rocketMQ 订阅关系

场景：2 个消费者进程中，创建了 2 个消费者，同属于 1 个消费组，但是订阅了不同的 topic，会因为订阅信息相互覆盖，导致拉不到消息。

原因是 rocketMQ 的订阅关系，是根据 group 来管理的，c1 订阅 t1，c2 订阅 t2，他们同属于 group，当 c1 拉取 t1 的消息时，broker 发现 group 订阅的是 t2，就不会返回消息给 c1。

client 在 MQClientInstance 中发送心跳给所有的 broker，心跳中包含 consumer 的订阅信息，

this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

    @Override
    public void run() {
        try {
            MQClientInstance.this.cleanOfflineBroker();
            MQClientInstance.this.sendHeartbeatToAllBrokerWithLock();
        } catch (Exception e) {
            log.error("ScheduledTask sendHeartbeatToAllBroker exception", e);
        }
    }
}, 1000, this.clientConfig.getHeartbeatBrokerInterval(), TimeUnit.MILLISECONDS);

// org.apache.rocketmq.client.impl.factory.MQClientInstance#prepareHeartbeatData
private HeartbeatData prepareHeartbeatData() {
    HeartbeatData heartbeatData = new HeartbeatData();

    // clientID
    heartbeatData.setClientID(this.clientId);

    // Consumer
    for (Map.Entry<String, MQConsumerInner> entry : this.consumerTable.entrySet()) {
        MQConsumerInner impl = entry.getValue();
        if (impl != null) {
            ConsumerData consumerData = new ConsumerData();
            consumerData.setGroupName(impl.groupName());
            consumerData.setConsumeType(impl.consumeType());
            consumerData.setMessageModel(impl.messageModel());
            consumerData.setConsumeFromWhere(impl.consumeFromWhere());
            consumerData.getSubscriptionDataSet().addAll(impl.subscriptions());
            consumerData.setUnitMode(impl.isUnitMode());

            heartbeatData.getConsumerDataSet().add(consumerData);
        }
    }

    // Producer
    for (Map.Entry<String/* group */, MQProducerInner> entry : this.producerTable.entrySet()) {
        MQProducerInner impl = entry.getValue();
        if (impl != null) {
            ProducerData producerData = new ProducerData();
            producerData.setGroupName(entry.getKey());

            heartbeatData.getProducerDataSet().add(producerData);
        }
    }

    return heartbeatData;
}

broker 处理心跳请求

// org.apache.rocketmq.broker.processor.ClientManageProcessor#heartBeat
public RemotingCommand heartBeat(ChannelHandlerContext ctx, RemotingCommand request) {
    RemotingCommand response = RemotingCommand.createResponseCommand(null);
    HeartbeatData heartbeatData = HeartbeatData.decode(request.getBody(), HeartbeatData.class);
    ClientChannelInfo clientChannelInfo = new ClientChannelInfo(
        ctx.channel(),
        heartbeatData.getClientID(),
        request.getLanguage(),
        request.getVersion()
    );

    for (ConsumerData data : heartbeatData.getConsumerDataSet()) {
        SubscriptionGroupConfig subscriptionGroupConfig =
            this.brokerController.getSubscriptionGroupManager().findSubscriptionGroupConfig(
                data.getGroupName());
        boolean isNotifyConsumerIdsChangedEnable = true;
        if (null != subscriptionGroupConfig) {
            isNotifyConsumerIdsChangedEnable = subscriptionGroupConfig.isNotifyConsumerIdsChangedEnable();
            int topicSysFlag = 0;
            if (data.isUnitMode()) {
                topicSysFlag = TopicSysFlag.buildSysFlag(false, true);
            }
            String newTopic = MixAll.getRetryTopic(data.getGroupName());
            this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(
                newTopic,
                subscriptionGroupConfig.getRetryQueueNums(),
                PermName.PERM_WRITE | PermName.PERM_READ, topicSysFlag);
        }

        boolean changed = this.brokerController.getConsumerManager().registerConsumer(
            data.getGroupName(),
            clientChannelInfo,
            data.getConsumeType(),
            data.getMessageModel(),
            data.getConsumeFromWhere(),
            data.getSubscriptionDataSet(),
            isNotifyConsumerIdsChangedEnable
        );

        if (changed) {
            log.info("registerConsumer info changed {} {}",
                data.toString(),
                RemotingHelper.parseChannelRemoteAddr(ctx.channel())
            );
        }
    }

    for (ProducerData data : heartbeatData.getProducerDataSet()) {
        this.brokerController.getProducerManager().registerProducer(data.getGroupName(),
            clientChannelInfo);
    }
    response.setCode(ResponseCode.SUCCESS);
    response.setRemark(null);
    return response;
}

broker 比较当前 consumer group 的订阅信息，如果当前消费组加入了新的消费者，或者订阅的 topic 发生了变化，则通知消费者进行 rebalance

// org.apache.rocketmq.broker.client.ConsumerManager#registerConsumer
public boolean registerConsumer(final String group, final ClientChannelInfo clientChannelInfo,
    ConsumeType consumeType, MessageModel messageModel, ConsumeFromWhere consumeFromWhere,
    final Set<SubscriptionData> subList, boolean isNotifyConsumerIdsChangedEnable) {

    ConsumerGroupInfo consumerGroupInfo = this.consumerTable.get(group);
    if (null == consumerGroupInfo) {
        ConsumerGroupInfo tmp = new ConsumerGroupInfo(group, consumeType, messageModel, consumeFromWhere);
        ConsumerGroupInfo prev = this.consumerTable.putIfAbsent(group, tmp);
        consumerGroupInfo = prev != null ? prev : tmp;
    }

    boolean r1 =
        consumerGroupInfo.updateChannel(clientChannelInfo, consumeType, messageModel,
            consumeFromWhere);
    // 本文讨论的场景，重点在这，group 的订阅数据被覆盖
    boolean r2 = consumerGroupInfo.updateSubscription(subList);

    if (r1 || r2) {
        if (isNotifyConsumerIdsChangedEnable) {
            this.consumerIdsChangeListener.handle(ConsumerGroupEvent.CHANGE, group, consumerGroupInfo.getAllChannel());
        }
    }

    this.consumerIdsChangeListener.handle(ConsumerGroupEvent.REGISTER, group, subList);

    return r1 || r2;
}

发生 ConsumerGroupEvent.CHANGE 事件，broker 通知 client 进行 rebalance

// org.apache.rocketmq.broker.client.DefaultConsumerIdsChangeListener#handle
public void handle(ConsumerGroupEvent event, String group, Object... args) {
    if (event == null) {
        return;
    }
    switch (event) {
        case CHANGE:
            if (args == null || args.length < 1) {
                return;
            }
            List<Channel> channels = (List<Channel>) args[0];
            if (channels != null && brokerController.getBrokerConfig().isNotifyConsumerIdsChangedEnable()) {
                for (Channel chl : channels) {
                    this.brokerController.getBroker2Client().notifyConsumerIdsChanged(chl, group);
                }
            }
            break;
        case UNREGISTER:
            this.brokerController.getConsumerFilterManager().unRegister(group);
            break;
        case REGISTER:
            if (args == null || args.length < 1) {
                return;
            }
            Collection<SubscriptionData> subscriptionDataList = (Collection<SubscriptionData>) args[0];
            this.brokerController.getConsumerFilterManager().register(group, subscriptionDataList);
            break;
        default:
            throw new RuntimeException("Unknown event " + event);
    }
}

broker 向 client 发送 NOTIFY_CONSUMER_IDS_CHANGED 请求

// org.apache.rocketmq.broker.client.net.Broker2Client#notifyConsumerIdsChanged
public void notifyConsumerIdsChanged(
    final Channel channel,
    final String consumerGroup) {
    if (null == consumerGroup) {
        log.error("notifyConsumerIdsChanged consumerGroup is null");
        return;
    }

    NotifyConsumerIdsChangedRequestHeader requestHeader = new NotifyConsumerIdsChangedRequestHeader();
    requestHeader.setConsumerGroup(consumerGroup);
    RemotingCommand request =
        RemotingCommand.createRequestCommand(RequestCode.NOTIFY_CONSUMER_IDS_CHANGED, requestHeader);

    try {
        this.brokerController.getRemotingServer().invokeOneway(channel, request, 10);
    } catch (Exception e) {
        log.error("notifyConsumerIdsChanged exception, " + consumerGroup, e.getMessage());
    }
}

client 收到 NOTIFY_CONSUMER_IDS_CHANGED 请求，触发 rebalance

// org.apache.rocketmq.client.impl.ClientRemotingProcessor#notifyConsumerIdsChanged
public RemotingCommand notifyConsumerIdsChanged(ChannelHandlerContext ctx,
    RemotingCommand request) throws RemotingCommandException {
    try {
        final NotifyConsumerIdsChangedRequestHeader requestHeader =
            (NotifyConsumerIdsChangedRequestHeader) request.decodeCommandCustomHeader(NotifyConsumerIdsChangedRequestHeader.class);
        log.info("receive broker's notification[{}], the consumer group: {} changed, rebalance immediately",
            RemotingHelper.parseChannelRemoteAddr(ctx.channel()),
            requestHeader.getConsumerGroup());
        this.mqClientFactory.rebalanceImmediately();
    } catch (Exception e) {
        log.error("notifyConsumerIdsChanged exception", RemotingHelper.exceptionSimpleDesc(e));
    }
    return null;
}

需要说明的是，client 一直在进行 rebalance，只是间隔 10s 而已。这里只是让 client 立马 rebalance，但其实并 没有任何效果，因为消费者数量没有变化。

但是由于 broker 保存的订阅关系一直被修改，导致 consumer 去拉数据时，会因为订阅关系不存在，而拉不到数据。

subscriptionData = consumerGroupInfo.findSubscriptionData(requestHeader.getTopic());
if (null == subscriptionData) {
    log.warn("the consumer's subscription not exist, group: {}, topic:{}", requestHeader.getConsumerGroup(), requestHeader.getTopic());
    response.setCode(ResponseCode.SUBSCRIPTION_NOT_EXIST);
    response.setRemark("the consumer's subscription not exist" + FAQUrl.suggestTodo(FAQUrl.SAME_GROUP_DIFFERENT_TOPIC));
    return response;
}