RocketMQ原理学习--死信消息实现原理

        上一篇博客​​《RocketMQ原理学习--失败消息实现原理》​​中我们了解到RocketMQ对于失败消息的处理原理，当消息一直失败的情况下RocketMQ是如何处理的，这篇博客我们通过分析源码简单了解一下。

        RocketMQ对于失败次数超过16次的消息设置为死信消息，消息最终被放到DLQ死信队列中，需要人工进行干预处理。处理代码还是在SendMessageProcessor的consumerSendMsgBack方法中，简单来说就是判断重试次数超过16或者延时级别小于0，则将消息设置为新的死信topic为：%DLQ%+consumerGroup

private RemotingCommand consumerSendMsgBack(final ChannelHandlerContext ctx, final RemotingCommand request)
        throws RemotingCommandException {
        final RemotingCommand response = RemotingCommand.createResponseCommand(null);
        final ConsumerSendMsgBackRequestHeader requestHeader =
            (ConsumerSendMsgBackRequestHeader)request.decodeCommandCustomHeader(ConsumerSendMsgBackRequestHeader.class);

        //省略部分代码

        //最大重试次数为16
        if (msgExt.getReconsumeTimes() >= maxReconsumeTimes
            || delayLevel < 0) {
            //死信队列 %DLQ%+consumerGroup
            newTopic = MixAll.getDLQTopic(requestHeader.getGroup());
            queueIdInt = Math.abs(this.random.nextInt() % 99999999) % DLQ_NUMS_PER_GROUP;

            topicConfig = this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(newTopic,
                DLQ_NUMS_PER_GROUP,
                PermName.PERM_WRITE, 0
            );
            if (null == topicConfig) {
                response.setCode(ResponseCode.SYSTEM_ERROR);
                response.setRemark("topic[" + newTopic + "] not exist");
                return response;
            }
        } else {
            if (0 == delayLevel) {
                delayLevel = 3 + msgExt.getReconsumeTimes();
            }

            msgExt.setDelayTimeLevel(delayLevel);
        }

        MessageExtBrokerInner msgInner = new MessageExtBrokerInner();
        msgInner.setTopic(newTopic);
        msgInner.setBody(msgExt.getBody());
        msgInner.setFlag(msgExt.getFlag());
        MessageAccessor.setProperties(msgInner, msgExt.getProperties());
        msgInner.setPropertiesString(MessageDecoder.messageProperties2String(msgExt.getProperties()));
        msgInner.setTagsCode(MessageExtBrokerInner.tagsString2tagsCode(null, msgExt.getTags()));

        msgInner.setQueueId(queueIdInt);
        msgInner.setSysFlag(msgExt.getSysFlag());
        msgInner.setBornTimestamp(msgExt.getBornTimestamp());
        msgInner.setBornHost(msgExt.getBornHost());
        msgInner.setStoreHost(this.getStoreHost());
        msgInner.setReconsumeTimes(msgExt.getReconsumeTimes() + 1);

        String originMsgId = MessageAccessor.getOriginMessageId(msgExt);
        MessageAccessor.setOriginMessageId(msgInner, UtilAll.isBlank(originMsgId) ? msgExt.getMsgId() : originMsgId);

        //消息被持久化到死信队列中
        PutMessageResult putMessageResult = this.brokerController.getMessageStore().putMessage(msgInner);
        //省略部分代码
        return response;
    }

总结：死信消息需要人为进行处理干预，可以通过RocketMQ控制台等重新发送