RocketMQ原理学习--延时消息实现原理

阳光穿透心脏的1/2处 2022-04-05 17:26 293阅读 0赞

RocketMQ提供了延时消息类型，简单来说就是生产者在发送消息的时候指定一个延时时间，当到达延时时间之后消息才能够被投送到消费者。

首先我们可以考虑一下RocketMQ的延时消息是如何实现：

（1）生产者将延时消息发送到Broker，Broker是如何区分普通消息和延时消息（消息类型）

（2）消息如何保证不被消费者拉取到的（RocketMQ将消息以SCHEDULE\_TOPIC\_XXXX为topic将延时消息持久化，等到达延时消息之后再以原有的topic重新保存）。

### 1、简单示例 ###

（1）RocketMQ目前指定的延时时间间隔有1s，5s，10s，30s，1m，2m，3m，4m，5m，6m，7m，8m，9m，10m，20m，30m，1h，2h，用等级来表示时间间隔。

public class DelayProducer {
    
        public static void main(String[] args) throws MQClientException, InterruptedException {
            DefaultMQProducer producer = new DefaultMQProducer("rmq-group");
            producer.setNamesrvAddr("localhost:9876");
            producer.start();
            try {
                for (int i = 0; i < 3; i++) {
                    Message msg = new Message("TopicA-test",// topic
                            "TagA",// tag
                            (new Date() + "Hello RocketMQ ,QuickStart 11" + i)
                                    .getBytes()// body
                    );
                    //1s，5s，10s，30s，1m，2m，3m，4m，5m，6m，7m，8m，9m，10m，20m，30m，1h，2h。
                    // level=0，表示不延时。level=1，表示 1 级延时，对应延时 1s。level=2 表示 2 级延时，对应5s，以此类推
                    msg.setDelayTimeLevel(2);
    
                    SendResult sendResult = producer.send(msg);
                    System.out.println(sendResult);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
           producer.shutdown();
        }
    
    }

### 2、Broker持久化延时消息 ###

Broker对于接收到的消息首先会判断一下是不是延时消息，如果是延时消息会将消息以SCHEDULE\_TOPIC\_XXXX为topic替换原有的topic名称进行持久化，实现方法在CommitLog的putMessage中。

首先会判断msg的延时标准如果大于0，则重新设置消息的topic名称和queueId，之后将消息以SCHEDULE\_TOPIC\_XXXX为topic，以延时时间的等级为queueId持久化到commitlog文件中。

// Delay Delivery
                if (msg.getDelayTimeLevel() > 0) {
                    if (msg.getDelayTimeLevel() > this.defaultMessageStore.getScheduleMessageService().getMaxDelayLevel()) {
                        msg.setDelayTimeLevel(this.defaultMessageStore.getScheduleMessageService().getMaxDelayLevel());
                    }
    
                    topic = ScheduleMessageService.SCHEDULE_TOPIC;
                    queueId = ScheduleMessageService.delayLevel2QueueId(msg.getDelayTimeLevel());
    
                    // Backup real topic, queueId
                    MessageAccessor.putProperty(msg, MessageConst.PROPERTY_REAL_TOPIC, msg.getTopic());
                    MessageAccessor.putProperty(msg, MessageConst.PROPERTY_REAL_QUEUE_ID, String.valueOf(msg.getQueueId()));
                    msg.setPropertiesString(MessageDecoder.messageProperties2String(msg.getProperties()));
    
                    msg.setTopic(topic);
                    msg.setQueueId(queueId);
                }

### 3、延时消息处理 ###

Broker将延时消息以SCHEDULE\_TOPIC\_XXXX为topic名称将消息进行持久化，接下来我们看看Broker是如何将消息在延时消息到达之后进行消息还原的。

RocketMQ提供了定时任务服务ScheduleMessageService，通过定时任务的方式不断的读取topic为SCHEDULE\_TOPIC\_XXXX何queueId为延时等级的消息进行消息还原处理，这样消息被还原之后消费者就可以拉取消息了。

每个消费等级有个定时任务DeliverDelayedMessageTimerTask：

for (Map.Entry<Integer, Long> entry : this.delayLevelTable.entrySet()) {
                Integer level = entry.getKey();
                Long timeDelay = entry.getValue();
                Long offset = this.offsetTable.get(level);
                if (null == offset) {
                    offset = 0L;
                }
    
                if (timeDelay != null) {
                    this.timer.schedule(new DeliverDelayedMessageTimerTask(level, offset), FIRST_DELAY_TIME);
                }
            }

在DeliverDelayedMessageTimerTask中根据SCHEDULE\_TOPIC\_XXXX名称和延时等级对应的queueId获取消息队列，然后从commitlog中读取消息，还原消息的原有信息（消息的原topic信息）再将消息持久化到commitlog文件中，这样消费者就可以拉取消息了。

public void executeOnTimeup() {
    			//获取消费者消息
                ConsumeQueue cq =
                    ScheduleMessageService.this.defaultMessageStore.findConsumeQueue(SCHEDULE_TOPIC,
                        delayLevel2QueueId(delayLevel));
    
                long failScheduleOffset = offset;
    
                if (cq != null) {
    				//读取消息
                    SelectMappedBufferResult bufferCQ = cq.getIndexBuffer(this.offset);
                    if (bufferCQ != null) {
                        try {
                            long nextOffset = offset;
                            int i = 0;
                            ConsumeQueueExt.CqExtUnit cqExtUnit = new ConsumeQueueExt.CqExtUnit();
                            for (; i < bufferCQ.getSize(); i += ConsumeQueue.CQ_STORE_UNIT_SIZE) {
                                long offsetPy = bufferCQ.getByteBuffer().getLong();
                                int sizePy = bufferCQ.getByteBuffer().getInt();
                                long tagsCode = bufferCQ.getByteBuffer().getLong();
    
                                if (cq.isExtAddr(tagsCode)) {
                                    if (cq.getExt(tagsCode, cqExtUnit)) {
                                        tagsCode = cqExtUnit.getTagsCode();
                                    } else {
                                        //can't find ext content.So re compute tags code.
                                        log.error("[BUG] can't find consume queue extend file content!addr={}, offsetPy={}, sizePy={}",
                                            tagsCode, offsetPy, sizePy);
                                        long msgStoreTime = defaultMessageStore.getCommitLog().pickupStoreTimestamp(offsetPy, sizePy);
                                        tagsCode = computeDeliverTimestamp(delayLevel, msgStoreTime);
                                    }
                                }
    
                                long now = System.currentTimeMillis();
                                long deliverTimestamp = this.correctDeliverTimestamp(now, tagsCode);
    
                                nextOffset = offset + (i / ConsumeQueue.CQ_STORE_UNIT_SIZE);
    
                                long countdown = deliverTimestamp - now;
    
                                if (countdown <= 0) {
    								//获取消息
                                    MessageExt msgExt =
                                        ScheduleMessageService.this.defaultMessageStore.lookMessageByOffset(
                                            offsetPy, sizePy);
    
                                    if (msgExt != null) {
                                        try {
    										//还原消息信息topic名称等
                                            MessageExtBrokerInner msgInner = this.messageTimeup(msgExt);
    										//重新将消息持久化到commitlog中
                                            PutMessageResult putMessageResult =
                                                ScheduleMessageService.this.defaultMessageStore
                                                    .putMessage(msgInner);
    
                                            if (putMessageResult != null
                                                && putMessageResult.getPutMessageStatus() == PutMessageStatus.PUT_OK) {
                                                continue;
                                            } else {
                                                // XXX: warn and notify me
                                                log.error(
                                                    "ScheduleMessageService, a message time up, but reput it failed, topic: {} msgId {}",
                                                    msgExt.getTopic(), msgExt.getMsgId());
                                                ScheduleMessageService.this.timer.schedule(
                                                    new DeliverDelayedMessageTimerTask(this.delayLevel,
                                                        nextOffset), DELAY_FOR_A_PERIOD);
                                                ScheduleMessageService.this.updateOffset(this.delayLevel,
                                                    nextOffset);
                                                return;
                                            }
                                        } catch (Exception e) {
                                            /*
                                             * XXX: warn and notify me
    
    
    
                                             */
                                            log.error(
                                                "ScheduleMessageService, messageTimeup execute error, drop it. msgExt="
                                                    + msgExt + ", nextOffset=" + nextOffset + ",offsetPy="
                                                    + offsetPy + ",sizePy=" + sizePy, e);
                                        }
                                    }
                                } else {
                                    ScheduleMessageService.this.timer.schedule(
                                        new DeliverDelayedMessageTimerTask(this.delayLevel, nextOffset),
                                        countdown);
                                    ScheduleMessageService.this.updateOffset(this.delayLevel, nextOffset);
                                    return;
                                }
                            } // end of for
    
                            nextOffset = offset + (i / ConsumeQueue.CQ_STORE_UNIT_SIZE);
                            ScheduleMessageService.this.timer.schedule(new DeliverDelayedMessageTimerTask(
                                this.delayLevel, nextOffset), DELAY_FOR_A_WHILE);
                            ScheduleMessageService.this.updateOffset(this.delayLevel, nextOffset);
                            return;
                        } finally {
    
                            bufferCQ.release();
                        }
                    } // end of if (bufferCQ != null)
                    else {
    
                        long cqMinOffset = cq.getMinOffsetInQueue();
                        if (offset < cqMinOffset) {
                            failScheduleOffset = cqMinOffset;
                            log.error("schedule CQ offset invalid. offset=" + offset + ", cqMinOffset="
                                + cqMinOffset + ", queueId=" + cq.getQueueId());
                        }
                    }
                } // end of if (cq != null)
    
                ScheduleMessageService.this.timer.schedule(new DeliverDelayedMessageTimerTask(this.delayLevel,
                    failScheduleOffset), DELAY_FOR_A_WHILE);
            }

总结：

延时消息的实现还是挺精巧的，首先将延时消息换了一个topic名称进行持久化，这样消费者就无法获取消息，然后有定时任务，会将消息还原到原有的topic信息，这样消费者又可以重新拉取消息了。