RocketMQ消费失败重试机制的示例分析，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。
现象：mq消费1次，重试3次，然后停止，如下实例展示
首次（reconsumeTimes=0）
MQ_CON_MSG gmcf-lsc-zhongbang-repu-calc-from-topic MSG MessageExt [queueId=1, storeSize=453, queueOffset=25, sysFlag=0, bornTimestamp=1566785215908, bornHost=/10.42.0.77:54608, storeTimestamp=1566785215908, storeHost=/10.42.0.244:10911, msgId=0A2A00F400002A9F000000000B77CE84, commitLogOffset=192401028, bodyCRC=53737244, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic='gmcf-lsc-zhongbang-repu-calc-from-topic', flag=0, properties={MIN_OFFSET=0, _catChildMessageId1=gmcf-lsc-job-0a2a004d-435218-15071, HASH_CODE=690132963, MAX_OFFSET=26, _catRootMessageId=lts-0a2a006b-434483-0, CONSUME_START_TIME=1566785215911, SERIALIZE_CLASS=java.lang.String, _catParentMessageId=lts-0a2a006b-435218-524, _catChildMessageId=gmcf-lsc-job-0a2a004d-435218-15072, UNIQ_KEY=0A2A004D000938AF386882EAA5A40112, WAIT=true}, body=[71, 77, 66, 69, 67, 49, 57, 48, 56, 50, 52, 49, 48, 52, 48, 52, 54, 56, 57, 52, 48, 52, 48, 56, 48], transactionId='null’}]第一次retry（reconsumeTimes=1,DELAY=3）
MQ_CON_MSG %RETRY%gmcf-lsc-consumerGroup MSG MessageExt [queueId=0, storeSize=608, queueOffset=1187, sysFlag=0, bornTimestamp=1566785215923, bornHost=/10.42.0.77:54608, storeTimestamp=1566785226241, storeHost=/10.42.0.244:10911, msgId=0A2A00F400002A9F000000000B785900, commitLogOffset=192436480, bodyCRC=893293938, reconsumeTimes=1, preparedTransactionOffset=0, toString()=Message{topic='gmcf-lsc-zhongbang-repu-calc-from-topic', flag=0, properties={_catChildMessageId1=gmcf-lsc-job-0a2a004d-435218-15075, _catRootMessageId=lts-0a2a006b-434483-0, CONSUME_START_TIME=1566785226242, SERIALIZE_CLASS=java.lang.String, _catParentMessageId=lts-0a2a006b-435218-524, _catChildMessageId=gmcf-lsc-job-0a2a004d-435218-15076, MIN_OFFSET=0, REAL_TOPIC=%RETRY%gmcf-lsc-consumerGroup, ORIGIN_MESSAGE_ID=0A2A00F400002A9F000000000B77D049, RETRY_TOPIC=gmcf-lsc-zhongbang-repu-calc-from-topic, HASH_CODE=1506102461, MAX_OFFSET=1188, UNIQ_KEY=0A2A004D000938AF386882EAA5B30113, WAIT=false, DELAY=3, REAL_QID=0}, body=[71, 77, 66, 69, 67, 49, 57, 48, 56, 50, 52, 49, 48, 52, 49, 49, 49, 50, 48, 55, 48, 52, 48, 56, 49], transactionId='null'}]第二次retry（reconsumeTimes=2, DELAY=4）
MQ_CON_MSG %RETRY%gmcf-lsc-consumerGroup MSG MessageExt [queueId=0, storeSize=608, queueOffset=1209, sysFlag=0, bornTimestamp=1566785215923, bornHost=/10.42.0.77:54608, storeTimestamp=1566785256680, storeHost=/10.42.0.244:10911, msgId=0A2A00F400002A9F000000000B791399, commitLogOffset=192484249, bodyCRC=893293938, reconsumeTimes=2, preparedTransactionOffset=0, toString()=Message{topic='gmcf-lsc-zhongbang-repu-calc-from-topic', flag=0, properties={_catChildMessageId1=gmcf-lsc-job-0a2a004d-435218-15075, _catRootMessageId=lts-0a2a006b-434483-0, CONSUME_START_TIME=1566785256728, SERIALIZE_CLASS=java.lang.String, _catParentMessageId=lts-0a2a006b-435218-524, _catChildMessageId=gmcf-lsc-job-0a2a004d-435218-15076, MIN_OFFSET=0, REAL_TOPIC=%RETRY%gmcf-lsc-consumerGroup, ORIGIN_MESSAGE_ID=0A2A00F400002A9F000000000B77D049, RETRY_TOPIC=gmcf-lsc-zhongbang-repu-calc-from-topic, HASH_CODE=1506102461, MAX_OFFSET=1210, UNIQ_KEY=0A2A004D000938AF386882EAA5B30113, WAIT=false, DELAY=4, REAL_QID=0}, body=[71, 77, 66, 69, 67, 49, 57, 48, 56, 50, 52, 49, 48, 52, 49, 49, 49, 50, 48, 55, 48, 52, 48, 56, 49], transactionId='null'}]第三次retry（reconsumeTimes=3, DELAY=5）
MQ_CON_MSG %RETRY%gmcf-lsc-consumerGroup MSG MessageExt [queueId=0, storeSize=608, queueOffset=1228, sysFlag=0, bornTimestamp=1566785215923, bornHost=/10.42.0.77:54608, storeTimestamp=1566785316978, storeHost=/10.42.0.244:10911, msgId=0A2A00F400002A9F000000000B79F598, commitLogOffset=192542104, bodyCRC=893293938, reconsumeTimes=3, preparedTransactionOffset=0, toString()=Message{topic='gmcf-lsc-zhongbang-repu-calc-from-topic', flag=0, properties={_catChildMessageId1=gmcf-lsc-job-0a2a004d-435218-15075, _catRootMessageId=lts-0a2a006b-434483-0, CONSUME_START_TIME=1566785316980, SERIALIZE_CLASS=java.lang.String, _catParentMessageId=lts-0a2a006b-435218-524, _catChildMessageId=gmcf-lsc-job-0a2a004d-435218-15076, MIN_OFFSET=0, REAL_TOPIC=%RETRY%gmcf-lsc-consumerGroup, ORIGIN_MESSAGE_ID=0A2A00F400002A9F000000000B77D049, RETRY_TOPIC=gmcf-lsc-zhongbang-repu-calc-from-topic, HASH_CODE=1506102461, MAX_OFFSET=1231, UNIQ_KEY=0A2A004D000938AF386882EAA5B30113, WAIT=false, DELAY=5, REAL_QID=0}, body=[71, 77, 66, 69, 67, 49, 57, 48, 56, 50, 52, 49, 48, 52, 49, 49, 49, 50, 48, 55, 48, 52, 48, 56, 49], transactionId='null'}]根据现象我们提出2个疑问？
1.为什么只会重试4次？而不是一直重试？
try { try { if (messageExtWrappers.size() > 0) { try { var22 = messageExtWrappers.iterator(); while(var22.hasNext()) { messageExt = (MessageExt)var22.next(); span.addEvent("MQConsumer.from", messageExt.getBornHostString()); } } catch (Throwable var16) { ; } this.consume(messageExtWrappers, context); } LOGGER.info("MQ_CON_SUCCESS {} BROKER {} QUEUE {}", new Object[]{topic, broker, queueId}); span.addEvent("MQConsumer", ConsumeConcurrentlyStatus.CONSUME_SUCCESS.name()); span.success(); ConsumeConcurrentlyStatus var23 = ConsumeConcurrentlyStatus.CONSUME_SUCCESS; return var23; } catch (MessageListenerConcurrentlyException var17) { LOGGER.error("MQ_CON_EX {} BROKER {} QUEUE {}", new Object[]{topic, broker, queueId, var17}); throw var17; } catch (Throwable var18) { LOGGER.error("MQ_CON_EX {} BROKER {} QUEUE {}", new Object[]{topic, broker, queueId, var18}); LOGGER.info("MQ_CON_RECONSUME {} BROKER {} QUEUE {}", new Object[]{topic, broker, queueId}); span.failed(var18); span.addEvent("MQConsumer", ConsumeConcurrentlyStatus.RECONSUME_LATER.name()); if (CollectionUtils.isNotEmpty(msgs) && ((MessageExt)msgs.get(0)).getDelayTimeLevel() >= 2 + this.retryTimes) { context.setDelayLevelWhenNextConsume(-1); } }从代码可以看出，如果消费失败了，我们自己的控制了重发次数，代码如下：
if (CollectionUtils.isNotEmpty(msgs) && ((MessageExt)msgs.get(0)).getDelayTimeLevel() >= 2 + this.retryTimes) { context.setDelayLevelWhenNextConsume(-1); }当重试达到满足条件的时候，不再重试，直接放到dlq队列里面。如果不控制的，会一直重试到最高DelayLevel 18
2.DelayTimeLeve默认的值为什么不是从0开始，而是从3开始？
我们知道RocketMQ的默认的配置是messageDelayLevel=1s 5s 10s 30s 1m 2m 3m 4m 5m 6m 7m 8m 9m 10m 20m 30m 1h 2h，分别代表延迟level1-level18，为什么不是从1开始呢？
带着疑问我们继续深挖源码，我们从DefaultMQPullConsumerImpl类里面找到一段代码
public void sendMessageBack(MessageExt msg, int delayLevel, String brokerName, String consumerGroup) throws RemotingException, MQBrokerException, InterruptedException, MQClientException { try { String brokerAddr = null != brokerName ? this.mQClientFactory.findBrokerAddressInPublish(brokerName) : RemotingHelper.parseSocketAddressAddr(msg.getStoreHost()); if (UtilAll.isBlank(consumerGroup)) { consumerGroup = this.defaultMQPullConsumer.getConsumerGroup(); } this.mQClientFactory.getMQClientAPIImpl().consumerSendMessageBack(brokerAddr, msg, consumerGroup, delayLevel, 3000L, this.defaultMQPullConsumer.getMaxReconsumeTimes()); } catch (Exception var8) { this.log.error("sendMessageBack Exception, " + this.defaultMQPullConsumer.getConsumerGroup(), var8); Message newMsg = new Message(MixAll.getRetryTopic(this.defaultMQPullConsumer.getConsumerGroup()), msg.getBody()); String originMsgId = MessageAccessor.getOriginMessageId(msg); MessageAccessor.setOriginMessageId(newMsg, UtilAll.isBlank(originMsgId) ? msg.getMsgId() : originMsgId); newMsg.setFlag(msg.getFlag()); MessageAccessor.setProperties(newMsg, msg.getProperties()); MessageAccessor.putProperty(newMsg, "RETRY_TOPIC", msg.getTopic()); MessageAccessor.setReconsumeTime(newMsg, String.valueOf(msg.getReconsumeTimes() + 1)); MessageAccessor.setMaxReconsumeTimes(newMsg, String.valueOf(this.defaultMQPullConsumer.getMaxReconsumeTimes())); newMsg.setDelayTimeLevel(3 + msg.getReconsumeTimes()); this.mQClientFactory.getDefaultMQProducer().send(newMsg); } }从代码中看到DelayTimeLevel =3+reconsumeTime
newMsg.setDelayTimeLevel(3 + msg.getReconsumeTimes());
所以默认重试时，实际是从3开始的，从时间的角度，也验证为什么会重试4次，而且每次间隔的时间是10s/30s/1m .
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