RabbitMQ之四 Routinig

在上篇文章Publish/Subscribe中我们构建了一个简单的日志系统，可以将日志消息广播给多个消费者。

这这篇文章中我们需要添加一个功能，使得日志消费者可以订阅日志的部分消息，比如我们需要将error的日志消息记录到日志文件中，同时将所有的日志通过控制台打印出来。

所谓的绑定(binding)就是一个queue与exchange之间的一种联系，即这个queue对特定exchange中的消息感兴趣。

比如代码channel.queueBind(queueName, EXCHANGE_NAME, "");

    /**
     * Bind a queue to an exchange, with no extra arguments.
     * @see com.rabbitmq.client.AMQP.Queue.Bind
     * @see com.rabbitmq.client.AMQP.Queue.BindOk
     * @param queue the name of the queue
     * @param exchange the name of the exchange
     * @param routingKey the routing key to use for the binding
     * @return a binding-confirm method if the binding was successfully created
     * @throws java.io.IOException if an error is encountered
     */
    Queue.BindOk queueBind(String queue, String exchange, String routingKey) throws IOException;

这个方法的第三个参数routingKey表示对交换机中的某种规则。routingKey对于fanout这种exchange type来说没有意义，因为fanout会将消息广播给所有的queue(绑定的queue)。

Direct exchange

上篇文章中的日志系统将所有的日志消息广播给绑定的queue，在这篇文章中，我们需要改造代码使得可以根据日志消息的重要性来过滤消息，比如我们希望将特定的error消息写到硬盘，对于warning或者info的日志消息不需要记录都硬盘。

可以使用direct exchange。direct exchange的算法就是根据routing key去匹配对应的binding key的消息。direct exchange的消息模型如下

在上图中我们可以看到，xechange为direct的交换机x有两个queue绑定。一个通过binding key orange绑定，另一个通过black green这两个binding key来绑定。这种情况下，消息带有orange将会被分发到第一个queue，消息带有black或者green将会分发到第二个quue。

当然，我们也可以为一个bindingkey绑定多个queue，这种情况下，就是exchange type为direct就跟exchange type为fanout一样，会将消息广播给绑定的queue

调整后的日志系统消息模型如下

代码如下：

客户端EmitLogDirect.java

package org.rabbitmq;

import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;

public class EmitLogDirect {
	private static final String EXCHANGE_NAME = "direct_logs";

	public static void main(String[] argv) throws Exception {

		ConnectionFactory factory = new ConnectionFactory();
		factory.setHost("localhost");
		factory.setPort(5672);
		Connection connection = factory.newConnection();
		Channel channel = connection.createChannel();

		channel.exchangeDeclare(EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
		String reqArgv[] = new String[]{"error","Run. Run. Or it will explode."};
		String severity = getSeverity(reqArgv);
		String message = getMessage(reqArgv);

		channel.basicPublish(EXCHANGE_NAME, severity, null, message.getBytes("UTF-8"));
		System.out.println(" [x] Sent '" + severity + "':'" + message + "'");

		channel.close();
		connection.close();
	}

	private static String getSeverity(String[] strings) {
		if (strings.length < 1)
			return "info";
		return strings[0];
	}

	private static String getMessage(String[] strings) {
		if (strings.length < 2)
			return "Hello World!";
		return joinStrings(strings, " ", 1);
	}

	private static String joinStrings(String[] strings, String delimiter, int startIndex) {
		int length = strings.length;
		if (length == 0)
			return "";
		if (length < startIndex)
			return "";
		StringBuilder words = new StringBuilder(strings[startIndex]);
		for (int i = startIndex + 1; i < length; i++) {
			words.append(delimiter).append(strings[i]);
		}
		return words.toString();
	}
}

服务端ReceiveLogsDirect.java

package org.rabbitmq;

import java.io.IOException;

import com.rabbitmq.client.AMQP;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Consumer;
import com.rabbitmq.client.DefaultConsumer;
import com.rabbitmq.client.Envelope;

public class ReceiveLogsDirect {
	 private static final String EXCHANGE_NAME = "direct_logs";

	  public static void main(String[] argv) throws Exception {
	    ConnectionFactory factory = new ConnectionFactory();
	    factory.setHost("localhost");
	    factory.setPort(5673);
	    Connection connection = factory.newConnection();
	    Channel channel = connection.createChannel();

	    channel.exchangeDeclare(EXCHANGE_NAME, "direct");
	    String queueName = channel.queueDeclare().getQueue();
	    String[] reqArgv = new String[]{"info","warning","error"};
	    if (reqArgv.length < 1){
	      System.err.println("Usage: ReceiveLogsDirect [info] [warning] [error]");
	      System.exit(1);
	    }

	    for(String severity : reqArgv){
	      channel.queueBind(queueName, EXCHANGE_NAME, severity);
	    }
	    System.out.println(" [*] Waiting for messages. To exit press CTRL+C");

	    Consumer consumer = new DefaultConsumer(channel) {
	      @Override
	      public void handleDelivery(String consumerTag, Envelope envelope,
	                                 AMQP.BasicProperties properties, byte[] body) throws IOException {
	        String message = new String(body, "UTF-8");
	        System.out.println(" [x] Received '" + envelope.getRoutingKey() + "':'" + message + "'");
	      }
	    };
	    channel.basicConsume(queueName, true, consumer);
	  }
}

运行结果如下：

参考：http://www.rabbitmq.com/tutorials/tutorial-four-java.html

RabbitMQ之四 Routinig

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