RocketMQ源码：延迟消息实现原理

我会带着你远行 2024-03-23 17:00 83阅读 0赞

这一节我们来看下RocketMQ的延迟消息

## 1.发送延迟消息 ##

public class Producer {
    
       public static void main(String[] args) throws Exception {
          // 实例化一个生产者来产生延时消息
          DefaultMQProducer producer = new DefaultMQProducer("DELAY_P_G");
    
          producer.setNamesrvAddr("127.0.0.1:9876");
    
          // 启动生产者
          producer.start();
    
    
          for (int i = 0; i < 1; i++) {
              Message message = new Message(MQConstant.DELAY_TOPIC, ("Hello scheduled message " + i).getBytes());
    
              /**
               * MessageStoreConfig
               * messageDelayLevel = "1s 5s 10s 30s 1m 2m 3m 4m 5m 6m 7m 8m 9m 10m 20m 30m 1h 2h";
               *
               * 共18个等级，依次是从1-18
               * 比如，level=3, 表示延迟10s 消费
               */
              message.setDelayTimeLevel(4);
    
              // 发送消息
              SendResult send = producer.send(message);
              System.out.println("send = " + send);
          }
           // 关闭生产者
          producer.shutdown();
      }
    }
    复制代码

延迟消息的标志就是在发送时,通过消息对象`Message`的`setDelayTimeLevel(int level)`方法设置一个延迟等级,这样该条消息就是一个延迟消息了。那么延迟等级与延迟时间是如何对应的呢？

![format_png][]

## 2.存储延迟消息 ##

其实延迟消息和普通消息并没有多大的差异，只不过broker在存储消息时，会判断消息的延迟属性是否为空，如果不为空，则判断是延迟消息，进而会做一些额外的处理，那么我们就看下broker存储时判断是否为延迟消息的逻辑:

> **CommitLog\#asyncPutMessage(..)**

public CompletableFuture<PutMessageResult> asyncPutMessage(final MessageExtBrokerInner msg) {
        // Set the storage time
        msg.setStoreTimestamp(System.currentTimeMillis());
        // Set the message body BODY CRC (consider the most appropriate setting
        // on the client)
        msg.setBodyCRC(UtilAll.crc32(msg.getBody()));
        // Back to Results
        AppendMessageResult result = null;
    
        StoreStatsService storeStatsService = this.defaultMessageStore.getStoreStatsService();
    
        String topic = msg.getTopic();
        int queueId = msg.getQueueId();
    
        final int tranType = MessageSysFlag.getTransactionValue(msg.getSysFlag());
        if (tranType == MessageSysFlag.TRANSACTION_NOT_TYPE
                || tranType == MessageSysFlag.TRANSACTION_COMMIT_TYPE) {
            //TODO:延迟消息的判定
            if (msg.getDelayTimeLevel() > 0) {
                if (msg.getDelayTimeLevel() > this.defaultMessageStore.getScheduleMessageService().getMaxDelayLevel()) {
                    msg.setDelayTimeLevel(this.defaultMessageStore.getScheduleMessageService().getMaxDelayLevel());
                }
    
                //TODO:将延迟消息的topic替换为broker固定的topic: SCHEDULE_TOPIC_XXXX
                topic = TopicValidator.RMQ_SYS_SCHEDULE_TOPIC;
                //TODO: 将queueid替换为（延迟级别-1)
                queueId = ScheduleMessageService.delayLevel2QueueId(msg.getDelayTimeLevel());
    
                //TODO:备份原始的topic/queueid, 留着后面解析
                MessageAccessor.putProperty(msg, MessageConst.PROPERTY_REAL_TOPIC, msg.getTopic());
                MessageAccessor.putProperty(msg, MessageConst.PROPERTY_REAL_QUEUE_ID,

[format_png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/23/a4b55784140c4903879c4e3a79ac8a68.png