding.UTF8.GetBytes(logMessage); channel.BasicPublish("direct-exchange-example", "", null, message);

Here, we use an empty string as our routing key [PunCha: 所以，假如想成功发送消息，必然有一个名叫direct-exchange-example的Exchange对象和一个空的RoutingKey用来绑定某个Queue。In this case，是在Consumer这端实现的，这也是为什么Consumer一定要先运行的原因。] and null for our message properties.

We end our Producer by closing the channel and connection:

channel.Close(); 
connection.Close();

Here’s the full listing:

using System;
using System.Diagnostics;
using System.Text;
using System.Threading;
using RabbitMQ.Client;

namespace Producer
{
  class Program
  {
    static void Main(string[] args)
    {
      Thread.Sleep(1000);
      var connectionFactory = new ConnectionFactory();
      IConnection connection = connectionFactory.CreateConnection();
      IModel channel = connection.CreateModel();

      channel.ExchangeDeclare("direct-exchange-example", ExchangeType.Direct);
      string value = DoSomethingInteresting();
      string logMessage = string.Format("{0}: {1}", TraceEventType.Information, value);

      byte[] message = Encoding.UTF8.GetBytes(logMessage);
      channel.BasicPublish("direct-exchange-example", "", null, message);

      channel.Close();
      connection.Close();
    }

    static string DoSomethingInteresting()
    {
      return Guid.NewGuid().ToString();
    }
  }
}

Note that our logging example’s Producer differs from our Hello World’s Producer in that we didn’t declare a queue this time.[PunCha: 特别注意，我们没有声明Queue! 这个不同于HelloWorld。]  In our Hello World example, we needed to run our Producer before the Consumer since the Consumer simply retrieved a single message and exited.  Had we published to the default exchange without declaring the queue first, our message would simply have been discarded by the server before the Consumer had an opportunity to declare and bind the queue.

Next, we’ll create our Consumer which starts the same way as our Producer code:

using RabbitMQ.Client;

namespace Consumer
{
  class Program
  {
    static void Main(string[] args)
    {
      var connectionFactory = new ConnectionFactory();
      IConnection connection = connectionFactory.CreateConnection();
      IModel channel = connection.CreateModel();

      channel.ExchangeDeclare("direct-exchange-example", ExchangeType.Direct);
    }
  }
}

Next, we need to declare a queue to bind to our exchange.  Let’s name our queue “logs”:

 
channel.QueueDeclare("logs", false, false, true, null);

To associate our logs queue with our exchange, we use the QueueBind() method providing the name of the queue, the name of the exchange, and the binding key to filter messages on: [PunCha：这里就产生了一个绑定，Exchange是direct-exchange-example，Queue是logs，纽带是空的RoutingKey。这个与Producer代码相匹配。]

channel.QueueBind("logs", "direct-exchange-example", "");

At this point we could consume messages using the pull API method BasicGet() as we did in the Hello World example, but this time we’ll use the push API.  To have messages pushed to us rather than us pulling messages, we first need to declare a consumer:

var consumer = new QueueingBasicConsumer(channel);

To start pushing messages to our consumer, we call the channel’s BasicConsume() method and tell it which consumer to start pushing messages to: [PunCha：注意，取数据永远只需要Queue，没有Exchange和RoutingKey。]

channel.BasicConsume(“logs”, true, consumer);

Here, we specify the queue to consume messages from, a boolean flag instructing messages to be auto-acknowledged (see discussion in the Getting the Message section of Hello World Review), and the consumer to push the messages to.  

Now, any messages placed on the queue will automatically be retrieved and placed in a local in-memory queue.  To dequeue a message from the local queue, we call the Dequeue() method on the consumer’s Queue property:

var eventArgs = (BasicDeliverEventArgs)consumer.Queue.Dequeue();

This method call blocks until a message is available to be dequeued, or until an EndOfStreamException is thrown indicating that the consumer was cancelled, the channel was closed, or the connection otherwise was terminated.[PunCha：说实话，我的感觉，无论是返回参数（需要强转），还是这个函数的取名，都很奇怪。。Dequeue的时候Block。。。]

Once the Dequeue() method returns, the BasicDeliverEventArgs contains the bytes published from the Producer in the Body property, so we can convert this value back into a string and print it to the console:

var message = Encoding.UTF8.GetString(eventArgs.Body);
Console.WriteLine(message);

We end our Consumer by closing the channel and connection:

channel.Close();
connection.Close();

Here’s the full listing:

using System;
using System.Text;
using RabbitMQ.Client;
using RabbitMQ.Client.Events;

namespace Consumer
{
  class Program
  {
    static void Main(string[] args)
    {
      var connectionFactory = new ConnectionFactory();
      IConnection connection = connectionFactory.CreateConnection();
      IModel channel = connection.CreateModel();

      channel.ExchangeDeclare("direct-exchange-example", ExchangeType.Direct);
      channel.QueueDeclare("logs", false, false, true, null);
      channel.QueueBind("logs", "direct-exchange-example", "");

      var consumer = new QueueingBasicConsumer(channel);
      channel.BasicConsume("logs", true, consumer);

      var eventArgs = (BasicDeliverEventArgs) consumer.Queue.Dequeue();

      string message = Encoding.UTF8.GetString(eventArgs.Body);
      Console.WriteLine(message);

      channel.Close();
      connection.Close();
      Console.ReadLine();
    }
  }
}

If we run the resulting Consumer.exe at this point, it will block until a message is routed to the queue.  Running the Producer.exe from another shell produces a message on the consumer console similar to the following:

Information: 610fe447-bf31-41d2-ae29-414b2d00087b

Note: For a convenient way to execute both the Consumer and Producer from within Visual Studio, go to the solution properties and choose “Set StartUp Projects …”.  Select the “Multiple startup projects:”[PunCha：这个功能蛮好的，我以前竟然不知道~] option and set both the Consumer and Producer to the Action: Start.  Use the arrows to the right of the projects to ensure the Consumer is started before the Producer.  In some cases, this can still result in the Producer publishing the message before the Consumer has time to declare and bind the queue, so putting a Thread.Sleep(1000) at the start of your Producer should ensure things happen in the required order.  After this, you can run your examples by using Ctrl+F5 (which automatically prompts to exit).

That concludes our direct exchange example.  Next time, we’ll take a look at the Fanout exchange type.

 http://blog.csdn.net/puncha/article/details/8449395