rocketmq：当同一消费组的消费方式被修改后，消费组将按什么方式消费。

一、Consumer 批量消费

可以通过

[java] view plain copy
consumer.setConsumeMessageBatchMaxSize(10);//每次拉取10条
这里需要分为2种情况1、Consumer端先启动 2、Consumer端后启动. 正常情况下：应该是Consumer需要先启动
1、Consumer端先启动

Consumer代码如下

[java] view plain copy
package quickstart;

import java.util.List;

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.consumer.ConsumeFromWhere;
import com.alibaba.rocketmq.common.message.MessageExt;

/**

• Consumer，订阅消息
  */

public class Consumer {

public static void main(String[] args) throws InterruptedException, MQClientException {  
    DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("please_rename_unique_group_name_4");  
    consumer.setNamesrvAddr("192.168.100.145:9876;192.168.100.146:9876");  
    consumer.setConsumeMessageBatchMaxSize(10);  
    /** 
     * 设置Consumer第一次启动是从队列头部开始消费还是队列尾部开始消费<br> 
     * 如果非第一次启动，那么按照上次消费的位置继续消费 
     */  
    consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET);  

    consumer.subscribe("TopicTest", "*");  

    consumer.registerMessageListener(new MessageListenerConcurrently() {  

        public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> msgs, ConsumeConcurrentlyContext context) {  
              
            try {  
                System.out.println("msgs的长度" + msgs.size());  
                System.out.println(Thread.currentThread().getName() + " Receive New Messages: " + msgs);  
            } catch (Exception e) {  
                e.printStackTrace();  
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;  
            }  
              
              
              
            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;  
        }  
    });  

    consumer.start();  

    System.out.println("Consumer Started.");  
}  

}

由于这里是Consumer先启动，所以他会去轮询MQ上是否有订阅队列的消息，由于每次producer插入一条，Consumer就拿一条所以测试结果如下（每次size都是1）：

2、Consumer端后启动，也就是Producer先启动

由于这里是Consumer后启动，所以MQ上也就堆积了一堆数据，Consumer的

consumer.setConsumeMessageBatchMaxSize(10);//每次拉取10条

所以这段代码就生效了测试结果如下（每次size最多是10）：

二、消息重试机制：消息重试分为2种1、Producer端重试 2、Consumer端重试

1、Producer端重试

也就是Producer往MQ上发消息没有发送成功，我们可以设置发送失败重试的次数

[java] view plain copy
package quickstart;

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;

/**

• Producer，发送消息
• */

public class Producer {

public static void main(String[] args) throws MQClientException, InterruptedException {  
    DefaultMQProducer producer = new DefaultMQProducer("please_rename_unique_group_name");  
    producer.setNamesrvAddr("192.168.100.145:9876;192.168.100.146:9876");  
    producer.setRetryTimesWhenSendFailed(10);//失败的 情况发送10次  
    producer.start();  

    for (int i = 0; i < 1000; i++) {  
        try {  
            Message msg = new Message("TopicTest",// topic  
                    "TagA",// tag  
                    ("Hello RocketMQ " + i).getBytes()// body  
            );  
            SendResult sendResult = producer.send(msg);  
            System.out.println(sendResult);  
        } catch (Exception e) {  
            e.printStackTrace();  
            Thread.sleep(1000);  
        }  
    }  

    producer.shutdown();  
}  

}

2、Consumer端重试

2.1、exception的情况，一般重复16次 10s、30s、1分钟、2分钟、3分钟等等

上面的代码中消费异常的情况返回

return ConsumeConcurrentlyStatus.RECONSUME_LATER;//重试

正常则返回：

return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;//成功

[java] view plain copy
package quickstart;

import java.util.List;

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.consumer.ConsumeFromWhere;
import com.alibaba.rocketmq.common.message.MessageExt;

/**

• Consumer，订阅消息
  */

public class Consumer {

public static void main(String[] args) throws InterruptedException, MQClientException {  
    DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("please_rename_unique_group_name_4");  
    consumer.setNamesrvAddr("192.168.100.145:9876;192.168.100.146:9876");  
    consumer.setConsumeMessageBatchMaxSize(10);  
    /** 
     * 设置Consumer第一次启动是从队列头部开始消费还是队列尾部开始消费<br> 
     * 如果非第一次启动，那么按照上次消费的位置继续消费 
     */  
    consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET);  

    consumer.subscribe("TopicTest", "*");  

    consumer.registerMessageListener(new MessageListenerConcurrently() {  

        public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> msgs, ConsumeConcurrentlyContext context) {  

            try {  
                // System.out.println("msgs的长度" + msgs.size());  
                System.out.println(Thread.currentThread().getName() + " Receive New Messages: " + msgs);  
                for (MessageExt msg : msgs) {  
                    String msgbody = new String(msg.getBody(), "utf-8");  
                    if (msgbody.equals("Hello RocketMQ 4")) {  
                        System.out.println("======错误=======");  
                        int a = 1 / 0;  
                    }  
                }  

            } catch (Exception e) {  
                e.printStackTrace();  
                if(msgs.get(0).getReconsumeTimes()==3){  
                    //记录日志  
                      
                    return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 成功  
                }else{  
                      
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;// 重试  
                }  
            }  

            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 成功  
        }  
    });  

    consumer.start();  

    System.out.println("Consumer Started.");  
}  

}
打印结果：

假如超过了多少次之后我们可以让他不再重试记录 日志。

if(msgs.get(0).getReconsumeTimes()==3){
//记录日志
return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 成功
}

2.2超时的情况，这种情况MQ会无限制的发送给消费端。

就是由于网络的情况，MQ发送数据之后，Consumer端并没有收到导致超时。也就是消费端没有给我返回return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;这样的就认为没有到达Consumer端。

这里模拟Producer只发送一条数据。consumer端暂停1分钟并且不发送接收状态给MQ

[java] view plain copy
package model;

import java.util.List;

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.consumer.ConsumeFromWhere;
import com.alibaba.rocketmq.common.message.MessageExt;

/**

• Consumer，订阅消息
  */

public class Consumer {

public static void main(String[] args) throws InterruptedException, MQClientException {  
    DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("message_consumer");  
    consumer.setNamesrvAddr("192.168.100.145:9876;192.168.100.146:9876");  
    consumer.setConsumeMessageBatchMaxSize(10);  
    /** 
     * 设置Consumer第一次启动是从队列头部开始消费还是队列尾部开始消费<br> 
     * 如果非第一次启动，那么按照上次消费的位置继续消费 
     */  
    consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET);  

    consumer.subscribe("TopicTest", "*");  

    consumer.registerMessageListener(new MessageListenerConcurrently() {  

        public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> msgs, ConsumeConcurrentlyContext context) {  

            try {  

                // 表示业务处理时间  
                System.out.println("=========开始暂停===============");  
                Thread.sleep(60000);  

                for (MessageExt msg : msgs) {  
                    System.out.println(" Receive New Messages: " + msg);  
                }  

            } catch (Exception e) {  
                e.printStackTrace();  
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;// 重试  
            }  

            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 成功  
        }  
    });  

    consumer.start();  

    System.out.println("Consumer Started.");  
}  

}

三、消费模式

广播消费：rocketMQ默认是集群消费,我们可以通过在Consumer来支持广播消费

consumer.setMessageModel(MessageModel.BROADCASTING);// 广播消费

[java] view plain copy
package model;

import java.util.List;

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.consumer.ConsumeFromWhere;
import com.alibaba.rocketmq.common.message.MessageExt;
import com.alibaba.rocketmq.common.protocol.heartbeat.MessageModel;

/**

• Consumer，订阅消息
  */

public class Consumer2 {

public static void main(String[] args) throws InterruptedException, MQClientException {  
    DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("message_consumer");  
    consumer.setNamesrvAddr("192.168.100.145:9876;192.168.100.146:9876");  
    consumer.setConsumeMessageBatchMaxSize(10);  
    consumer.setMessageModel(MessageModel.BROADCASTING);// 广播消费  
  
    consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET);  

    consumer.subscribe("TopicTest", "*");  

    consumer.registerMessageListener(new MessageListenerConcurrently() {  

        public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> msgs, ConsumeConcurrentlyContext context) {  

            try {  

                for (MessageExt msg : msgs) {  
                    System.out.println(" Receive New Messages: " + msg);  
                }  

            } catch (Exception e) {  
                e.printStackTrace();  
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;// 重试  
            }  

            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 成功  
        }  
    });  

    consumer.start();  

    System.out.println("Consumer Started.");  
}  

}
如果我们有2台节点(非主关系),2个节点物理上是分开的，Producer往MQ上写入20条数据 其中broker1中拉取了12条 。broker2中拉取了8 条，这种情况下，假如broker1宕机，那么我们消费数据的时候，只能消费到broker2中的8条，broker1中的12条已经持久化到中。需要broker1回复之后这12条数据才能继续被消费。

异步复制和同步双写主要是主和从的关系。消息需要实时消费的，就需要采用主从模式部署

异步复制:比如这里有一主一从，我们发送一条消息到主节点之后，这样消息就算从producer端发送成功了，然后通过异步复制的方法将数据复制到从节点

同步双写:比如这里有一主一从，我们发送一条消息到主节点之后，这样消息就并不算从producer端发送成功了，需要通过同步双写的方法将数据同步到从节点后， 才算数据发送成功。

四、刷盘方式

同步刷盘：在消息到达MQ后，RocketMQ需要将数据持久化，同步刷盘是指数据到达内存之后，必须刷到commitlog日志之后才算成功，然后返回producer数据已经发送成功。

异步刷盘：，同步刷盘是指数据到达内存之后,返回producer说数据已经发送成功。，然后再写入commitlog日志。

commitlog：

commitlog就是来存储所有的元信息，包含消息体，类似于Mysql、Oracle的redolog,所以主要有CommitLog在，Consume Queue即使数据丢失，仍然可以恢复出来。

consumequeue：记录数据的位置,以便Consume快速通过consumequeue找到commitlog中的数据

不能，这样会错乱的。出现订阅关系不一致的情况