RocketMQ原理解析-Name Server

1. 介绍
    Namesrv的功能，就相当于RPC中的注册中心。对于MQ而言，broker启动，将自身创建的topic等信息注册到Namesrv上。consumer和producer需要配置namesrv的地址，启动后，首先和namesrv建立长连接，并获取相应的topic信息(比如，哪些broker有topic路由信息)，然后再和broker建立长连接。Namesrv本身无状态，可集群部署。所有的注册信息，都保存在namesrv的类似map内存数据结构中。

2. 启动
    Namesrv启动后，首先会加载KVConfig服务，然后监听本地端口(默认9876),等待客户端连接，并定时清理非活跃broker和打印KVConfig值信息。代码清晰易懂。

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();


        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);

        return true;
    }

3. 请求处理
    Namesrv能处理的所有请求都在DefaultRequestProcessor#processRequest方法：

public RemotingCommand processRequest(ChannelHandlerContext ctx, RemotingCommand request) throws RemotingCommandException {
        if (log.isDebugEnabled()) {
            log.debug("receive request, {} {} {}",//
                    request.getCode(), //
                    RemotingHelper.parseChannelRemoteAddr(ctx.channel()), //
                    request);
        }

        switch (request.getCode()) {
            case RequestCode.PUT_KV_CONFIG:
                return this.putKVConfig(ctx, request);
            case RequestCode.GET_KV_CONFIG:
                return this.getKVConfig(ctx, request);
            case RequestCode.DELETE_KV_CONFIG:
                return this.deleteKVConfig(ctx, request);
            case RequestCode.REGISTER_BROKER:
                Version brokerVersion = MQVersion.value2Version(request.getVersion());
                if (brokerVersion.ordinal() >= MQVersion.Version.V3_0_11.ordinal()) {
                    return this.registerBrokerWithFilterServer(ctx, request);
                }
                else {
                    return this.registerBroker(ctx, request);
                }
            case RequestCode.UNREGISTER_BROKER:
                return this.unregisterBroker(ctx, request);
            case RequestCode.GET_ROUTEINTO_BY_TOPIC:
                return this.getRouteInfoByTopic(ctx, request);
            case RequestCode.GET_BROKER_CLUSTER_INFO:
                return this.getBrokerClusterInfo(ctx, request);
            case RequestCode.WIPE_WRITE_PERM_OF_BROKER:
                return this.wipeWritePermOfBroker(ctx, request);
            case RequestCode.GET_ALL_TOPIC_LIST_FROM_NAMESERVER:
                return getAllTopicListFromNameserver(ctx, request);
            case RequestCode.DELETE_TOPIC_IN_NAMESRV:
                return deleteTopicInNamesrv(ctx, request);
            case RequestCode.GET_KVLIST_BY_NAMESPACE:
                return this.getKVListByNamespace(ctx, request);
            case RequestCode.GET_TOPICS_BY_CLUSTER:
                return this.getTopicsByCluster(ctx, request);
            case RequestCode.GET_SYSTEM_TOPIC_LIST_FROM_NS:
                return this.getSystemTopicListFromNs(ctx, request);
            case RequestCode.GET_UNIT_TOPIC_LIST:
                return this.getUnitTopicList(ctx, request);
            case RequestCode.GET_HAS_UNIT_SUB_TOPIC_LIST:
                return this.getHasUnitSubTopicList(ctx, request);
            case RequestCode.GET_HAS_UNIT_SUB_UNUNIT_TOPIC_LIST:
                return this.getHasUnitSubUnUnitTopicList(ctx, request);
            default:
                break;
        }
        return null;
    }

    broker启动后，会调用REGISTER_BROKER注册topic等信息，其心跳也是直接调用REGISTER_BROKER的，broker下线会调用UNREGISTER_BROKER方法。

4. 数据结构
    Namesrv的数据都保存在RouteInfoManager类中:

private final static long BrokerChannelExpiredTime = 1000 * 60 * 2;
    private final ReadWriteLock lock = new ReentrantReadWriteLock();
    private final HashMap<String/* topic */, List<QueueData>> topicQueueTable;
    private final HashMap<String/* brokerName */, BrokerData> brokerAddrTable;
    private final HashMap<String/* clusterName */, Set<String/* brokerName */>> clusterAddrTable;
    private final HashMap<String/* brokerAddr */, BrokerLiveInfo> brokerLiveTable;
    private final HashMap<String/* brokerAddr */, List<String>/* Filter Server */> filterServerTable;

    可以看到，broker非活跃时间为2分钟。

5. KVConfig作用
    猜测MQ作者，想通过KVConfig做类似configserver的简易功能，将信息配置在namesrv后，客户端获取后进行业务处理。其数据结构为：

private final HashMap<String/* Namespace */, HashMap<String/* Key */, String/* Value */>> configTable =
            new HashMap<String, HashMap<String, String>>();

    KVConfig的数据最终会保存在kvConfig.json文件中，每次的put，delete操作都会直接修改kvConfig.json文件，并在namesrv启动时，加载数据。

6. 控制台
    MQ的mqadmin脚本可以直接获取namesrv存储的信息，在MQ的bin目录下：

    这里面，最重要的一个命令为wipeWritePerm，该命令可清除指定broker的写权限。当某台broker需要重启，将会导致向这台broker发消息失败，消息消费失败。通过该命令清除broker写权限后，由于producer只会向带有写权限的master broker发消息，当producer从namesrv更新(定时30s更新topic路由信息)到该broker无写权限后，将会将消息发往其他broker，然后再根据业务情况（譬如30分钟后），让consumer消费完消息再停掉该broker。