第三章 （8-9）消费端限流

什么是消费端限流

• 假设一个场景，首先，我们Rabbitmq服务器有上万条未处理的消息，我们随便打开一个消费者客户端，会出现下面情况
• 巨量消息瞬间全部推送过来，但是我们单个客户端无法同时处理这么多数据

消费端限流方法

• RabbitMQ提供了一种qos（服务质量保证）功能，即在非自动确认消息的前提下，如果一定数目的消息（通过基于consume或者channel设置Qos的值）未被确认前，不进行消费新的消息
• void BasicQos(unit prefetchSize, ushort prefetchCount, bool global);
  • prefetchSize：0
  • prefetchCount：会告诉RabbitMQ不要同时给一个消费者推送多于N个消息，即一旦有N个消息还没有ack，则该consumer将block掉，直到有消息ack
  • global：true\false 是否将上面设置应用于channel，简单点说，就是上面限制时channel级别的还是consumer级别
  • prefetchSize和global这两项，rabbitmq没有实现，暂且不研究prefetch_count在no_ack=false的情况下生效，即在自动应答的情况下这两个值时不生效的

代码实现

生产端

package club.yunqiang.rabbitmqapi.limit;

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

import java.io.IOException;
import java.util.concurrent.TimeoutException;

/**
 * @author 张云强
 * @date 2019/12/7
 */
public class Producer {

    public static void main(String[] args) throws IOException, TimeoutException {
        ConnectionFactory connectionFactory = new ConnectionFactory();
        connectionFactory.setPort(5672);
        connectionFactory.setVirtualHost("/");
        connectionFactory.setHost("localhost");

        Connection connection = connectionFactory.newConnection();

        Channel channel = connection.createChannel();

        String exchange = "test_qos_exchange";
        String routingKey = "qos.save";

        String msg = "Hello Rabbit MQ Consumer Message";
        for (int i = 0; i < 5; i++) {
            channel.basicPublish(exchange, routingKey, true, null, msg.getBytes());
        }
    }

}

消费端

package club.yunqiang.rabbitmqapi.limit;

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

import java.io.IOException;
import java.util.concurrent.TimeoutException;

/**
 * @author 张云强
 * @date 2019/12/7
 */
public class Consumer {

    public static void main(String[] args) throws IOException, TimeoutException {
        ConnectionFactory connectionFactory = new ConnectionFactory();
        connectionFactory.setPort(5672);
        connectionFactory.setVirtualHost("/");
        connectionFactory.setHost("localhost");

        Connection connection = connectionFactory.newConnection();

        Channel channel = connection.createChannel();

        String exchangeName = "test_qos_exchange";
        String routingKey = "qos.#";
        String queueName = "test_qos_queue";

        channel.exchangeDeclare(exchangeName, "topic", true, false, null);
        channel.queueDeclare(queueName, true, false, false, null);
        channel.queueBind(queueName, exchangeName, routingKey);

        // 1、如果要限流，第一件事必须关掉 autoAck
        // 2、设置 qos
        channel.basicQos(0, 1, false);
        channel.basicConsume(queueName, false, new MyConsumer(channel));
    }
}

消费者

package club.yunqiang.rabbitmqapi.limit;

import com.rabbitmq.client.AMQP;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.DefaultConsumer;
import com.rabbitmq.client.Envelope;

import java.io.IOException;

/**
 * @author 张云强
 * @date 2019/12/7
 */
public class MyConsumer extends DefaultConsumer {

    private Channel channel;

    /**
     * Constructs a new instance and records its association to the passed-in channel.
     *
     * @param channel the channel to which this consumer is attached
     */
    public MyConsumer(Channel channel) {
        super(channel);
        this.channel = channel;
    }

    @Override
    public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body) throws IOException {
        System.out.println("---- consume message ----");
        System.err.println("consumerTag:" + consumerTag);
        System.err.println("envelope:" + envelope);
        System.err.println("properties:" + properties);
        System.err.println("body:" + new String(body));

        // 3、确认签收消息
        channel.basicAck(envelope.getDeliveryTag(), false);
    }
}

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