RabbitMQ学习之Routing(4)

上一节，是广播日志message到很多的receivers.

这节，我们讲订阅其中的一个子集。例如，我们想可以把危机的error message导到log file。而仍然可以打印所有的log messages到控制台。

这里使用到Direct exchage

Direct exchange

在使用fanout exchange时，没有很多的灵活性，它只是广播。

这节，我们将使用direct exchange . 在direct exchange背后的路由算法是简单的，即message会发送到一个binding key 正好匹配message的routing key的queue.

如图

我们可以看到，有两个queue绑定到exchange了。第一个queue是和binding key为orange的绑定的。并且第二个有两个bindings.一个是black，另一个是green.

带有routing key 为orange的发送到exchange的 message将会发送到queue Q1；

而routing key为black 和green的messages将会发送到Q2. 其他的messages会被丢弃。

Multiple binding(多重绑定)

如图，多重绑定，即一个binding key为black绑定到两个queue.

Emitting logs

我们会把日志严重级别(log severity)作为routing key. 那样，接收脚本将会选择它想要接收的严重级别进行接收。

channel.ExchangeDeclare(exchange: "direct_logs", type: "direct");

var body = Encoding.UTF8.GetBytes(message);
channel.BasicPublish(exchange: "direct_logs",
                     routingKey: severity,
                     basicProperties: null,
                     body: body);

严重级别（Severity）

info, warning, error

Subscribing

接收程序跟之前大致一样，除了一个例外，我们将会为我们感兴趣的严重级别(serverity)创建一个新的binding. 

var queueName = channel.QueueDeclare().QueueName;
foreach(var severity in args)
{
    channel.QueueBind(queue: queueName,
                      exchange: "direct_logs",
                      routingKey: severity);
}

代码

EmitLogDirect.cs

using System;using System.Linq;using RabbitMQ.Client;using System.Text;
class EmitLogDirect
{
    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: "direct_logs", //声明direct类型exchange
                                    type: "direct");

            var severity = (args.Length > 0) ? args[0] : "info";
            var message = (args.Length > 1)
                          ? string.Join(" ", args.Skip( 1 ).ToArray())
                          : "Hello World!";
            var body = Encoding.UTF8.GetBytes(message);
            channel.BasicPublish(exchange: "direct_logs", //发送routingkey 为severity的message
                                 routingKey: severity,
                                 basicProperties: null,
                                 body: body);
            Console.WriteLine(" [x] Sent '{0}':'{1}'", severity, message);
        }
　　}
        Console.WriteLine(" Press [enter] to exit.");
        Console.ReadLine();
}

运行示例：

cd EmitLogDirect
dotnet run error "Run. Run. Or it will explode."# => [x] Sent 'error':'Run. Run. Or it will explode.'

ReceiveLogsDirect.cs

using System;using RabbitMQ.Client;using RabbitMQ.Client.Events;using System.Text;
class ReceiveLogsDirect
{
    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: "direct_logs",  //声明direct类型exchange
                                    type: "direct");
            var queueName = channel.QueueDeclare().QueueName; //声明带随机queue name的queue

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

            foreach(var severity in args)
            {
                channel.QueueBind(queue: queueName,  //绑定queue和exchange和特定值的routingkey
                                  exchange: "direct_logs",
                                  routingKey: severity);
            }

            Console.WriteLine(" [*] Waiting for messages.");

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

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

使用示例：

cd ReceiveLogsDirect
dotnet run warning error > logs_from_rabbit.log

示例2

cd ReceiveLogsDirect
dotnet run info warning error# => [*] Waiting for logs. To exit press CTRL+C

 参考网址：

https://www.rabbitmq.com/tutorials/tutorial-four-dotnet.html