RocketMQ重试机制和消息幂

一.重试机制

1.由于MQ经常处于复杂的分布式系统中,考虑网络波动,服务宕机,程序异常因素,很有可能出现消息发送或者消费失败的问题。因此,消息的重试就是所有MQ中间件必须考虑到的一个关键点。如果没有消息重试,就可能产生消息丢失的问题,可能对系统产生很大的影响。所以,秉承宁可多发消息,也不可丢失消息的原则,大部分MQ都对消息重试提供了很好的支持。

2.RocketMQ为了使用者封装了消息重试的处理流程,无需开发人员手动处理。RocketMQ支持了生产端和消费端两类重试机制。

模拟异常

  Consumer端消息消费两种状态:

 
package com.alibaba.rocketmq.client.consumer.listener;

public enum ConsumeConcurrentlyStatus {
    CONSUME_SUCCESS,
    RECONSUME_LATER;

    private ConsumeConcurrentlyStatus() {
    }
}
 

一个是成功(CONSUME_SUCCESS),一个是失败&重试(RECONSUME_LATER);

Consumer为了保证消息消费成功,只有使用方明确表示消费成功,返回CONSUME_SUCCESS,RocketMQ才会认为消息消费成功。

如果消息消费失败,只要返回ConsumeConcurrentlyStatus.RECONSUME_LATER,RocketMQ就会认为消息消费失败了,需要重新投递。

  1.出现异常

 
package com.wn.consumer;

import com.alibaba.rocketmq.client.consumer.DefaultMQPushConsumer;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyContext;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyStatus;
import com.alibaba.rocketmq.client.consumer.listener.MessageListenerConcurrently;
import com.alibaba.rocketmq.client.exception.MQClientException;
import com.alibaba.rocketmq.common.message.MessageExt;

import java.util.List;

public class MQConsumer {
    public static void main(String[] args) throws MQClientException {
        //创建消费者
        DefaultMQPushConsumer consumer=new DefaultMQPushConsumer("rmq-group");
        //设置NameServer地址
        consumer.setNamesrvAddr("192.168.138.187:9876;192.168.138.188:9876");
        //设置消费者实例名称
        consumer.setInstanceName("consumer");
        //订阅topic
        consumer.subscribe("wn02","TagA");
        //监听消息
        consumer.registerMessageListener(new MessageListenerConcurrently() {
            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> list, ConsumeConcurrentlyContext consumeConcurrentlyContext) {
                //获取消息
                for (MessageExt msg:list){
                    System.out.println(msg.getMsgId()+"---"+new String(msg.getBody()));
                }

                try {
                    int i=1/0;
                }catch (Exception e){
                    e.printStackTrace();
                    //需要重试
                    return ConsumeConcurrentlyStatus.RECONSUME_LATER;
                }//消息成功
                return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
            }
        });
        consumer.start();
        System.out.println("Consumer Started...");

    }
}
 

  2.网络延迟

 
package com.wn.consumer;

import com.alibaba.rocketmq.client.consumer.DefaultMQPushConsumer;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyContext;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyStatus;
import com.alibaba.rocketmq.client.consumer.listener.MessageListenerConcurrently;
import com.alibaba.rocketmq.client.exception.MQClientException;
import com.alibaba.rocketmq.common.message.MessageExt;

import java.util.List;

public class MQConsumer {
    public static void main(String[] args) throws MQClientException {
        //创建消费者
        DefaultMQPushConsumer consumer=new DefaultMQPushConsumer("rmq-group");
        //设置NameServer地址
        consumer.setNamesrvAddr("192.168.138.187:9876;192.168.138.188:9876");
        //设置消费者实例名称
        consumer.setInstanceName("consumer");
        //订阅topic
        consumer.subscribe("wn03","TagA");
        //监听消息
        consumer.registerMessageListener(new MessageListenerConcurrently() {
            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> list, ConsumeConcurrentlyContext consumeConcurrentlyContext) {
                //获取消息
                for (MessageExt msg:list){
                    System.out.println(msg.getMsgId()+"---"+new String(msg.getBody()));
                }
                //网络延迟
                try {
                    Thread.sleep(600000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
//消息成功
                return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
            }
        });
        consumer.start();
        System.out.println("Consumer Started...");

    }
}

 

二、消息幂等

1、在什么情况下会发生RocketMQ的消息重复消费

 ①、当系统的调用链路比较长的时候,比如系统A调用系统B,系统B再把消息发送到RocketMQ中,在系统A调用系统B的时候,如果系统B处理成功,但是迟迟没有将调用成功的结果返回给系统A的时候,系统A就会尝试重新发起请求给系统B,造成系统B重复处理,发起多条消息给RocketMQ造成重复消费;

②、在系统B发送给RocketMQ的时候,也有可能会发生和上面一样的问题,消息发送超时,节骨系统B重试,导致RocketMQ接收到了重读消息;

③、当RocketMQ成功接收到消息,并将消息交给消费者处理,如果消费者消费完成后还没来得及提交offset给RocketMQ,自己宕机或者重启了,那么RocketMQ没有接收到offset,就会认为消费失败了,会重发消息给消费者再次消费;

2、如何解决消息的重复消费

通过幂等性来保证,只要保证重复消息不对结果产生影响,就完美地解决这个问题。

在生产者端保证幂等性,一下两种方式:

①、RocketMQ支持消息查询的功能,只要去RocketMQ查询一下是否已经发送过该条消息就可以了,不存在则发送,存在则不发送;

②、引入Redis,在发送消息到RocketMQ成功之后,向Redis中插入一条数据,如果发送重试,则先去Redis查询一个该条消息是否已经发送过了,存在的话就不重复发送消息了;

方法一:RocketMQ消息查询的性能不是特别好,如果在高并发的场景下,每条消息在发送到RocketMQ时都去查询一下,可能会影响接口的性能;

方法二:在一些极端的场景下,Redis也无法保证消息发送成功之后,就一定能写入Redis成功,比如写入消息成功而Redis此时宕机,那么再次查询Redis判断消息是否已经发送过,是无法得到正确结果的;

3、生产者

 
package com.zn.idempotent;

import com.alibaba.rocketmq.client.exception.MQBrokerException;
import com.alibaba.rocketmq.client.exception.MQClientException;
import com.alibaba.rocketmq.client.producer.DefaultMQProducer;
import com.alibaba.rocketmq.client.producer.SendResult;
import com.alibaba.rocketmq.common.message.Message;
import com.alibaba.rocketmq.remoting.exception.RemotingException;

/**
 * 消息幂等生产者
 */
public class IdempotentProvider {
    public static void main(String[] args) throws MQClientException, InterruptedException, RemotingException, MQBrokerException {
        //创建一个生产者
        DefaultMQProducer producer=new DefaultMQProducer("rmq-group");
        //设置NameServer地址
        producer.setNamesrvAddr("192.168.33.135:9876;192.168.33.136:9876");
        //设置生产者实例名称
        producer.setInstanceName("producer");
        //启动生产者
        producer.start();

            //发送消息
            for (int i=1;i<=1;i++){
                //模拟网络延迟,每秒发送一次MQ
                Thread.sleep(1000);
                //创建消息,topic主题名称  tags临时值代表小分类, body代表消息体
                Message message=new Message("itmayiedu-topic03","TagA",("itmayiedu-"+i).getBytes());
                //消息的唯一标识
                message.setKeys("订单消息:"+i);
                //发送消息
                SendResult sendResult=producer.send(message);
                System.out.println("信息幂等问题来了:"+sendResult.toString());
            }
        producer.shutdown();
    }
}
 

4、消费者

 
package com.zn.idempotent;

import com.alibaba.rocketmq.client.consumer.DefaultMQPushConsumer;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyContext;
import com.alibaba.rocketmq.client.consumer.listener.ConsumeConcurrentlyStatus;
import com.alibaba.rocketmq.client.consumer.listener.MessageListenerConcurrently;
import com.alibaba.rocketmq.client.exception.MQClientException;
import com.alibaba.rocketmq.common.message.MessageExt;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.logging.LogManager;
import java.util.logging.Logger;

/**
 * 消息幂等消费者
 */
public class IdempotentConsumer {

    static private Map<String, Object> logMap = new HashMap<>();

    public static void main(String[] args) throws MQClientException {
        //创建消费者
        DefaultMQPushConsumer consumer=new DefaultMQPushConsumer("rmq-group");
        //设置NameServer地址
        consumer.setNamesrvAddr("192.168.33.135:9876;192.168.33.136:9876");
        //设置实例名称
        consumer.setInstanceName("consumer");
        //订阅topic
        consumer.subscribe("itmayiedu-topic03","TagA");

        //监听消息
        consumer.registerMessageListener(new MessageListenerConcurrently() {
            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> list, ConsumeConcurrentlyContext consumeConcurrentlyContext) {
                String key=null;
                String msgId=null;

                    for (MessageExt messageExt:list){
                        key=messageExt.getKeys();
                        //判读redis中有没有当前消息key
                        if (logMap.containsKey(key)) {
                            // 无需继续重试。
                            System.out.println("key:"+key+",已经消费,无需重试...");
                            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
                        }
                        //RocketMQ由于是集群环境,所以产生的消息ID可能会重复
                        msgId = messageExt.getMsgId();
                        System.out.println("key:" + key + ",msgid:" + msgId + "---" + new String(messageExt.getBody()));
                        //将当前key保存在redis中
                        logMap.put(messageExt.getKeys(),messageExt);
                    }
                try {
                    int i=5/0;
                }catch (Exception e){
                    e.printStackTrace();
                    //人工补偿
                    return ConsumeConcurrentlyStatus.RECONSUME_LATER;
                }
                return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
        }
    });
        //启动消费者
        consumer.start();
        System.out.println("Consumer Started!");
    }
}
 

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