什么是延迟队列
指消息发送到某个队列后，在指定多长时间之后才能被消费。
应用场景
RocketMQ 延迟队列
定时消息（延迟队列）是指消息发送到broker后，不会立即被消费，等待特定时间投递给真正的topic。
broker有配置项messageDelayLevel，默认值为“1s 5s 10s 30s 1m 2m 3m 4m 5m 6m 7m
8m 9m 10m 20m 30m 1h 2h”，18个level。
可以配置自定义messageDelayLevel。需要注意的是 messageDelayLevel是broker的属性，不属于某个topic。发消息时，设置delayLevel等级即可：
msg.setDelayLevel(level)level有以下三种情况：
level == 0，消息为非延迟消息1<=level<=maxLevel，消息延迟特定时间，例如level1，延迟1s
level > maxLevel，则level maxLevel，例如level==20，延迟2h
在 RocketMQ中定时消息会暂存在名为SCHEDULE_TOPIC_XXXX的topic中，并根据delayTimeLevel存入特定的queue，queueId = delayTimeLevel – 1，即一个queue只存相同延迟的消息，保证具有相同发送延迟的消息能够顺序消费。broker会调度地消费SCHEDULE_TOPIC_XXXX，将消息写入真实的topic。需要注意的是，定时消息会在第一次写入和调度写入真实topic时都会计数，因此发送数量、tps都会变高。
RocketMQ 延迟队列和RabbitMQ延迟队列相比
RocketMQ直接一步到位，功能类似于RabbitMQ的延迟交换器插件、而RabbitMQ提供了延迟队列，但如果是基于消息设置每个消息设置不同的延迟时间，会产生前面的消息早已过期，但后面的消息还存在消息队列中，故此RabbitMQ提供了延迟交换器插件，而RocketMQ 延迟队列设计就比较好了。
具体示例
生产者
// 实例化生产者，并指定生产组名称DefaultMQProducer producer =newDefaultMQProducer("myproducer_group_topic_name_delay_01");//设置实例名称，一个jvm中有多个生产者可以根据实例名区分//默认default
producer.setInstanceName("topic_delay");// 指定nameserver的地址
producer.setNamesrvAddr("localhost:9876");//设置同步重试次数
producer.setRetryTimesWhenSendFailed(2);//设置异步发送次数//producer.setRetryTimesWhenSendAsyncFailed(2);// 初始化生产者
producer.start();for(int i =0; i <20; i++){Message message =newMessage("topic_name_delay",("key="+ i).getBytes("utf-8"));//设置延迟消费时间 设置延迟时间级别0,18,0表示不延迟，18表示延迟2h，大于18的都是2h
message.setDelayTimeLevel(i);// 1 同步发送 如果发送失败会根据重试次数重试SendResult send = producer.send(message);SendStatus sendStatus = send.getSendStatus();System.out.println(sendStatus.toString());}消费者
/**
* 推消息消费
*/DefaultMQPushConsumer defaultMQPushConsumer =newDefaultMQPushConsumer("consumer_group_delay_01");// 指定nameserver的地址
defaultMQPushConsumer.setNamesrvAddr("localhost:9876");
defaultMQPushConsumer.subscribe("topic_name_delay","*");// 1 提高消费并行度
defaultMQPushConsumer.setConsumeThreadMax(10);
defaultMQPushConsumer.setConsumeThreadMin(1);// 2 以批量方式进行 消费// 设置消息批处理的一个批次中消息的最大个数
defaultMQPushConsumer.setConsumeMessageBatchMaxSize(10);//设置重试次数 默认16次
defaultMQPushConsumer.setMaxReconsumeTimes(1);// 添加消息监听器，一旦有消息推送过来，就进行消费
defaultMQPushConsumer.setMessageListener(newMessageListenerConcurrently(){@OverridepublicConsumeConcurrentlyStatusconsumeMessage(List<MessageExt> msgs,ConsumeConcurrentlyContext context){//final MessageQueue messageQueue = context.getMessageQueue();for(MessageExt msg : msgs){System.out.println(msg);try{System.out.println(newString(msg.getBody(),"utf-8"));}catch(UnsupportedEncodingException e){
e.printStackTrace();}}// 消息消费成功returnConsumeConcurrentlyStatus.CONSUME_SUCCESS;// 消息消费失败// return ConsumeConcurrentlyStatus.RECONSUME_LATER;}});消费消息，按照0到18级别来，0 表示不延迟，1表示延迟1s，大于等于18表示延迟2h
按照级别一次类推
默认值为“1s 5s 10s 30s 1m 2m 3m 4m 5m 6m 7m8m 9m 10m 20m 30m 1h 2h”，18个level。
这里只拷贝0-8 的打印日志，可以自己等待确认。
MessageExt [queueId=0, storeSize=195, queueOffset=19, sysFlag=0, bornTimestamp=1628949643548, bornHost=/192.168.0.103:55518, storeTimestamp=1628949643554, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F0000000000029A8A, commitLogOffset=170634, bodyCRC=858365373, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, MAX_OFFSET=20, CONSUME_START_TIME=1628949650433, UNIQ_KEY=C0A8006748D07C53A9EB47ABD51C0000, CLUSTER=DefaultCluster, WAIT=true, DELAY=0}, body=[107, 101, 121, 61, 48], transactionId=‘null’}]
key=0
MessageExt [queueId=1, storeSize=234, queueOffset=22, sysFlag=0, bornTimestamp=1628949643558, bornHost=/192.168.0.103:55518, storeTimestamp=1628949644566, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002ACF8, commitLogOffset=175352, bodyCRC=1143909675, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=23, CONSUME_START_TIME=1628949650435, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5260001, CLUSTER=DefaultCluster, WAIT=true, DELAY=1, REAL_QID=1}, body=[107, 101, 121, 61, 49], transactionId=‘null’}]
key=1
MessageExt [queueId=2, storeSize=234, queueOffset=18, sysFlag=0, bornTimestamp=1628949643561, bornHost=/192.168.0.103:55518, storeTimestamp=1628949648566, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002ADE2, commitLogOffset=175586, bodyCRC=1562901649, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=19, CONSUME_START_TIME=1628949650436, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5290002, CLUSTER=DefaultCluster, WAIT=true, DELAY=2, REAL_QID=2}, body=[107, 101, 121, 61, 50], transactionId=‘null’}]
key=2
MessageExt [queueId=3, storeSize=234, queueOffset=17, sysFlag=0, bornTimestamp=1628949643566, bornHost=/192.168.0.103:55518, storeTimestamp=1628949653569, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002AECC, commitLogOffset=175820, bodyCRC=706792455, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=18, CONSUME_START_TIME=1628949653572, UNIQ_KEY=C0A8006748D07C53A9EB47ABD52E0003, CLUSTER=DefaultCluster, WAIT=true, DELAY=3, REAL_QID=3}, body=[107, 101, 121, 61, 51], transactionId=‘null’}]
key=3
MessageExt [queueId=0, storeSize=234, queueOffset=20, sysFlag=0, bornTimestamp=1628949643568, bornHost=/192.168.0.103:55518, storeTimestamp=1628949673574, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002B0A0, commitLogOffset=176288, bodyCRC=876894628, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=21, CONSUME_START_TIME=1628949673577, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5300004, CLUSTER=DefaultCluster, WAIT=true, DELAY=4, REAL_QID=0}, body=[107, 101, 121, 61, 52], transactionId=‘null’}]
key=4
MessageExt [queueId=1, storeSize=234, queueOffset=23, sysFlag=0, bornTimestamp=1628949643570, bornHost=/192.168.0.103:55518, storeTimestamp=1628949703574, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002B18A, commitLogOffset=176522, bodyCRC=1128491314, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=24, CONSUME_START_TIME=1628949703577, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5320005, CLUSTER=DefaultCluster, WAIT=true, DELAY=5, REAL_QID=1}, body=[107, 101, 121, 61, 53], transactionId=‘null’}]
key=5
MessageExt [queueId=2, storeSize=234, queueOffset=20, sysFlag=0, bornTimestamp=1628949643572, bornHost=/192.168.0.103:55518, storeTimestamp=1628949763575, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002B35E, commitLogOffset=176990, bodyCRC=1514813576, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=21, CONSUME_START_TIME=1628949763580, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5340006, CLUSTER=DefaultCluster, WAIT=true, DELAY=6, REAL_QID=2}, body=[107, 101, 121, 61, 54], transactionId=‘null’}]
key=6
MessageExt [queueId=3, storeSize=234, queueOffset=19, sysFlag=0, bornTimestamp=1628949643574, bornHost=/192.168.0.103:55518, storeTimestamp=1628949823580, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002B532, commitLogOffset=177458, bodyCRC=760023070, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=20, CONSUME_START_TIME=1628949823583, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5360007, CLUSTER=DefaultCluster, WAIT=true, DELAY=7, REAL_QID=3}, body=[107, 101, 121, 61, 55], transactionId=‘null’}]
key=7
MessageExt [queueId=0, storeSize=234, queueOffset=22, sysFlag=0, bornTimestamp=1628949643576, bornHost=/192.168.0.103:55518, storeTimestamp=1628949883582, storeHost=/192.168.0.103:10911, msgId=C0A8006700002A9F000000000002B706, commitLogOffset=177926, bodyCRC=1039275407, reconsumeTimes=0, preparedTransactionOffset=0, toString()=Message{topic=‘topic_name_delay’, flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=topic_name_delay, MAX_OFFSET=23, CONSUME_START_TIME=1628949883586, UNIQ_KEY=C0A8006748D07C53A9EB47ABD5380008, CLUSTER=DefaultCluster, WAIT=true, DELAY=8, REAL_QID=0}, body=[107, 101, 121, 61, 56], transactionId=‘null’}]
key=8同样我们在控制台可以看到，存放的消息
本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
