rabbitmq入门-RPC

在项目中引入RabbitMQ通常会考虑它会带来的好处:解耦应用程序,实现不同编程语言之间的互通,解除对特定通信协议的依赖,解除应用程序在时序上执行的依赖(异步).落实到代码层面就是两种常用应用模式:"发后即忘"(fire-and-forget)和RPC.

fire-and-forget

RabbitMQ解决的是应用程序之间互联(connect)和规模(scale)的问题,消息发送和接收是隔离,发送方不知道消息最终由谁接收,接收方也不必关心消息是谁步发出的;发送和接收是隔离的,消息本质上就是异步的.这种隔离也就解耦了应用程序之间的依赖.RabbitMQ的角色就是应用程序中间的路由器.对于消息的发布方来讲这是一种"发后即忘"(fire-and_forget)的发布方式.

fire-and-forget模式发送消息,消息的发送方和接收方彼此隔离

RPC

RPC需要双向通信,或者说RPC Server需要明确知道要把消息发送给谁.我们可以在payload的数据部分附加 "发给谁" 这种EndPoint信息. RabbitMQ提供的解决方案:在每一个AMQP的消息头上有一个reply_to字段.这样消息的producer就可以指定Queue name,RPC Server接受到消息检查reply_to字段,创建一个消息包含Response并把queue name作为routing key,订阅了这个队列的Client就拿到了消息.

这里有两件事情要保证:

1.要为队列创建随机Name

2.即使Name随机还是有可能冲突,还需要保证消息通信的独占性,看看RabbitMQ是怎么满足这两点的:

（1）如果创建的队列不指定queue name，rabbitMQ就会创建一个随机的Name

（2）独占只需要exclusive参数即可

总而言之,需要做的就是Client创建一个temporary,exclusive,anonymou的queue,并把queue name设置在RPC消息的reply_to字段即可.注意这里RPC Server已经知道要投递到哪个Queue,所以不需要指Exchange

RabbitMq RPC与传统RPC区别

传统的RPC调用Client和Server紧密依赖,客户端连接上服务器,发送一个请求然后阻塞等待服务器响应.这样的做的特点是客户端和服务器端是知道对方的.如果RPC Server崩溃掉,客户端需要重连,如果Server彻底崩掉就要重新找一个提供同样服务的Server,然后客户端重连过去.

用RabbitMQ来实现RPC,依然保持Client Server信息隐藏的特点,Client依赖的不是特定的Server而是特定的消息,在有多个等效Server的情况下,一个Server的状态是否正常不会影响到客户端的状态.

总结一下,使用RabbitMQ是先RPC,客观上还实现了下面的效果:

容错一个Server崩溃不影响 Client
解耦了对特定通信协议和接口的依赖,统一走AMQP消息.
在多个RPC Server之间的负载均衡由RabbitMQ完成

生产者代码

public class RPCClient {
    
  private Connection connection;
  private Channel channel;
  private String requestQueueName = "rpc_queue";
  private String replyQueueName;
  private QueueingConsumer consumer;
    
  public RPCClient() throws Exception {
    ConnectionFactory factory = new ConnectionFactory();
    factory.setHost("localhost");
    connection = factory.newConnection();
    channel = connection.createChannel();

    replyQueueName = channel.queueDeclare().getQueue(); 
    consumer = new QueueingConsumer(channel);
    channel.basicConsume(replyQueueName, true, consumer);
  }
  
  public String call(String message) throws Exception {     
    String response = null;
    String corrId = UUID.randomUUID().toString();
    
    BasicProperties props = new BasicProperties
                                .Builder()
                                .correlationId(corrId)
                                .replyTo(replyQueueName)
                                .build();
    
    channel.basicPublish("", requestQueueName, props, message.getBytes());
    
    while (true) {
      QueueingConsumer.Delivery delivery = consumer.nextDelivery();
      if (delivery.getProperties().getCorrelationId().equals(corrId)) {
        response = new String(delivery.getBody(),"UTF-8");
        break;
      }
    }

    return response; 
  }
    
  public void close() throws Exception {
    connection.close();
  }
  
  public static void main(String[] argv) {
    RPCClient fibonacciRpc = null;
    String response = null;
    try {
      fibonacciRpc = new RPCClient();
  
      System.out.println(" [x] Requesting fib(30)");   
      response = fibonacciRpc.call("30");
      System.out.println(" [.] Got '" + response + "'");
    }
    catch  (Exception e) {
      e.printStackTrace();
    }
    finally {
      if (fibonacciRpc!= null) {
        try {
          fibonacciRpc.close();
        }
        catch (Exception ignore) {}
      }
    }
  }
}

消费者代码

public class RPCServer {
  
  private static final String RPC_QUEUE_NAME = "rpc_queue";
  
  private static int fib(int n) {
    if (n ==0) return 0;
    if (n == 1) return 1;
    return fib(n-1) + fib(n-2);
  }
    
  public static void main(String[] argv) {
    Connection connection = null;
    Channel channel = null;
    try {
      ConnectionFactory factory = new ConnectionFactory();
      factory.setHost("localhost");
  
      connection = factory.newConnection();
      channel = connection.createChannel();
      
      channel.queueDeclare(RPC_QUEUE_NAME, false, false, false, null);
  
      channel.basicQos(1);
  
      QueueingConsumer consumer = new QueueingConsumer(channel);
      channel.basicConsume(RPC_QUEUE_NAME, false, consumer);
  
      System.out.println(" [x] Awaiting RPC requests");
  
      while (true) {
        String response = null;
        
        QueueingConsumer.Delivery delivery = consumer.nextDelivery();
        
        BasicProperties props = delivery.getProperties();
        BasicProperties replyProps = new BasicProperties
                                         .Builder()
                                         .correlationId(props.getCorrelationId())
                                         .build();
        
        try {
          String message = new String(delivery.getBody(),"UTF-8");
          int n = Integer.parseInt(message);
  
          System.out.println(" [.] fib(" + message + ")");
          response = "" + fib(n);
        }
        catch (Exception e){
          System.out.println(" [.] " + e.toString());
          response = "";
        }
        finally {  
          channel.basicPublish( "", props.getReplyTo(), replyProps, response.getBytes("UTF-8"));
  
          channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
        }
      }
    }
    catch  (Exception e) {
      e.printStackTrace();
    }
    finally {
      if (connection != null) {
        try {
          connection.close();
        }
        catch (Exception ignore) {}
      }
    }                            
  }
}

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