一個 RabbitMQ 隊列上的多個消費者

Question

我正在遵循 RabbitMQ 的本指南： https : //www.rabbitmq.com/tutorials/tutorial-two-java.html 。 我想用一個隊列上的多個線程來模擬這個功能。

如果我在啟動發送器之前啟動接收器，它會按預期工作，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，首先啟動我的接收器會導致只有一個線程接收消息（要啟動的最后一個線程）：

[*] 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，如果我啟動發送器，然后啟動接收器，如上所述，然后再次啟動發送器（而接收器正在處理第一批）。 發送的第一條消息是串行處理的，而第二批消息是並行處理的，或者至少與剩余的線程一起處理。：

 [*] 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'

RabbitMQ 顯然可以做到這一點，我不確定我做錯了什么。 我的簡單代碼如下：

發件人

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 + "'");
            }
        }
    }
}

接收者

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);

    }
}

我也綁定了這個例子，正如他們描述的那樣，看到了相同的結果。 https://self-learning-java-tutorial.blogspot.com/2015/09/rabbitmq-one-producer-and-multiple.html

我假設我的設置有問題。

Answer 1

根據RabbitMQ api：

“雖然一個 Channel 可以被多個線程使用，但確保一次只有一個線程執行一個命令很重要。同時執行命令可能會導致拋出 UnexpectedFrameError”

首先，我認為您應該為不同的線程使用不同的通道。

最后，我認為第一個線程因為空閑而終止，所以只有第二個線程處於活動狀態並完成整個工作。 在這種情況下，一根線程就足夠了。

看看 java 8 的 ThreadPoolExecutor api：

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

例如，您可以找到：

“默認情況下，即使是核心線程也只有在新任務到達時才最初創建和啟動，但這可以使用方法 prestartCoreThread() 或 prestartAllCoreThreads() 動態覆蓋。如果您使用非空構造池，您可能想要預啟動線程隊列”

和

“如果池中當前有超過 corePoolSize 的線程，如果空閑時間超過 keepAliveTime（請參閱 getKeepAliveTime(TimeUnit)），多余的線程將被終止。”

您應該使用 prestartAllCoreThreads() 或 prestartCoreThreads() 使核心線程即使在空閑時也能啟動，或者使用 getKeepAliveTime(TimeUnit) 使它們即使空閑也能保持活動狀態。

Answer 2

看起來我缺少一個關鍵的通道配置。 這解決了我的問題：

channel.basicQos(1);

這就是 RabbitMQ 所說的。

公平調度

您可能已經注意到調度仍然不能完全按照我們想要的方式工作。 例如，在有兩個 worker 的情況下，當所有奇數消息都很重，偶數消息很輕時，一個 worker 會一直很忙，而另一個 worker 幾乎不做任何工作。 好吧，RabbitMQ 對此一無所知，仍然會均勻地發送消息。

發生這種情況是因為 RabbitMQ 只是在消息進入隊列時分派消息。 它不考慮消費者未確認消息的數量。 它只是盲目地將每條第 n 條消息分派給第 n 條消費者。

為了解決這個問題，我們可以使用帶有 prefetchCount = 1 設置的 basicQos 方法。 這告訴 RabbitMQ 一次不要給一個工人多個消息。 或者，換句話說，在處理並確認前一條消息之前，不要向工作人員發送新消息。 相反，它會將它分派給下一個不忙的工人。