一个RabbitMQ队列上的多个使用者

Question

我正在遵循来自RabbitMQ的指南：https://www.rabbitmq.com/tutorials/tutorial-two-java.html。我想用一个队列上的多个线程来模拟这个功能。

如果我在启动发送器之前启动接收器，它会按预期工作，如下所示：

[*] Rcvr1 Waiting for messages...
[*] Rcvr2 Waiting for messages...
[x] Rcvr1 Received 'Hello 0'
[x] Rcvr2 Received 'Hello 1'
[x] Rcvr1 Received 'Hello 2'
[x] Rcvr2 Received 'Hello 3'
[x] Rcvr1 Received 'Hello 4'
[x] Rcvr2 Received 'Hello 5'
[x] Rcvr1 Received 'Hello 6'
[x] Rcvr2 Received 'Hello 7'
[x] Rcvr1 Received 'Hello 8'
...

但是，首先启动我的接收器会导致只有一个线程接收消息(启动的最后一个线程)：

[*] Rcvr2 Waiting for messages...
[*] Rcvr1 Waiting for messages...
[x] Rcvr1 Received 'Hello 9'
[x] Rcvr1 Received 'Hello 10'
[x] Rcvr1 Received 'Hello 11'
[x] Rcvr1 Received 'Hello 12'
[x] Rcvr1 Received 'Hello 13'
[x] Rcvr1 Received 'Hello 14'
[x] Rcvr1 Received 'Hello 15'
...

有趣的是，如果我启动发送器，然后启动接收器，如上所述，然后再次启动发送器(而接收器正在处理第一批)。发送的第一个消息是串行处理的，而第二批消息是并行处理的，或者至少与其余线程一起处理。

 [*] Rcvr1 Waiting for messages...
 [*] Rcvr2 Waiting for messages...
 [x] Rcvr1 Received '[Batch 1] Hello 0'
 [x] Rcvr1 Received '[Batch 1] Hello 1'
 [x] Rcvr1 Received '[Batch 1] Hello 2'
 [x] Rcvr1 Received '[Batch 1] Hello 3'
 [x] Rcvr1 Received '[Batch 1] Hello 4'
 [x] Rcvr1 Received '[Batch 1] Hello 5'
 [x] Rcvr1 Received '[Batch 1] Hello 6'
 [x] Rcvr1 Received '[Batch 1] Hello 7'
 [x] Rcvr1 Received '[Batch 1] Hello 8'
 [x] Rcvr2 Received '[Batch 2] Hello 1'
 [x] Rcvr1 Received '[Batch 1] Hello 9'
 [x] Rcvr2 Received '[Batch 2] Hello 3'
 [x] Rcvr1 Received '[Batch 1] Hello 10'
 [x] Rcvr2 Received '[Batch 2] Hello 5'
 [x] Rcvr1 Received '[Batch 1] Hello 11'
 [x] Rcvr2 Received '[Batch 2] Hello 7'
 [x] Rcvr1 Received '[Batch 1] Hello 12'
 [x] Rcvr2 Received '[Batch 2] Hello 9'
 [x] Rcvr1 Received '[Batch 1] Hello 13'
 [x] Rcvr2 Received '[Batch 2] Hello 11'

对于RabbitMQ，这显然是可能的，我不确定我做错了什么。我的简单代码如下：

发件人

public class Send {

    private final static String QUEUE_NAME = "hello";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {
            channel.queueDeclare(QUEUE_NAME, true, false, false, null);
            for(int x=0; x<100; x++) {
                String message = "Hello "+x;
                channel.basicPublish("", QUEUE_NAME, null, message.getBytes("UTF-8"));
                System.out.println(" [x] Sent '" + message + "'");
            }
        }
    }
}

接收器

package com.mawv.ingest.rabbitmq;

import com.rabbitmq.client.*;

import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;

public class Recv {

    private final static String QUEUE_NAME = "hello";

    public static void main(String[] argv) throws Exception {
        ThreadPoolExecutor rcvrPool = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");

        Runnable rcvr1 = () -> {
            try {
                Connection connection = factory.newConnection();
                Channel channel = connection.createChannel();
                channel.queueDeclare(QUEUE_NAME, true, false, false, null);

                System.out.println(" [*] Rcvr1 Waiting for messages...");
                DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                    Envelope envelope = delivery.getEnvelope();
                    String message = new String(delivery.getBody(), "UTF-8");
                    System.out.println(" [x] Rcvr1 Received '" + message + "'");
                    long deliveryTag = envelope.getDeliveryTag();
                    channel.basicAck(deliveryTag, true);
                    try {
                        Thread.sleep(1000);
                    } catch (Exception ex) { }

                };
                channel.basicConsume(QUEUE_NAME, false, deliverCallback, consumerTag -> {  });

            } catch(Exception ex){
                ex.printStackTrace();
            }
        };
        Runnable rcvr2 = () -> {
            try {
                Connection connection = factory.newConnection();
                Channel channel = connection.createChannel();
                channel.queueDeclare(QUEUE_NAME, true, false, false, null);

                System.out.println(" [*] Rcvr2 Waiting for messages...");
                DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                    Envelope envelope = delivery.getEnvelope();
                    String message = new String(delivery.getBody(), "UTF-8");
                    System.out.println(" [x] Rcvr2 Received '" + message + "'");
                    long deliveryTag = envelope.getDeliveryTag();
                    channel.basicAck(deliveryTag, true);
                    try {
                        Thread.sleep(1000);
                    } catch (Exception ex) {
                    }
                };
                channel.basicConsume(QUEUE_NAME, false, deliverCallback, consumerTag -> {
                });
            } catch(Exception ex){
                ex.printStackTrace();
            }
        };
        rcvrPool.execute(rcvr1);
        rcvrPool.execute(rcvr2);

    }
}

我还将这个示例与他们描述的完全相同，并看到了相同的结果。https://self-learning-java-tutorial.blogspot.com/2015/09/rabbitmq-one-producer-and-multiple.html

我假设我的设置有些地方不对劲。

Answer 1

根据RabbitMQ接口：

“虽然通道可以由多个线程使用，但确保一次只有一个线程执行命令很重要。并发执行命令可能会导致抛出UnexpectedFrameError。”

首先，我认为你应该对不同的线程使用不同的通道。

最后，我认为第一个线程被终止了，因为它是空闲的，所以只有第二个线程是活动的，并且完成了所有的工作。在这种情况下，一个线程就足够了。

看看Java8的ThreadPoolExecutor应用编程接口：

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html

例如，您可以找到：

“默认情况下，即使是核心线程也只在新任务到达时初始创建和启动，但这可以使用方法prestartCoreThread()或prestartAllCoreThreads()动态覆盖。如果使用非空队列构造池，则可能需要预先启动线程。”

和

如果池当前具有超过corePoolSize的线程数，则超过的线程在空闲时间超过keepAliveTime时将被终止(请参见getKeepAliveTime(TimeUnit))。

即使在空闲时，也应该使用prestartAllCoreThreads()或prestartCoreThreads()来启动核心线程，或者使用getKeepAliveTime(TimeUnit)来使核心线程在空闲时保持活动状态。

Answer 2

看起来我错过了一个关键的通道配置。这解决了我的问题：

channel.basicQos(1)；

这就是RabbitMQ对此的看法。

公平调度

您可能已经注意到，分派仍然不能完全按照我们的要求工作。例如，在有两个工作者的情况下，当所有的奇数消息都很重，而偶数消息都很轻时，一个工作者将经常忙碌，而另一个工作者几乎不会做任何工作。嗯，RabbitMQ对此一无所知，仍然会均匀地分发消息。

这是因为当消息进入队列时，RabbitMQ只调度消息。它不会为消费者查看未确认消息的数量。它只是盲目地将每第n条消息分派到第n条消费者。

为了克服这一点，我们可以使用prefetchCount =1设置的basicQos方法。这告诉RabbitMQ不要一次向一个worker发送多条消息。或者，换句话说，在worker处理并确认前一条消息之前，不要向worker发送新消息。相反，它会将其分派给下一个不忙的worker。