A. 多线程消费同一队列
消费一条消息往往比产生一条消息慢很多，为了防止消息积压，一般需要开启多个工作线程同时消费消息。在 RabbitMQ 中，我们可以创建多个 Consumer 消费同一队列。示意图如下：
gordon.study.rabbitmq.workqueue.Sender.java
public class Sender {
private static final String QUEUE_NAME = "tasks";
private String name;
public Sender(String name) {
this.name = name;
}
public void work() throws Exception {
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost");
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME, false, false, false, null);
for (int i = 0; i < 10;) {
String message = "NO. " + ++i;
TimeUnit.MILLISECONDS.sleep(100);
channel.basicPublish("", QUEUE_NAME, null, message.getBytes("UTF-8"));
System.out.printf("(%1$s)[===>%2$s ] %3$s\n", name, ":" + QUEUE_NAME, message);
}
channel.close();
connection.close();
}
}
gordon.study.rabbitmq.workqueue.Receiver.java
public class Receiver {
private static final String QUEUE_NAME = "tasks";
private String name;
private int sleepTime;
public Receiver(String name, int sleepTime) {
this.name = name;
this.sleepTime = sleepTime;
}
public void work() throws Exception {
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost");
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME, false, false, false, null);
Consumer consumer = new DefaultConsumer(channel) {
@Override
public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body)
throws IOException {
String message = new String(body, "UTF-8");
System.out.printf(" [ %2$s<===](%1$s) %3$s\n", name, QUEUE_NAME, message);
try {
TimeUnit.MILLISECONDS.sleep(sleepTime);
} catch (InterruptedException e) {
}
}
};
channel.basicConsume(QUEUE_NAME, true, consumer);
}
}
gordon.study.rabbitmq.workqueue.Test01.java
public class Test01 {
public static void main(String[] args) throws Exception {
Receiver recv1 = new Receiver("A", 200);
recv1.work();
Receiver recv2 = new Receiver("B", 200);
recv2.work();
Sender sender = new Sender("S");
sender.work();
}
}
运行 Test01，发现 A、B 两个消费者轮流获取 S 发送的消息。
RabbitMQ 默认将消息顺序发送给下一个消费者，这样，每个消费者会得到相同数量的消息。即，轮询(round-robin)分发消息。
轮询很好，可是如果两个消费者消费能力不一样呢？
gordon.study.rabbitmq.workqueue.Test02SlowConsumer.java
public class Test02SlowConsumer {
public static void main(String[] args) throws Exception {
Receiver recv1 = new Receiver("A", 200);
recv1.work();
Receiver recv2 = new Receiver("B", 800);
recv2.work();
Sender sender = new Sender("S");
sender.work();
}
}
将消费者B 的消费时间提高到800毫秒，问题就出现了：B 依然分到了一半消息，需要运行很久才能处理完。
B. 公平分发(fair dispatch)
怎样才能做到按照每个消费者的能力分配消息呢？联合使用 Qos 和 Acknowledge 就可以做到。
gordon.study.rabbitmq.workqueue.QosAcknowledgeReceiver.java
public class QosAcknowledgeReceiver {
private static final String QUEUE_NAME = "tasks";
private String name;
private int sleepTime;
public QosAcknowledgeReceiver(String name, int sleepTime) {
this.name = name;
this.sleepTime = sleepTime;
}
public void work() throws Exception {
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost");
Connection connection = factory.newConnection();
final Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME, false, false, false, null);
channel.basicQos(1);
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.printf(" [ %2$s<===](%1$s) %3$s\n", name, QUEUE_NAME, message);
try {
TimeUnit.MILLISECONDS.sleep(sleepTime);
} catch (InterruptedException e) {
}
channel.basicAck(envelope.getDeliveryTag(), false);
}
};
channel.basicConsume(QUEUE_NAME, false, consumer);
}
}
代码第22行的 basicQos 方法设置了当前信道最大预获取(prefetch)消息数量为1。消息从队列异步推送给消费者，消费者的 ack 也是异步发送给队列，从队列的视角去看，总是会有一批消息已推送但尚未获得 ack 确认，Qos 的 prefetchCount 参数就是用来限制这批未确认消息数量的。设为1时，队列只有在收到消费者发回的上一条消息 ack 确认后，才会向该消费者发送下一条消息。prefetchCount 的默认值为0，即没有限制，队列会将所有消息尽快发给消费者。
basicQos 中 prefetchSize 参数通过消息的总字节数来限制队列推送消息的速度
prefetchSize 与 prefetchCount 可以同时设置，达到任何一个限制，则队列暂停推送消息
global 参数表示前两个参数的作用域，true 表示限制是针对信道的，false 表示限制是针对消费者的(我还没试过一个信道支持多个消费者的例子，样例代码见下方)
可以对同一个信道同时设置 global 为 true 和 false 的 Qos，表示队列要考虑每个消费者的限制，同时还要考虑整个信道的限制
看起来API注释是错的，因为 global 默认是 false，所以第22行代码应该是把当前信道上每个消费者(当然，上面的例子中只有一个)的 prefetchCount 设为 1
Channel channel = ...;
Consumer consumer1 = ...;
Consumer consumer2 = ...;
channel.basicQos(10, false); // Per consumer limit
channel.basicQos(15, true); // Per channel limit
channel.basicConsume("my-queue1", false, consumer1);
channel.basicConsume("my-queue2", false, consumer2);
第37行代码将 autoAck 设为 false，向 Broker 发送 ack 响应的任务就交给开发人员了。
第34行代码在任务真正完成后，调用 basicAck 方法主动通知队列消息已成功消费。当队列收到 ack 确认后，会把下一条消息推送过来，并将该消息从队列中删除。
Qos 方案示意图如下：
gordon.study.rabbitmq.workqueue.Test03FairDispatch.java
public class Test03FairDispatch {
public static void main(String[] args) throws Exception {
QosAcknowledgeReceiver recv1 = new QosAcknowledgeReceiver("A", 200);
recv1.work();
QosAcknowledgeReceiver recv2 = new QosAcknowledgeReceiver("B", 800);
recv2.work();
Sender sender = new Sender("S");
sender.work();
}
}
运行Test03，可以看到 RabbitMQ 按照消费者的实际能力分配消息。