使用Rocketmq一定要注意，如果项目中要订阅两个topic，一定要保证consumeGroup是两个不同的。

　　这是因为，Consumer会定期发送心跳，默认是30s一次。心跳会像全部broker发送，心跳包内容包括groupname，topicname1。然后broker端会缓存这个信息，以groupname为key

　　代码在 ClientManagerProcessor # 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(@1
                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;
    }

　　代码很长，就不一一分析了。重点是 心跳里带着数据 

　　heartbeatData.getConsumerDataSet()。

public class ConsumerData {
    private String groupName;
    private ConsumeType consumeType;
    private MessageModel messageModel;
    private ConsumeFromWhere consumeFromWhere;
    private Set<SubscriptionData> subscriptionDataSet = new HashSet<SubscriptionData>();
    private boolean unitMode;

　　SubscriptionData 是关于topic的相关信息，里面最重要的就是topic

　　@1处的代码

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

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);
        boolean r2 = consumerGroupInfo.updateSubscription(subList);

public boolean updateSubscription(final Set<SubscriptionData> subList) {
        boolean updated = false;

        for (SubscriptionData sub : subList) {
            SubscriptionData old = this.subscriptionTable.get(sub.getTopic());
            if (old == null) {
                SubscriptionData prev = this.subscriptionTable.putIfAbsent(sub.getTopic(), sub);
                if (null == prev) {
                    updated = true;
                    log.info("subscription changed, add new topic, group: {} {}",
                        this.groupName,
                        sub.toString());
                }
            } else if (sub.getSubVersion() > old.getSubVersion()) {
                if (this.consumeType == ConsumeType.CONSUME_PASSIVELY) {
                    log.info("subscription changed, group: {} OLD: {} NEW: {}",
                        this.groupName,
                        old.toString(),
                        sub.toString()
                    );
                }

                this.subscriptionTable.put(sub.getTopic(), sub);
            }
        }

　　从上面的代码看得出来，如果两个不同的topic，consmer客户端不小心配成了相同的consumeGroup，在broker端的缓存里，每次心跳就有可能覆盖之前的订阅信息。导致某一个consume消费不到自己要订阅的topic了