《RabbitMQ Tutorial》译文 第 5 章 主题

原文来自 RabbitMQ 英文官网的教程（5.Topics），其示例代码采用了 .NET C# 语言。

In the previous tutorial we improved our logging system. Instead of using a fanout exchange only capable of dummy broadcasting, we used a direct one, and gained a possibility of selectively receiving the logs.

在以前的教程中，咱们改进了日志系统。比起只可以单一广播的 fanout 型交换机，咱们如今采用了 direct 型，从而得到了选择性接收日志的可能性。

Although using the direct exchange improved our system, it still has limitations - it can't do routing based on multiple criteria.

尽管使用 direct 型交换机对系统有所改进，但仍然有其局限性 - 即在多重条件下没办法进行路由。

In our logging system we might want to subscribe to not only logs based on severity, but also based on the source which emitted the log. You might know this concept from the syslog unix tool, which routes logs based on both severity (info/warn/crit...) and facility (auth/cron/kern...).

在该日志系统中，咱们可能但愿不单单订阅基于严重性（这一项指标）的日志，也但愿能基于产生日志的源头。你大概会知道这个概念来自于 syslog unix 工具，它能够针对严重性（info/warn/crit...）和设备（auth/cron/kern...）进行路由。

That would give us a lot of flexibility - we may want to listen to just critical errors coming from 'cron' but also all logs from 'kern'.

这便给了咱们许多灵活性 - 咱们能够作到监听“cron”类型的严重错误，以及全部来自于“kern”设备的日志。

To implement that in our logging system we need to learn about a more complex topic exchange.

为了在咱们的日志系统中实现上述功能，咱们须要学习更为复杂的话题（Topic）交换机。

Topic exchange

话题交换机

Messages sent to a topic exchange can't have an arbitrary routing_key - it must be a list of words, delimited by dots. The words can be anything, but usually they specify some features connected to the message. A few valid routing key examples: "stock.usd.nyse", "nyse.vmw", "quick.orange.rabbit". There can be as many words in the routing key as you like, up to the limit of 255 bytes.

发送至 topic 型交换机的消息已不能携带任意的 routing_key - 而必须是一个字符串列表，并基于小数点来分隔。

The binding key must also be in the same form. The logic behind the topic exchange is similar to a direct one - a message sent with a particular routing key will be delivered to all the queues that are bound with a matching binding key. However there are two important special cases for binding keys:

• * (star) can substitute for exactly one word.
• # (hash) can substitute for zero or more words.

“绑定键”也必须是相同的格式。topic 型交换机背后的逻辑与 direct 型交换机类似 - 采用指定 routing key 所发送的消息将会被递送到与 binding key 相匹配的队列中，然而针对 binding key 有两个很是重要且特殊的情形：

• * （星号）能够替代一个明确的字符串。
• # （哈希，井号）能够替代 0 至多个字符串。

It's easiest to explain this in an example:

在示例中能够很容易的对其解释：

In this example, we're going to send messages which all describe animals. The messages will be sent with a routing key that consists of three words (two dots). The first word in the routing key will describe speed, second a colour and third a species: " . . ".

在这个示例中，咱们打算发送用来描述动物的消息，它是基于三个字符串（包含两个小数点）组成的 routing key，其第一个字符串将用来描述速度，第二个是颜色，第三个是物种，以下：“ <速度> . <颜色> . <物种> ”。

We created three bindings: Q1 is bound with binding key ".orange." and Q2 with "..rabbit" and "lazy.#".

咱们建立了三个绑定，队列 Q1 将关联到绑定键“*.orange.*”，队列 Q2 则关联到“*.*.rabbit”和“lazy.#”。

These bindings can be summarised as:

• Q1 is interested in all the orange animals.
• Q2 wants to hear everything about rabbits, and everything about lazy animals.

这些绑定能够归纳为：

• 队列 Q1 对全部桔黄色（orange）的动物感兴趣。
• 队列 Q2 指望监听到全部 rabbit 后缀，以及全部 lazy 前缀的动物。

A message with a routing key set to "quick.orange.rabbit" will be delivered to both queues. Message "lazy.orange.elephant" also will go to both of them. On the other hand "quick.orange.fox" will only go to the first queue, and "lazy.brown.fox" only to the second. "lazy.pink.rabbit" will be delivered to the second queue only once, even though it matches two bindings. "quick.brown.fox" doesn't match any binding so it will be discarded.

routing key 设置为"quick.orange.rabbit"的消息将会被递送到两个队列。消息"lazy.orange.elephant"一样也会去往两个队列。另外一方面，"quick.orange.fox"将只会去往第一个队列，"lazy.brown.fox"则只会去第二个队列。"lazy.pink.rabbit"，尽管它匹配了两个绑定，但只会被递送到第二个队列一次（而不会是两次）。"quick.brown.fox"则不匹配任何绑定，因此将会被丢弃。

What happens if we break our contract and send a message with one or four words, like "orange" or "quick.orange.male.rabbit"? Well, these messages won't match any bindings and will be lost.

若是咱们打破约定将会发生什么呢？好比发送一个字符串或者四个字符串，相似于“orange”或者“quick.orange.male.rabbit”。

On the other hand "lazy.orange.male.rabbit", even though it has four words, will match the last binding and will be delivered to the second queue.

另外一方面像“lazy.orange.male.rabbit”，即便它有四个字符串，但仍然会匹配最后一个绑定（lazy.#）并被递送到第二个队列。

Topic exchange

话题交换机

Topic exchange is powerful and can behave like other exchanges.

话题（Topic）交换机很强大，它能够表现得相似其余交换机。

When a queue is bound with "#" (hash) binding key - it will receive all the messages, regardless of the routing key - like in fanout exchange.

当一个队列采用"#"做为绑定键时，它将接收到全部的消息而忽略掉路由键，这就像是 fanout 型交换机同样。

When special characters "*" (star) and "#" (hash) aren't used in bindings, the topic exchange will behave just like a direct one.

当特殊字符"*" (星号) and "#" (哈希，井号)都没有应用到绑定中时，topic 型交换机将表现得与 direct 型同样。

Putting it all together

融合一块儿

We're going to use a topic exchange in our logging system. We'll start off with a working assumption that the routing keys of logs will have two words: " . ".

咱们即将在日志系统中使用 topic 型交换机，首先从一个假定的工做前替起步，即拟定日志的路由键为两个字符串：" . "

The code is almost the same as in the previous tutorial.

其代码与以前教程几乎相同。

The code for EmitLogTopic.cs:

EmitLogTopic.cs 类文件代码：

using System;
using System.Linq;
using RabbitMQ.Client;
using System.Text;

class EmitLogTopic
{
    public static void Main(string[] args)
    {
        var factory = new ConnectionFactory() { HostName = "localhost" };
        using(var connection = factory.CreateConnection())
        using(var channel = connection.CreateModel())
        {
            channel.ExchangeDeclare(exchange: "topic_logs",
                                    type: "topic");

            var routingKey = (args.Length > 0) ? args[0] : "anonymous.info";
            var message = (args.Length > 1)
                          ? string.Join(" ", args.Skip( 1 ).ToArray())
                          : "Hello World!";
            var body = Encoding.UTF8.GetBytes(message);
            channel.BasicPublish(exchange: "topic_logs",
                                 routingKey: routingKey,
                                 basicProperties: null,
                                 body: body);
            Console.WriteLine(" [x] Sent '{0}':'{1}'", routingKey, message);
        }
    }
}

The code for ReceiveLogsTopic.cs:

ReceiveLogsTopic.cs 类文件代码：

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

class ReceiveLogsTopic
{
    public static void Main(string[] args)
    {
        var factory = new ConnectionFactory() { HostName = "localhost" };
        using(var connection = factory.CreateConnection())
        using(var channel = connection.CreateModel())
        {
            channel.ExchangeDeclare(exchange: "topic_logs", type: "topic");
            var queueName = channel.QueueDeclare().QueueName;

            if(args.Length < 1)
            {
                Console.Error.WriteLine("Usage: {0} [binding_key...]",
                                        Environment.GetCommandLineArgs()[0]);
                Console.WriteLine(" Press [enter] to exit.");
                Console.ReadLine();
                Environment.ExitCode = 1;
                return;
            }

            foreach(var bindingKey in args)
            {
                channel.QueueBind(queue: queueName,
                                  exchange: "topic_logs",
                                  routingKey: bindingKey);
            }

            Console.WriteLine(" [*] Waiting for messages. To exit press CTRL+C");

            var consumer = new EventingBasicConsumer(channel);
            consumer.Received += (model, ea) =>
            {
                var body = ea.Body;
                var message = Encoding.UTF8.GetString(body);
                var routingKey = ea.RoutingKey;
                Console.WriteLine(" [x] Received '{0}':'{1}'",
                                  routingKey,
                                  message);
            };
            channel.BasicConsume(queue: queueName,
                                 autoAck: true,
                                 consumer: consumer);

            Console.WriteLine(" Press [enter] to exit.");
            Console.ReadLine();
        }
    }
}

Run the following examples:

运行示例：

To receive all the logs:

接收全部的日志：

cd ReceiveLogsTopic
dotnet run "#"

To receive all logs from the facility "kern":

只接收来自“kern”设备的日志：

cd ReceiveLogsTopic
dotnet run "kern.*"

Or if you want to hear only about "critical" logs:

或者你只想关心“critical”级别的日志：

ReceiveLogsTopic.exe "*.critical"

You can create multiple bindings:

你能够建立多重绑定：

cd ReceiveLogsTopic
dotnet run "kern.*" "*.critical"

And to emit a log with a routing key "kern.critical" type:

基于路由键“kern.critical”来产生日志，能够输入：

cd EmitLogTopic
dotnet run "kern.critical" "A critical kernel error"

Have fun playing with these programs. Note that the code doesn't make any assumption about the routing or binding keys, you may want to play with more than two routing key parameters.

程序执行起来仍是很使人愉快的，要注意的是，这些代码并无针对路由键或者绑定键作任何预设 ，你可能会须要用到超出两个以上的路由键参数。

(Full source code for EmitLogTopic.cs and ReceiveLogsTopic.cs)

( EmitLogTopic.cs 和 ReceiveLogsTopic.cs 完整代码)