RabbitMQ消息模式之消费端限流

为什么要进行消费端限流

假设一个场景，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：巨量的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，所以我们需要进行消费端限流

针对消费端限流RabbitMQ提供的解决方案

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Consumer或者Channel设置Qos的值）未被确认前，不进行消费新的消息，即不管你来的数据有多少，但我只接收规定数目的消息

通过调用这个方法来实现：

Void BasicQos(uint prefetchSize, ushort prefetchCount, bool global);

参数解释：
#prefetchSize：为0则不限制消息大小

#prefetchSize：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（阻塞）掉，直到有消息ack

#Global：true\false是否将上面设置应用于Channel；简单来说，就是上面限制是Channel级别的还是Consumer级别

注意：

prefetchSize和global这两项，RabbitMQ没有实现，暂且不研究

prefetch_count在no_ask=false的情况下生效，即在自动应答的情况下，这两个值是不生效的

自定义消费端代码：

package com.ue.limit;

import com.rabbitmq.client.*;

import java.io.IOException;

public class MyConsumer extends DefaultConsumer {
    private Channel channel ;

    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.err.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));

        channel.basicAck(envelope.getDeliveryTag(), false);
    }
}

消费端代码：

package com.ue.limit;

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

public class Consumer {
    public static void main(String[] args) throws Exception {
        ConnectionFactory connectionFactory = new ConnectionFactory();
        connectionFactory.setHost("192.168.14.758");
        connectionFactory.setPort(5672);
        connectionFactory.setVirtualHost("/");

        Connection connection = connectionFactory.newConnection();
        Channel channel = connection.createChannel();

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

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

        //限流方式：第一件事就是将autoAck设置为false
        channel.basicQos(0, 1, false);

        channel.basicConsume(queueName, false, new MyConsumer(channel));
    }
}

生产端代码：

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package com.ue.limit;

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

public class Producer {
    public static void main(String[] args) throws Exception {
        ConnectionFactory connectionFactory = new ConnectionFactory();
        connectionFactory.setHost("192.168.14.758");
        connectionFactory.setPort(5672);
        connectionFactory.setVirtualHost("/");

        Connection connection = connectionFactory.newConnection();
        Channel channel = connection.createChannel();

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

        String msg = "Hello RabbitMQ QOS Message";

        for(int i = 0; i < 5; i ++){
            channel.basicPublish(exchange, routingKey, true, null, msg.getBytes());
        }
    }
}