一. 前言

RocketMQ支持消息消费失败后重新消费，具体代码如下：

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/*
         *  Register callback to execute on arrival of messages fetched from brokers.
         */
        consumer.registerMessageListener(new MessageListenerConcurrently() {
            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List msgs,
                ConsumeConcurrentlyContext context) {
                System.out.printf(Thread.currentThread().getName()  + "%n");
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;
            }
        });

也就是需要返回ConsumeConcurrentlyStatus.RECONSUME_LATER，那这个怎么实现的呢？

二. 代码流程

以push 非同步消息为例，消息消费过程可以参考https://blog.51cto.com/483181/2056301

我们从客户端成功获取到一条消息开始，也就是DefaultMQPushConsumerImpl.pullMessage

2.1. 获取消息回调

public void pullMessage(final PullRequest pullRequest) {
        ...

        PullCallback pullCallback = new PullCallback() {
            @Override
            public void onSuccess(PullResult pullResult) {
                if (pullResult != null) {
                    pullResult = DefaultMQPushConsumerImpl.this.pullAPIWrapper.processPullResult(pullRequest.getMessageQueue(), pullResult,
                        subscriptionData);

                    switch (pullResult.getPullStatus()) {
                        case FOUND:
                             ...
                                DefaultMQPushConsumerImpl.this.consumeMessageService.submitConsumeRequest(
                                    pullResult.getMsgFoundList(),
                                    processQueue,
                                    pullRequest.getMessageQueue(),
                                    dispathToConsume);

                            ....

                            break;

                    }
                }
            }

            @Override
            public void onException(Throwable e) {
                ...
            }
        };

        ...

        try {
            this.pullAPIWrapper.pullKernelImpl(
                pullRequest.getMessageQueue(),
                subExpression,
                subscriptionData.getExpressionType(),
                subscriptionData.getSubVersion(),
                pullRequest.getNextOffset(),
                this.defaultMQPushConsumer.getPullBatchSize(),
                sysFlag,
                commitOffsetValue,
                BROKER_SUSPEND_MAX_TIME_MILLIS,
                CONSUMER_TIMEOUT_MILLIS_WHEN_SUSPEND,
                CommunicationMode.ASYNC,
                pullCallback
            );
        } catch (Exception e) {

        }
    }

pullKernelImpl获取消息后，如果是异步请求，那么将会回调pullCallback，我们假设成功拿到消息，也就是FOUND分支。
那么就会调用submit 消费请求：consumeMessageService.submitConsumeRequest

2.2 submit消息请求

submitConsumeRequest有两个实现类，一个是pull, 一个是push。

我们以push为例。

2.3 push submit消息请求

ConsumeMessageConcurrentlyService.java

public void submitConsumeRequest(
        final List msgs,
        final ProcessQueue processQueue,
        final MessageQueue messageQueue,
        final boolean dispatchToConsume) {
        final int consumeBatchSize = this.defaultMQPushConsumer.getConsumeMessageBatchMaxSize();
                          ...

                ConsumeRequest consumeRequest = new ConsumeRequest(msgThis, processQueue, messageQueue);
                try {
                    this.consumeExecutor.submit(consumeRequest);
                } catch (RejectedExecutionException e) {

                }
            }
        }
    }

初始化一个ConsumeRequest Runnable对象，然后提交到线程池consumeExecutor里面，那么我们继续看ConsumeRequest。

2.4 ConsumeRequest

class ConsumeRequest implements Runnable {
        private final List msgs;
        private final ProcessQueue processQueue;
        private final MessageQueue messageQueue;

        @Override
        public void run() {
            ...

            try {
                ...
                status = listener.consumeMessage(Collections.unmodifiableList(msgs), context);
            } catch (Throwable e) {
                ....
            }

                        ....

            if (!processQueue.isDropped()) {
                ConsumeMessageConcurrentlyService.this.processConsumeResult(status, context, this);
            } else {
                log.warn("processQueue is dropped without process consume result. messageQueue={}, msgs={}", messageQueue, msgs);
            }
        }

    }

首先看try-catch里面，这里面就是回调客户端来消费消息。

listener.consumeMessage(Collections.unmodifiableList(msgs), context);

就像我们的消息消费重写如下：

/*
         *  Register callback to execute on arrival of messages fetched from brokers.
         */
        consumer.registerMessageListener(new MessageListenerConcurrently() {
            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List msgs,
                ConsumeConcurrentlyContext context) {
                System.out.printf(Thread.currentThread().getName()  + "%n");
                return ConsumeConcurrentlyStatus.RECONSUME_LATER;
            }
        });

然后根据返回的status调用processConsumeResult来处理返回结果

2.5 processConsumeResult

public void processConsumeResult(
        final ConsumeConcurrentlyStatus status,
        final ConsumeConcurrentlyContext context,
        final ConsumeRequest consumeRequest
    ) {
       ...

        switch (this.defaultMQPushConsumer.getMessageModel()) {
            case BROADCASTING:
                for (int i = ackIndex + 1; i < consumeRequest.getMsgs().size(); i++) {
                    MessageExt msg = consumeRequest.getMsgs().get(i);
                    log.warn("BROADCASTING, the message consume failed, drop it, {}", msg.toString());
                }
                break;
            case CLUSTERING:
                log.info("CLUSTERING...");
                List msgBackFailed = new ArrayList(consumeRequest.getMsgs().size());
                for (int i = ackIndex + 1; i < consumeRequest.getMsgs().size(); i++) {
                    MessageExt msg = consumeRequest.getMsgs().get(i);
                    boolean result = this.sendMessageBack(msg, context);
                    if (!result) {
                        msg.setReconsumeTimes(msg.getReconsumeTimes() + 1);
                        msgBackFailed.add(msg);
                    }
                }

                log.info("msgBackFailed.isEmpty() [{}]", msgBackFailed.isEmpty());
                if (!msgBackFailed.isEmpty()) {
                    log.info("msgBackFailed [{}]", msgBackFailed);
                    consumeRequest.getMsgs().removeAll(msgBackFailed);

                    this.submitConsumeRequestLater(msgBackFailed, consumeRequest.getProcessQueue(), consumeRequest.getMessageQueue());
                }
                break;
            default:
                break;
        }

        ...
    }

2.5.1

首先会判断消费模式是集群还是广播模式，如果广播模式，就日志记录下不处理了。
如果是集群模式，那么把调用sendMessageBack发送消息到broker。等待下一次broker重新消费消息。
如果发送失败，那么立即就会消费消息。

2.6 发送消息到Broker

public boolean sendMessageBack(final MessageExt msg, final ConsumeConcurrentlyContext context) {
        int delayLevel = context.getDelayLevelWhenNextConsume();

        try {
            this.defaultMQPushConsumerImpl.sendMessageBack(msg, delayLevel, context.getMessageQueue().getBrokerName());
            return true;
        } catch (Exception e) {
            ...
        }

        return false;
    }

public void sendMessageBack(MessageExt msg, int delayLevel, final String brokerName)
        throws RemotingException, MQBrokerException, InterruptedException, MQClientException {
        try {
            String brokerAddr = (null != brokerName) ? this.mQClientFactory.findBrokerAddressInPublish(brokerName)
                : RemotingHelper.parseSocketAddressAddr(msg.getStoreHost());
            this.mQClientFactory.getMQClientAPIImpl().consumerSendMessageBack(brokerAddr, msg,
                this.defaultMQPushConsumer.getConsumerGroup(), delayLevel, 5000, getMaxReconsumeTimes());
        } catch (Exception e) {
            ....
        }
    }

public void consumerSendMessageBack(
        final String addr,
        final MessageExt msg,
        final String consumerGroup,
        final int delayLevel,
        final long timeoutMillis,
        final int maxConsumeRetryTimes
    ) throws RemotingException, MQBrokerException, InterruptedException {
        ConsumerSendMsgBackRequestHeader requestHeader = new ConsumerSendMsgBackRequestHeader();
        RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.CONSUMER_SEND_MSG_BACK, requestHeader);

        requestHeader.setGroup(consumerGroup);
        requestHeader.setOriginTopic(msg.getTopic());
        requestHeader.setOffset(msg.getCommitLogOffset());
        requestHeader.setDelayLevel(delayLevel);
        requestHeader.setOriginMsgId(msg.getMsgId());
        requestHeader.setMaxReconsumeTimes(maxConsumeRetryTimes);

        log.info("addr [{}] request [{}] timeoutMillis [{}]", addr, request, timeoutMillis);
        RemotingCommand response = this.remotingClient.invokeSync(MixAll.brokerVIPChannel(this.clientConfig.isVipChannelEnabled(), addr),
            request, timeoutMillis);
        assert response != null;
        switch (response.getCode()) {
            case ResponseCode.SUCCESS: {
                return;
            }
            default:
                break;
        }

        throw new MQBrokerException(response.getCode(), response.getRemark());
    }

简单的调用api通过netty发到broker而已，请求码是RequestCode.CONSUMER_SEND_MSG_BACK

public static final int CONSUMER_SEND_MSG_BACK = 36;

Broker会把消息存储到文件中，当然也会让它的reconsume次数+1

具体可以参考SendMessageProcessor.proce***equest方法，这个后续再讲