seata源码解析：TM RM 客户端的初始化过程

上一篇文章说过，在Spring启动的过程中就会就会初始化TM和RM，建立与TC的长连接。TM，RM，TC都是用netty来处理网络连接的，初始化netty客户端和服务端的过程也非常类似。

本篇文章只分析TM的初始化过程，RM和TM复用了很多方法

// TmNettyRemotingClient
public void init() {
    // registry processor
    // 注册消息处理器
    registerProcessor();
    if (initialized.compareAndSet(false, true)) {
        super.init();
    }
}

// AbstractNettyRemotingClient
public void init() {
    // 不断连接seata server
    timerExecutor.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            clientChannelManager.reconnect(getTransactionServiceGroup());
        }
    }, SCHEDULE_DELAY_MILLS, SCHEDULE_INTERVAL_MILLS, TimeUnit.MILLISECONDS);
    // 是否允许批量发送请求
    if (NettyClientConfig.isEnableClientBatchSendRequest()) {
        mergeSendExecutorService = new ThreadPoolExecutor(MAX_MERGE_SEND_THREAD,
            MAX_MERGE_SEND_THREAD,
            KEEP_ALIVE_TIME, TimeUnit.MILLISECONDS,
            new linkedBlockingQueue<>(),
            new NamedThreadFactory(getThreadPrefix(), MAX_MERGE_SEND_THREAD));
        mergeSendExecutorService.submit(new MergedSendRunnable());
    }
    // 移除发送超时的消息
    super.init();
    clientBootstrap.start();
}

clientBootstrap#start是netty启动的模版代码，注册消息处理器和处理消息的套路我在seata server启动的文章分析的比较详细，本篇文章就不深入分析了

TM和RM每隔10s都要TC集群的每个地址建立长连接

// NettyClientChannelManager#reconnect
void reconnect(String transactionServiceGroup) {
    List availList = null;
    try {
        // 获得事务分组对应的集群中每台机器地址
        availList = getAvailServerList(transactionServiceGroup);
    } catch (Exception e) {
        LOGGER.error("Failed to get available servers: {}", e.getMessage(), e);
        return;
    }
    if (CollectionUtils.isEmpty(availList)) {
        RegistryService registryService = RegistryFactory.getInstance();
        String clusterName = registryService.getServiceGroup(transactionServiceGroup);

        if (StringUtils.isBlank(clusterName)) {
            LOGGER.error("can not get cluster name in registry config '{}{}', please make sure registry config correct",
                    ConfigurationKeys.SERVICE_GROUP_MAPPING_PREFIX,
                    transactionServiceGroup);
            return;
        }

        if (!(registryService instanceof FileRegistryServiceImpl)) {
            LOGGER.error("no available service found in cluster '{}', please make sure registry config correct and keep your seata server running", clusterName);
        }
        return;
    }
    // 遍历tc服务器地址
    for (String serverAddress : availList) {
        try {
            // 建立与tc的连接
            acquireChannel(serverAddress);
        } catch (Exception e) {
            LOGGER.error("{} can not connect to {} cause:{}",frameworkErrorCode.NetConnect.getErrCode(), serverAddress, e.getMessage(), e);
        }
    }
}

Channel acquireChannel(String serverAddress) {
    Channel channelToServer = channels.get(serverAddress);
    // 与当前serverAddress已经存在连接，直接返回
    if (channelToServer != null) {
        channelToServer = getExistAliveChannel(channelToServer, serverAddress);
        if (channelToServer != null) {
            return channelToServer;
        }
    }
    if (LOGGER.isInfoEnabled()) {
        LOGGER.info("will connect to " + serverAddress);
    }
    // 与当前serverAddress不存在连接，新建连接
    Object lockObj = CollectionUtils.computeIfAbsent(channelLocks, serverAddress, key -> new Object());
    synchronized (lockObj) {
        return doConnect(serverAddress);
    }
}

private Channel doConnect(String serverAddress) {
    Channel channelToServer = channels.get(serverAddress);
    // 当前地址已经存在连接
    if (channelToServer != null && channelToServer.isActive()) {
        return channelToServer;
    }
    Channel channelFromPool;
    try {
        NettyPoolKey currentPoolKey = poolKeyFunction.apply(serverAddress);
        NettyPoolKey previousPoolKey = poolKeyMap.putIfAbsent(serverAddress, currentPoolKey);
        if (previousPoolKey != null && previousPoolKey.getMessage() instanceof RegisterRMRequest) {
            RegisterRMRequest registerRMRequest = (RegisterRMRequest) currentPoolKey.getMessage();
            ((RegisterRMRequest) previousPoolKey.getMessage()).setResourceIds(registerRMRequest.getResourceIds());
        }
        channelFromPool = nettyClientKeyPool.borrowObject(poolKeyMap.get(serverAddress));
        channels.put(serverAddress, channelFromPool);
    } catch (Exception exx) {
        LOGGER.error("{} register RM failed.",frameworkErrorCode.RegisterRM.getErrCode(), exx);
        throw new frameworkException("can not register RM,err:" + exx.getMessage());
    }
    return channelFromPool;
}

TM和RM客户端在启动的时候会和集群中的的每台seata server建立长连接，但是在后续发送请求的时候，比如开启全局事务，注册分支事务只会和其中的一台机器通讯，TM或RM首先根据事务分组找到集群列表，然后根据负载均衡策略从列表中选出一台机器发起请求。具体代码可参见AbstractNettyRemotingClient#sendSyncRequest方法

[1]https://blog.csdn.net/zjj2006/category_10310426.html
[2]https://blog.csdn.net/weixin_38308374/article/details/108944877