RocketMQ源码分析（五）——Broker心跳原理

本章，我们来讲讲Broker是如何定时发送心跳到NameServer，让NameServer感知到Broker一直都存活着的。如果Broker一段时间内没有发送心跳到NameServer，那么NameServer是如何感知到Broker已经挂掉了呢？

一、心跳原理

首先，我们来回顾下BrokerController的启动，BrokerController启动的时候，其实并不是仅仅发送一次注册请求，而是启动了一个定时任务，会每隔一段时间就发送一次注册请求。

public void start() throws Exception {
    //...忽略无关代码

    // 启动一个定时调度任务，每隔一段时间进行一次注册，默认30s
    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            try {
                BrokerController.this.registerBrokerAll(true, false, brokerConfig.isForceRegister());
            } catch (Throwable e) {
                log.error("registerBrokerAll Exception", e);
            }
        }
    }, 1000 * 10, Math.max(10000, Math.min(brokerConfig.getRegisterNameServerPeriod(), 60000)), TimeUnit.MILLISECONDS);

    //...忽略无关代码
}

我们通过上一章已经知道，第一次发送注册请求就是把Broker路由数据放入到NameServer的RouteInfoManager的路由数据表里去。

但是后续每隔30秒Broker都会发送一次注册请求，这些后续定时发送的注册请求本质就是Broker发送的心跳，那么，NameServer是如何处理这些后续重复发送过来的注册请求（心跳）呢？

1.1 RouteInfoManager

我们来看下RouteInfoManager的注册方法registerBroker的逻辑。下面的代码，有几个核心要点：

• Broker的路由信息全部维护在brokerAddrTable这个Map里面，然后Broker会以集群为维度被管理；
• 心跳机制的关键是用了一个brokerLiveTable管理Broker的最新心跳，它的key就是Broker，Value是BrokerLiveInfo对象。Broker每上送一次心跳，就会创建一个BrokerLiveInfo对象覆盖掉brokerLiveTable里面老的，BrokerLiveInfo里面有当前时间戳，表示最近一次心跳的时间。

public RegisterBrokerResult registerBroker(
    final String clusterName,
    final String brokerAddr,
    final String brokerName,
    final long brokerId,
    final String haServerAddr,
    final TopicConfigSerializeWrapper topicConfigWrapper,
    final List<String> filterServerList,
    final Channel channel) {
    RegisterBrokerResult result = new RegisterBrokerResult();
    try {
        try {
            // 加写锁，保证同一时刻只有一个线程能进行修改
            this.lock.writeLock().lockInterruptibly();

            // 根据clusterName获取这个集群下的Broker集合
            Set<String> brokerNames = this.clusterAddrTable.get(clusterName);
            if (null == brokerNames) {
                brokerNames = new HashSet<String>();
                this.clusterAddrTable.put(clusterName, brokerNames);
            }
            // 添加到集群
            brokerNames.add(brokerName);

            boolean registerFirst = false;

            // Broker相关数据放在brokerAddrTable这个Map里，路由信息都在里面
            BrokerData brokerData = this.brokerAddrTable.get(brokerName);
            // 这里首次注册的情况
            if (null == brokerData) {
                registerFirst = true;
                brokerData = new BrokerData(clusterName, brokerName, new HashMap<Long, String>());
                this.brokerAddrTable.put(brokerName, brokerData);
            }

            // 对路由数据做处理，忽略
            Map<Long, String> brokerAddrsMap = brokerData.getBrokerAddrs();
            //Switch slave to master: first remove <1, IP:PORT> in namesrv, then add <0, IP:PORT>
            //The same IP:PORT must only have one record in brokerAddrTable
            Iterator<Entry<Long, String>> it = brokerAddrsMap.entrySet().iterator();
            while (it.hasNext()) {
                Entry<Long, String> item = it.next();
                if (null != brokerAddr && brokerAddr.equals(item.getValue()) && brokerId != item.getKey()) {
                    it.remove();
                }
            }

            String oldAddr = brokerData.getBrokerAddrs().put(brokerId, brokerAddr);
            registerFirst = registerFirst || (null == oldAddr);

            if (null != topicConfigWrapper
                && MixAll.MASTER_ID == brokerId) {
                if (this.isBrokerTopicConfigChanged(brokerAddr, topicConfigWrapper.getDataVersion())
                    || registerFirst) {
                    ConcurrentMap<String, TopicConfig> tcTable =
                        topicConfigWrapper.getTopicConfigTable();
                    if (tcTable != null) {
                        for (Map.Entry<String, TopicConfig> entry : tcTable.entrySet()) {
                            this.createAndUpdateQueueData(brokerName, entry.getValue());
                        }
                    }
                }
            }

            // 这里是关键，Broker心跳管理：每次接受到心跳请求后，这里会封装一个BrokerLiveInfo，放到brokerLiveTable中，替换掉老的
            // 这个BrokerLiveInfo里面，有一个当前时间戳，代表最近一次心跳的时间
            BrokerLiveInfo prevBrokerLiveInfo = this.brokerLiveTable.put(brokerAddr,
                new BrokerLiveInfo(
                    System.currentTimeMillis(),
                    topicConfigWrapper.getDataVersion(),
                    channel,
                    haServerAddr));
            if (null == prevBrokerLiveInfo) {
                log.info("new broker registered, {} HAServer: {}", brokerAddr, haServerAddr);
            }

            // 下面的代码忽略
            if (filterServerList != null) {
                if (filterServerList.isEmpty()) {
                    this.filterServerTable.remove(brokerAddr);
                } else {
                    this.filterServerTable.put(brokerAddr, filterServerList);
                }
            }

            if (MixAll.MASTER_ID != brokerId) {
                String masterAddr = brokerData.getBrokerAddrs().get(MixAll.MASTER_ID);
                if (masterAddr != null) {
                    BrokerLiveInfo brokerLiveInfo = this.brokerLiveTable.get(masterAddr);
                    if (brokerLiveInfo != null) {
                        result.setHaServerAddr(brokerLiveInfo.getHaServerAddr());
                        result.setMasterAddr(masterAddr);
                    }
                }
            }
        } finally {
            this.lock.writeLock().unlock();
        }
    } catch (Exception e) {
        log.error("registerBroker Exception", e);
    }

    return result;
}

二、故障感知

了解了Broker的心跳机制，我们再来思考一个问题，如果当前的Broker挂掉了，NameServer是如何检测到的？
我们重新回到NamesrvController的initialize()方法里去，里面启动了一个定时调度任务，调用RouteInfoManager的scanNotActiveBroker方法去定时扫描不活跃的Broker。

public boolean initialize() {
    this.kvConfigManager.load();

    this.remotingServer = new NettyRemotingServer(this.nettyServerConfig, this.brokerHousekeepingService);

    this.remotingExecutor =
        Executors.newFixedThreadPool(nettyServerConfig.getServerWorkerThreads(), new ThreadFactoryImpl("RemotingExecutorThread_"));

    this.registerProcessor();

    // 后台定时任务，扫码不活跃的Broker
    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            NamesrvController.this.routeInfoManager.scanNotActiveBroker();
        }
    }, 5, 10, TimeUnit.SECONDS);

    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            NamesrvController.this.kvConfigManager.printAllPeriodically();
        }
    }, 1, 10, TimeUnit.MINUTES);

    //...忽略无关代码

    return true;
}

1.1 RouteInfoManager

我们来看下RouteInfoManager的scanNotActiveBroker方法：

public void scanNotActiveBroker() {
    // 遍历brokerLiveTable
    Iterator<Entry<String, BrokerLiveInfo>> it = this.brokerLiveTable.entrySet().iterator();
    while (it.hasNext()) {
        Entry<String, BrokerLiveInfo> next = it.next();
        // 查看每个Broker的BrokerLiveInfo，也就是Broker的最新心跳时间
        long last = next.getValue().getLastUpdateTimestamp();
        // 如果心跳超时，就移除掉，默认120s
        if ((last + BROKER_CHANNEL_EXPIRED_TIME) < System.currentTimeMillis()) {
            // 断开与该超时Broker的连接
            RemotingUtil.closeChannel(next.getValue().getChannel());
            it.remove();
            log.warn("The broker channel expired, {} {}ms", next.getKey(), BROKER_CHANNEL_EXPIRED_TIME);
            this.onChannelDestroy(next.getKey(), next.getValue().getChannel());
        }
    }
}

该方法很简单，就是遍历brokerLiveTable，找到那些超过120s（默认）还没发送心跳的Broker，将它们移除，同时断开连接。

三、总结

本章，我们讲解了Broker的心跳机制，本质就是NameServer中的RouteInfoManager组件对其中的Broker路由信息的管理。