分布式事务(Seata)原理 详解篇,建议收藏

Posted 牧小农

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篇首语:本文由小常识网(cha138.com)小编为大家整理,主要介绍了分布式事务(Seata)原理 详解篇,建议收藏相关的知识,希望对你有一定的参考价值。

前言

在之前的系列中,我们讲解了关于Seata基本介绍和实际应用,今天带来的这篇,就给大家分析一下Seata的源码是如何一步一步实现的。读源码的时候我们需要俯瞰起全貌,不要去扣一个一个的细节,这样我们学习起来会快捷而且有效率,我们学习源码需要掌握的是整体思路和核心点。

首先 Seata 客户端启动一般分为以下几个流程:

  1. 自动加载Bean属性和配置信息
  2. 初始化TM
  3. 初始化RM
  4. 初始化分布式事务客户端完成,完成代理数据库配置
  5. 连接TC(Seata服务端),注册RM和TM
  6. 开启全局事务

在这篇源码的讲解中,我们主要以AT模式为主导,官网也是主推AT模式,我们在上篇的文章中也讲解过,感兴趣的小伙伴可以去看一看分布式事务(Seata) 四大模式详解,在官网中也提供了对应的流程地址:https://seata.io/zh-cn/docs/dev/mode/at-mode.html ,在这里我们只是做一些简单的介绍,AT模式主要分为两个阶段:

一阶段:

  • 解析SQL,获取SQL类型(CRUD)、表信息、条件(where) 等相关信息
  • 查询前镜像(改变之前的数据),根据解析得到的条件信息,生成查询语句,定位数据
  • 执行业务SQL,更新数据
  • 查询后镜像(改变后的数据),根据前镜像的结果,通过主键都给你为数据
  • 插入回滚日志,将前后镜像数据以及业务SQL等信息,组织成一条回滚日志记录,插入到undo Log表中
  • 提交前,向TC注册分支,申请全局锁
  • 本地事务提交,业务数据的更细腻和生成的undoLog一起提交
  • 将本地事务提交的结果通知给TC

二阶段:

如果TC收到的是回滚请求

  • 开启本地事务,通过XID和BranchID查找到对应的undo Log记录
  • 根据undoLog中的前镜像和业务SQL的相关信息生成并执行回滚语句
  • 提交本地事务,将本地事务的执行结果(分支事务回滚的信息)通知给TC

如果没问题,执行提交操作

  • 收到TC分支提交请求,将请求放入到一个异步任务的队列中,马上返回提交成功的结果给TC
  • 异步任务阶段的分支提交请求删除undoLog中记录

源码入口

接下来,我们就需要从官网中去下载源码,下载地址:https://seata.io/zh-cn/blog/download.html,选择 source 即可,下载完成之后,通过IDEA打开项目。

源码下载下来之后,我们应该如何去找入口呢?首先我们需要找到对应引入的 Seataspring-alibaba-seata,我们在回想一下,我们开启事务的时候,是不是添加过一个@GlobalTransactional的注解,这个注解就是我们入手的一个点,我们在 spring.factories 中看到有一个 GlobalTransactionAutoConfiguration,这个就是我们需要关注的点,也就是我们源码的入口

GlobalTransactionAutoConfiguration 中我们找到一个用Bean注入的方法 globalTransactionScanner ,这个就是全局事务扫描器,这个类型主要负责加载配置,注入相关的Bean

这里给大家展示了当前GlobalTransactionScanner的类关系图,其中我们现在继承了Aop的AbstractAutoProxyCreator类型,在这其中有一个重点方法,这个方法就是判断Bean对象是否需要代理,是否需要增强。

@Configuration
@EnableConfigurationProperties(SeataProperties.class)
public class GlobalTransactionAutoConfiguration 

    //全局事务扫描器
    @Bean
    public GlobalTransactionScanner globalTransactionScanner() 

      String applicationName = applicationContext.getEnvironment()
          .getProperty("spring.application.name");

      String txServiceGroup = seataProperties.getTxServiceGroup();

      if (StringUtils.isEmpty(txServiceGroup)) 
        txServiceGroup = applicationName + "-fescar-service-group";
        seataProperties.setTxServiceGroup(txServiceGroup);
      
      // 构建全局扫描器,传入参数:应用名、事务分组名,失败处理器
      return new GlobalTransactionScanner(applicationName, txServiceGroup);
    

在这其中我们要关心的是 GlobalTransactionScanner 这个类型,这个类型扫描 @GlobalTransactional 注解,并对代理方法进行拦截增强事务的功能。我们就从源码中搜索这个GlobalTransactionScanner类,看看里面具体是做了什么

/**
 * The type Global transaction scanner.
 * 全局事务扫描器
 * @author slievrly
 */
public class GlobalTransactionScanner
        //AbstractAutoProxyCreator AOP动态代理 增强Bean
        extends AbstractAutoProxyCreator
        /**
         * ConfigurationChangeListener: 监听器基准接口
         * InitializingBean: Bean初始化
         * ApplicationContextAware: Spring容器
         * DisposableBean: Spring 容器销毁
         */
        implements ConfigurationChangeListener, InitializingBean, ApplicationContextAware, DisposableBean 

    private final String applicationId;//服务名
    private final String txServiceGroup;//事务分组        

  private void initClient() 
        //启动日志
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Initializing Global Transaction Clients ... ");
        
        //检查应用名以及事务分组名,为空抛出异常IllegalArgumentException
        if (DEFAULT_TX_GROUP_OLD.equals(txServiceGroup)) 
            LOGGER.warn("the default value of seata.tx-service-group:  has already changed to  since Seata 1.5, " +
                    "please change your default configuration as soon as possible " +
                    "and we dont recommend you to use default tx-service-groups value provided by seata",
                    DEFAULT_TX_GROUP_OLD, DEFAULT_TX_GROUP);
        
        if (StringUtils.isNullOrEmpty(applicationId) || StringUtils.isNullOrEmpty(txServiceGroup)) 
            throw new IllegalArgumentException(String.format("applicationId: %s, txServiceGroup: %s", applicationId, txServiceGroup));
        
        //init TM
        //初始化TM
        TMClient.init(applicationId, txServiceGroup, accessKey, secretKey);
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Transaction Manager Client is initialized. applicationId[] txServiceGroup[]", applicationId, txServiceGroup);
        
        //init RM
        //初始化RM
        RMClient.init(applicationId, txServiceGroup);
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Resource Manager is initialized. applicationId[] txServiceGroup[]", applicationId, txServiceGroup);
        

        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Global Transaction Clients are initialized. ");
        
        registerSpringShutdownHook();

    

    @Override
    public void afterPropertiesSet() 
        if (disableGlobalTransaction) 
            if (LOGGER.isInfoEnabled()) 
                LOGGER.info("Global transaction is disabled.");
            
            ConfigurationCache.addConfigListener(ConfigurationKeys.DISABLE_GLOBAL_TRANSACTION,
                    (ConfigurationChangeListener)this);
            return;
        
        if (initialized.compareAndSet(false, true)) 
            initClient();
        
    

   private void initClient() 
        //启动日志
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Initializing Global Transaction Clients ... ");
        
        //检查应用名以及事务分组名,为空抛出异常IllegalArgumentException
        if (DEFAULT_TX_GROUP_OLD.equals(txServiceGroup)) 
            LOGGER.warn("the default value of seata.tx-service-group:  has already changed to  since Seata 1.5, " +
                    "please change your default configuration as soon as possible " +
                    "and we dont recommend you to use default tx-service-groups value provided by seata",
                    DEFAULT_TX_GROUP_OLD, DEFAULT_TX_GROUP);
        

        //检查应用名以及事务分组名,为空抛出异常IllegalArgumentException
        if (StringUtils.isNullOrEmpty(applicationId) || StringUtils.isNullOrEmpty(txServiceGroup)) 
            throw new IllegalArgumentException(String.format("applicationId: %s, txServiceGroup: %s", applicationId, txServiceGroup));
        
        //init TM
        //初始化TM
        TMClient.init(applicationId, txServiceGroup, accessKey, secretKey);
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Transaction Manager Client is initialized. applicationId[] txServiceGroup[]", applicationId, txServiceGroup);
        
        //init RM
        //初始化RM
        RMClient.init(applicationId, txServiceGroup);
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Resource Manager is initialized. applicationId[] txServiceGroup[]", applicationId, txServiceGroup);
        

        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Global Transaction Clients are initialized. ");
        
        registerSpringShutdownHook();

    

    //代理增强,Spring 所有的Bean都会经过这个方法
    @Override
    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) 
        // do checkers
        //检查bean和beanName
        if (!doCheckers(bean, beanName)) 
            return bean;
        

        try 
            //加锁防止并发
            synchronized (PROXYED_SET) 
                if (PROXYED_SET.contains(beanName)) 
                    return bean;
                
                interceptor = null;
                //check TCC proxy
                //检查是否为TCC模式
                if (TCCBeanParserUtils.isTccAutoProxy(bean, beanName, applicationContext)) 
                    // init tcc fence clean task if enable useTccFence
                    //如果启用useTccFence 失败 ,则初始化TCC清理任务
                    TCCBeanParserUtils.initTccFenceCleanTask(TCCBeanParserUtils.getRemotingDesc(beanName), applicationContext);
                    //TCC interceptor, proxy bean of sofa:reference/dubbo:reference, and LocalTCC
                    //如果是,添加TCC拦截器
                    interceptor = new TccActionInterceptor(TCCBeanParserUtils.getRemotingDesc(beanName));
                    ConfigurationCache.addConfigListener(ConfigurationKeys.DISABLE_GLOBAL_TRANSACTION,
                            (ConfigurationChangeListener)interceptor);
                 else 
                    //不是TCC
                    Class<?> serviceInterface = SpringProxyUtils.findTargetClass(bean);
                    Class<?>[] interfacesIfJdk = SpringProxyUtils.findInterfaces(bean);

                    //判断是否有相关事务注解,如果没有不进行代理
                    if (!existsAnnotation(new Class[]serviceInterface)
                        && !existsAnnotation(interfacesIfJdk)) 
                        return bean;
                    

                    //发现存在全局事务注解标注的Bean对象,添加拦截器
                    if (globalTransactionalInterceptor == null) 
                        //添加拦截器
                        globalTransactionalInterceptor = new GlobalTransactionalInterceptor(failureHandlerHook);
                        ConfigurationCache.addConfigListener(
                                ConfigurationKeys.DISABLE_GLOBAL_TRANSACTION,
                                (ConfigurationChangeListener)globalTransactionalInterceptor);
                    
                    interceptor = globalTransactionalInterceptor;
                

                LOGGER.info("Bean[] with name [] would use interceptor []", bean.getClass().getName(), beanName, interceptor.getClass().getName());
                //检查是否为代理对象
                if (!AopUtils.isAopProxy(bean)) 
                    //不是代理对象,调用父级
                    bean = super.wrapIfNecessary(bean, beanName, cacheKey);
                 else 
                    //是代理对象,反射获取代理类中已经存在的拦截器组合,然后添加到这个集合中
                    AdvisedSupport advised = SpringProxyUtils.getAdvisedSupport(bean);
                    Advisor[] advisor = buildAdvisors(beanName, getAdvicesAndAdvisorsForBean(null, null, null));
                    int pos;
                    for (Advisor avr : advisor) 
                        // Find the position based on the advisors order, and add to advisors by pos
                        pos = findAddSeataAdvisorPosition(advised, avr);
                        advised.addAdvisor(pos, avr);
                    
                
                PROXYED_SET.add(beanName);
                return bean;
            
         catch (Exception exx) 
            throw new RuntimeException(exx);
        
    

InitializingBean:中实现了一个 afterPropertiesSet()方法,在这个方法中,调用了initClient()

AbstractAutoProxyCreator:APO动态代理,在之前的的Nacos和Sentiel中都有这个代理类,AOP在我们越往深入学习,在学习源码的会见到的越来越多,越来越重要,很多相关代理,都是通过AOP进行增强,在这个类中,我们需要关注有一个wrapIfNecessary()方法, 这个方法主要是判断被代理的bean或者类是否需要代理增强,在这个方法中会调用GlobalTransactionalInterceptor.invoke()进行带来增强。

具体代码如下:

public class GlobalTransactionalInterceptor implements ConfigurationChangeListener, MethodInterceptor, SeataInterceptor 

    public GlobalTransactionalInterceptor(FailureHandler failureHandler) 
        this.failureHandler = failureHandler == null ? DEFAULT_FAIL_HANDLER : failureHandler;
        this.disable = ConfigurationFactory.getInstance().getBoolean(ConfigurationKeys.DISABLE_GLOBAL_TRANSACTION,
            DEFAULT_DISABLE_GLOBAL_TRANSACTION);
        this.order =
            ConfigurationFactory.getInstance().getInt(ConfigurationKeys.TM_INTERCEPTOR_ORDER, TM_INTERCEPTOR_ORDER);
        degradeCheck = ConfigurationFactory.getInstance().getBoolean(ConfigurationKeys.CLIENT_DEGRADE_CHECK,
            DEFAULT_TM_DEGRADE_CHECK);
        if (degradeCheck) 
            ConfigurationCache.addConfigListener(ConfigurationKeys.CLIENT_DEGRADE_CHECK, this);
            degradeCheckPeriod = ConfigurationFactory.getInstance()
                .getInt(ConfigurationKeys.CLIENT_DEGRADE_CHECK_PERIOD, DEFAULT_TM_DEGRADE_CHECK_PERIOD);
            degradeCheckAllowTimes = ConfigurationFactory.getInstance()
                .getInt(ConfigurationKeys.CLIENT_DEGRADE_CHECK_ALLOW_TIMES, DEFAULT_TM_DEGRADE_CHECK_ALLOW_TIMES);
            EVENT_BUS.register(this);
            if (degradeCheckPeriod > 0 && degradeCheckAllowTimes > 0) 
                startDegradeCheck();
            
        
        this.initDefaultGlobalTransactionTimeout();
    

    @Override
    public Object invoke(final MethodInvocation methodInvocation) throws Throwable 
        //获取执行的方法
        Class<?> targetClass =
            methodInvocation.getThis() != null ? AopUtils.getTargetClass(methodInvocation.getThis()) : null;
        Method specificMethod = ClassUtils.getMostSpecificMethod(methodInvocation.getMethod(), targetClass);
        if (specificMethod != null && !specificMethod.getDeclaringClass().equals(Object.class)) 
            final Method method = BridgeMethodResolver.findBridgedMethod(specificMethod);
            //获取GlobalTransactional(全局事务)、GlobalLock(全局锁)元数据
            final GlobalTransactional globalTransactionalAnnotation =
                getAnnotation(method, targetClass, GlobalTransactional.class);
            //GlobalLock会将本地事务的执行纳入Seata分布式事务的管理,共同竞争全局锁
            //保证全局事务在执行的时候,本地事务不可以操作全局事务的记录
            final GlobalLock globalLockAnnotation = getAnnotation(method, targetClass, GlobalLock.class);//获取全局锁
            boolean localDisable = disable || (degradeCheck && degradeNum >= degradeCheckAllowTimes);
            if (!localDisable) 
                if (globalTransactionalAnnotation != null || this.aspectTransactional != null) 
                    AspectTransactional transactional;
                    if (globalTransactionalAnnotation != null) 
                        transactional = new AspectTransactional(globalTransactionalAnnotation.timeoutMills(),
                            globalTransactionalAnnotation.name(), globalTransactionalAnnotation.rollbackFor(),
                            globalTransactionalAnnotation.noRollbackForClassName(),
                            globalTransactionalAnnotation.noRollbackFor(),
                            globalTransactionalAnnotation.noRollbackForClassName(),
                            globalTransactionalAnnotation.propagation(),
                            globalTransactionalAnnotation.lockRetryInterval(),
                            globalTransactionalAnnotation.lockRetryTimes());
                     else 
                        transactional = this.aspectTransactional;
                    
                    //执行全局事务
                    return handleGlobalTransaction(methodInvocation, transactional);
                 else if (globalLockAnnotation != null) 
                    //执行全局锁
                    return handleGlobalLock(methodInvocation, globalLockAnnotation);
                
            
        
        return methodInvocation.proceed();
    

具体流程图如下所示:

核心源码

在上面我们讲解到 GlobalTransactionalInterceptor 作为全局事务拦截器,一旦执行拦截,就会进入invoke方法,其中,我们会做 @GlobalTransactional 注解的判断,如果有这个注解的存在,会执行全局事务和全局锁,再执行全局事务的时候会调用 handleGlobalTransaction 全局事务处理器,获取事务信息,那我们接下来就来看一下 GlobalTransactionalInterceptor.handleGlobalTransaction 到底是如何执行全局事务的

Object handleGlobalTransaction(final MethodInvocation methodInvocation,
        final AspectTransactional aspectTransactional) throws Throwable 
        boolean succeed = true;
        try 
            return transactionalTemplate.execute(new TransactionalExecutor() 
                @Override
                public Object execute() throws Throwable 
                    return methodInvocation.proceed();
                

                //获取事务名称,默认获取方法名
                public String name() 
                    String name = aspectTransactional.getName();
                    if (!StringUtils.isNullOrEmpty(name)) 
                        return name;
                    
                    return formatMethod(methodInvocation.getMethod());
                

                /**
                 * 解析GlobalTransation注解属性,封装对对象
                 * @return
                 */
                @Override
                public TransactionInfo getTransactionInfo() 
                    // reset the value of timeout
                    //获取超时时间,默认60秒
                    int timeout = aspectTransactional.getTimeoutMills();
                    if (timeout <= 0 || timeout == DEFAULT_GLOBAL_TRANSACTION_TIMEOUT) 
                        timeout = defaultGlobalTransactionTimeout;
                    

                    //构建事务信息对象
                    TransactionInfo transactionInfo = new TransactionInfo();
                    transactionInfo.setTimeOut(timeout);//超时时间
                    transactionInfo.setName(name());//事务名称
                    transactionInfo.setPropagation(aspectTransactional.getPropagation());//事务传播
                    transactionInfo.setLockRetryInterval(aspectTransactional.getLockRetryInterval());//校验或占用全局锁重试间隔
                    transactionInfo.setLockRetryTimes(aspectTransactional.getLockRetryTimes());//校验或占用全局锁重试次数
                    Set<RollbackRule> rollbackRules = new LinkedHashSet<>();
                    //其他构建信息
                    for (Class<?> rbRule : aspectTransactional.getRollbackFor()) 
                        rollbackRules.add(new RollbackRule(rbRule));
                    
                    for (String rbRule : aspectTransactional.getRollbackForClassName()) 
                        rollbackRules.add(new RollbackRule(rbRule));
                    
                    for (Class<?> rbRule : aspectTransactional.getNoRollbackFor()) 
                        rollbackRules.add(new NoRollbackRule(rbRule));
                    
                    for (String rbRule : aspectTransactional.getNoRollbackForClassName()) 
                        rollbackRules.add(new NoRollbackRule(rbRule));
                    
                    transactionInfo.setRollbackRules(rollbackRules);
                    return transactionInfo;
                
            );
         catch (TransactionalExecutor.ExecutionException e) 
            //执行异常
            TransactionalExecutor.Code code = e.getCode();
            switch (code) 
                case RollbackDone:
                    throw e.getOriginalException();
                case BeginFailure:
                    succeed = false;
                    failureHandler.onBeginFailure(e.getTransaction(), e.getCause());
                    throw e.getCause();
                case CommitFailure:
                    succeed = false;
                    failureHandler.onCommitFailure(e.getTransaction(), e.getCause());
                    throw e.getCause();
                case RollbackFailure:
                    failureHandler.onRollbackFailure(e.getTransaction(), e.getOriginalException());
                    throw e.getOriginalException();
                case RollbackRetrying:
                    failureHandler.onRollbackRetrying(e.getTransaction(), e.getOriginalException());
                    throw e.getOriginalException();
                default:
                    throw new ShouldNeverHappenException(String.format("Unknown TransactionalExecutor.Code: %s", code));
            
         finally 
            if (degradeCheck) 
                EVENT_BUS.post(new DegradeCheckEvent(succeed));
            
        
    

在这里我们,主要关注一个重点方法 execute() ,这个方法主要用来执行事务的具体流程:

  • 获取事务信息
  • 执行全局事务
  • 发生异常全局回滚,各个数据通过UndoLog进行事务补偿
  • 全局事务提交
  • 清除所有资源

这个位置也是一个非常核心的一个位置,因为我们所有的业务进来以后都会去走这个位置,具体源码如下所示:

    public Object execute(TransactionalExecutor business) throws Throwable 
        // 1. Get transactionInfo
        //获取事务信息
        TransactionInfo txInfo = business.getTransactionInfo();
        if (txInfo == null) 
            throw new ShouldNeverHappenException("transactionInfo does not exist");
        
        // 1.1 Get current transaction, if not null, the tx role is GlobalTransactionRole.Participant.
        //获取当前事务,主要获取XID
        GlobalTransaction tx = GlobalTransactionContext.getCurrent();

        // 1.2 Handle the transaction propagation.
        //根据配置的不同事务传播行为,执行不同的逻辑
        Propagation propagation = txInfo.getPropagation();
        SuspendedResourcesHolder suspendedResourcesHolder = null;
        try 
            switch (propagation) 
                case NOT_SUPPORTED:
                    // If transaction is existing, suspend it.
                    if (existingTransaction(tx)) 
                        suspendedResourcesHolder = tx.suspend();
                    
                    // Execute without transaction and return.
                    return business.execute();
                case REQUIRES_NEW:
                    // If transaction is existing, suspend it, and then begin new transaction.
                    if (existingTransaction(tx)) 
                        suspendedResourcesHolder = tx.suspend();
                        tx = GlobalTransactionContext.createNew();
                    
                    // Continue and execute with new transaction
                    break;
                case SUPPORTS:
                    // If transaction is not existing, execute without transaction.
                    if (notExistingTransaction(tx)) 
                        return business.execute();
                    
                    // Continue and execute with new transaction
                    break;
                case REQUIRED:
                    // If current transaction is existing, execute with current transaction,
                    // else continue and execute with new transaction.
                    break;
                case NEVER:
                    // If transaction is existing, throw exception.
                    if (existingTransaction(tx)) 
                        throw new TransactionException(
                            String.format("Existing transaction found for transaction marked with propagation never, xid = %s"
                                    , tx.getXid()));
                     else 
                        // Execute without transaction and return.
                        return business.execute();
                    
                case MANDATORY:
                    // If transaction is not existing, throw exception.
                    if (notExistingTransaction(tx)) 
                        throw new TransactionException("No existing transaction found for transaction marked with propagation mandatory");
                    
                    // Continue and execute with current transaction.
                    break;
                default:
                    throw new TransactionException("Not Supported Propagation:" + propagation);
            

            // 1.3 If null, create new transaction with role GlobalTransactionRole.Launcher.
            //如果当前事务为空,创建一个新的事务
            if (tx == null) 
                tx = GlobalTransactionContext.createNew();
            

            // set current tx config to holder
            GlobalLockConfig previousConfig = replaceGlobalLockConfig(txInfo);

            try 
                // 2. If the tx role is GlobalTransactionRole.Launcher, send the request of beginTransaction to TC,
                //    else do nothing. Of course, the hooks will still be triggered.
                //开始执行全局事务
                beginTransaction(txInfo, tx);

                Object rs;
                try 
                    // Do Your Business
                    // 执行当前业务逻辑
                    //1、在TC注册当前分支事务,TC会在branch_table中插入一条分支事务数据
                    //2、执行本地update语句,并在执行前后查询数据状态,并把数据前后镜像存入到undo_log中
                    //3、远程调用其他应用,远程应用接收到XID,也会注册分支事务,写入branch_table以及本地undo_log表
                    //4、会在lock_table表中插入全局锁数据(一个分支一条)
                    rs = business.execute();
                 catch (Throwable ex) 
                    // 3. The needed business exception to rollback.
                    //发生异常全局回滚,每个事务通过undo_log表进行事务补偿
                    completeTransactionAfterThrowing(txInfo, tx, ex);
                    throw ex;
                

                // 4. everything is fine, commit.
                //全局提交
                commitTransaction(tx);

                return rs;
             finally 
                //5. clear
                //清理所有资源
                resumeGlobalLockConfig(previousConfig);
                triggerAfterCompletion();
                cleanUp();
            
         finally 
            // If the transaction is suspended, resume it.
            if (suspendedResourcesHolder != null) 
                tx.resume(suspendedResourcesHolder);
            
        
    

这其中的第三步和第四步其实在向 TC(Seata-Server)发起全局事务的提交或者回滚,在这里我们首先关注执行全局事务的 beginTransaction() 方法

    // 向TC发起请求,采用模板模式
    private void beginTransaction(TransactionInfo txInfo, GlobalTransaction tx) throws TransactionalExecutor.ExecutionException 
        try 
            triggerBeforeBegin();
            //对TC发起请求
            tx.begin(txInfo.getTimeOut(), txInfo.getName());
            triggerAfterBegin();
         catch (TransactionException txe) 
            throw new TransactionalExecutor.ExecutionException(tx, txe,
                TransactionalExecutor.Code.BeginFailure);

        
    

在来关注其中,向TC发起请求的 tx.begin() 方法,而调用begin()方法的类为:DefaultGlobalTransaction

 @Override
    public void begin(int timeout, String name) throws TransactionException 
        //判断调用者是否为TM
        if (role != GlobalTransactionRole.Launcher) 
            assertXIDNotNull();
            if (LOGGER.isDebugEnabled()) 
                LOGGER.debug("Ignore Begin(): just involved in global transaction []", xid);
            
            return;
        
        assertXIDNull();
        String currentXid = RootContext.getXID();
        if (currentXid != null) 
            throw new IllegalStateException("Global transaction already exists," +
                " cant begin a new global transaction, currentXid = " + currentXid);
        
        //获取XID
        xid = transactionManager.begin(null, null, name, timeout);
        status = GlobalStatus.Begin;
        //绑定XID
        RootContext.bind(xid);
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Begin new global transaction []", xid);
        
    

再来看一下 transactionManager.begin() 方法,这个时候使用的是 DefaultTransactionManager.begin 默认的事务管理者,来获取XID,传入事务相关的信息 ,最好TC返回对应的全局事务XID,它调用的是DefaultTransactionManager.begin()方法

    public String begin(String applicationId, String transactionServiceGroup, String name, int timeout)
        throws TransactionException 
        GlobalBeginRequest request = new GlobalBeginRequest();
        request.setTransactionName(name);
        request.setTimeout(timeout);
        //发送请求得到响应
        GlobalBeginResponse response = (GlobalBeginResponse) syncCall(request);
        if (response.getResultCode() == ResultCode.Failed) 
            throw new TmTransactionException(TransactionExceptionCode.BeginFailed, response.getMsg());
        
        //返回XID
        return response.getXid();
    

在这里我们需要关注一个syncCall,在这里采用的是Netty通讯方式

    private AbstractTransactionResponse syncCall(AbstractTransactionRequest request) throws TransactionException 
        try 
            // 通过Netty发送请求
            return (AbstractTransactionResponse) TmNettyRemotingClient.getInstance().sendSyncRequest(request);
         catch (TimeoutException toe) 
            throw new TmTransactionException(TransactionExceptionCode.IO, "RPC timeout", toe);
        
    

具体图解如下:

在这里我们需要重点了解 GlobalTransactionScanner 这个类型,在这个类型中继承了一些接口和抽象类,这个类主要作用就是扫描有注解的Bean,并做AOP增强。

  • ApplicationContextAware:继承这个类型以后,需要实现其方法 setApplicationContext(),当Spring启动完成以后,会自动调用这个类型,将 ApplicationContextbean,也就是说, GlobalTransactionScanner 能够很自然的使用Spring环境

  • InitializingBean: 继承这个接口,需要实现 afterPropertiesSet() ,但凡是继承这个接口的类,在初始化的时候,当所有的 properties 设置完成以后,会执行这个方法

  • DisposableBean : 这个类,实现了一个 destroy() 这个方法是在销毁的时候去调用

  • AbstractAutoProxyCreator: 这个类是Spring实现AOP的一种方式,本质上是一个 BeanPostProcessor ,在Bean初始化至去年,调用内部 createProxy() ,创建一个Bean的AOP代理Bean并返回,对Bean进行增强。

Seata数据源代理

在上面的环节中,我们讲解了Seata AT模式2PC的执行流程,那么现在我们就来带大家了解一下关于AT数据源代理的信息,这也是AT模式中非常关键的一个重要知识点,大家可以拿起小本子,记下来。

首先AT模式的核心主要分为一下两个

  • 开启全局事务,获取全局锁。
  • 解析SQL并写入undoLog中。

关于第一点我们已经分析清楚了,第二点就是关于AT模式如何解析SQL并写入undoLog中,但是在这之前,我们需要知道Seata是如何选择数据源,并进行数据源代理的。虽然全局事务拦截成功后最终还是执行了业务方法进行SQL提交和操作,但是由于Seata对数据源进行了代理,所以SQL的解析和undoLog的操作,是在数据源代理中进行完成的。

数据源代理是Seata中一个非常重要的知识点,在分布式事务运行过程中,undoLog的记录、资源的锁定,用户都是无感知的,因为这些操作都是数据源的代理中完成了,恰恰是这样,我们才要去了解,这样不仅有利于我们了解Seata的核心操作,还能对以后源码阅读有所帮助,因为其实很多底层代码都会去使用这样用户无感知的方式(代理)去实现。

同样,我们在之前的寻找源码入口的时候,通过我们项目中引入的jar找到一个 SeataAutoConfiguration 类,我们在里面找到一个SeataDataSourceBeanPostProcessor(),这个就是我们数据源代理的入口方法

我们进入SeataDataSourceBeanPostProcessor类里面,发现继承了一个 BeanPostProcessor ,这个接口我们应该很熟悉,这个是Sprng的拓展接口,所有的Bean对象,都有进入两个方法 postProcessAfterInitialization()postProcessBeforeInitialization() 这两个方法都是由 BeanPostProcessor提供的,这两个方法,一个是初始化之前执行Before。一个是在初始化之后执行After,主要用来对比我们的的Bean是否为数据源代理对象。

在这里我们需要关注到一个postProcessAfterInitialization.proxyDataSource() 方法,这个里面

    private Object proxyDataSource(Object originBean) 
        DataSourceProxy dataSourceProxy = DataSourceProxyHolder.get().putDataSource((DataSource) originBean);
        if (this.useJdkProxy) 
            return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), SpringProxyUtils.getAllInterfaces(originBean), (proxy, method, args) -> handleMethodProxy(dataSourceProxy, method, args, originBean));
         else 
            return Enhancer.create(originBean.getClass(), (MethodInterceptor) (proxy, method, args, methodProxy) -> handleMethodProxy(dataSourceProxy, method, args, originBean));
        

    

这里有一个DataSourceProxy代理对象,我们需要看的就是这个类,这个就是我们数据库代理的对象,我们从我们下载的源码项目中,搜索这个代理对象,当我们打开这个类的目录时发现,除了这个,还有ConnectionProxy 连接对象、StatementProxyPreparedStatementProxy SQL执行对象,这些都被Seata进行了代理,为什么要对这些都进行代理,代理的目的其实为了执行Seata的业务逻辑,生成undoLog,全局事务的开启,事务的提交回滚等操作

DataSourceProxy 具体做了什么,主要功能有哪些,我们来看一下。他在源码中是如何体现的,我们需要关注的是init()

public class DataSourceProxy extends AbstractDataSourceProxy implements Resource 

    private String resourceGroupId;

    private void init(DataSource dataSource, String resourceGroupId) 
        //资源组ID,默认是“default”这个默认值
        this.resourceGroupId = resourceGroupId;
        try (Connection connection = dataSource.getConnection()) 
            //根据原始数据源得到JDBC连接和数据库类型
            jdbcUrl = connection.getMetaData().getURL();
            dbType = JdbcUtils.getDbType(jdbcUrl);
            if (JdbcConstants.ORACLE.equals(dbType)) 
                userName = connection.getMetaData().getUserName();
             else if (JdbcConstants.MARIADB.equals(dbType)) 
                dbType = JdbcConstants.mysql;
            
         catch (SQLException e) 
            throw new IllegalStateException("can not init dataSource", e);
        
        initResourceId();
        DefaultResourceManager.get().registerResource(this);
        if (ENABLE_TABLE_META_CHECKER_ENABLE) 
            //如果配置开关打开,会定时在线程池不断更新表的元数据缓存信息
            tableMetaExecutor.scheduleAtFixedRate(() -> 
                try (Connection connection = dataSource.getConnection()) 
                    TableMetaCacheFactory.getTableMetaCache(DataSourceProxy.this.getDbType())
                        .refresh(connection, DataSourceProxy.this.getResourceId());
                 catch (Exception ignore) 
                
            , 0, TABLE_META_CHECKER_INTERVAL, TimeUnit.MILLISECONDS);
        

        //Set the default branch type to AT in the RootContext.
        RootContext.setDefaultBranchType(this.getBranchType());
    

从上面我们可以看出,他主要做了以下几点的增强:

  1. 给每个数据源标识资源组ID
  2. 如果打开配置,会有一个定时线程池定时更新表的元数据信息并缓存到本地
  3. 生成代理连接 ConnectionProxy 对象

在这三个增强功能里面,第三个是最重要的,在AT模式里面,会自动记录undoLog,资源锁定,都是通过ConnectionProxy完成的,除此之外 DataSrouceProxy重写了一个方法 getConnection,因为这里返回的是一个 ConnectionProxy,而不是原生的Connection

    @Override
    public ConnectionProxy getConnection() throws SQLException 
        Connection targetConnection = targetDataSource.getConnection();
        return new ConnectionProxy(this, targetConnection);
    

    @Override
    public ConnectionProxy getConnection(String username, String password) throws SQLException 
        Connection targetConnection = targetDataSource.getConnection(username, password);
        return new ConnectionProxy(this, targetConnection);
    

ConnectionProxy

ConnectionProxy 继承 AbstractConnectionProxy ,在这个父类中有很多公用的方法,在这个父类有 PreparedStatementProxyStatementProxyDataSourceProxy

所以我们需要先来看一下AbstractConnectionProxy,因为这里封装了需要我们用到的通用方法和逻辑,在其中我们需要关注的主要在于 PreparedStatementProxyStatementProxy ,在这里的逻辑主要是数据源连接的步骤,连接获取,创建执行对象等等

    @Override
    public Statement createStatement() throws SQLException 
        //调用真实连接对象获取Statement对象
        Statement targetStatement = getTargetConnection().createStatement();
        //创建Statement的代理
        return new StatementProxy(this, targetStatement);
    

    @Override
    public PreparedStatement prepareStatement(String sql) throws SQLException 
        //获取数据库类型 mysql/oracle
        String dbType = getDbType();
        // support oracle 10.2+
        PreparedStatement targetPreparedStatement = null;
        //如果是AT模式且开启全局事务
        if (BranchType.AT == RootContext.getBranchType()) 
            List<SQLRecognizer> sqlRecognizers = SQLVisitorFactory.get(sql, dbType);
            if (sqlRecognizers != null && sqlRecognizers.size() == 1) 
                SQLRecognizer sqlRecognizer = sqlRecognizers.get(0);
                if (sqlRecognizer != null && sqlRecognizer.getSQLType() == SQLType.INSERT) 
                    //获取表的元数据
                    TableMeta tableMeta = TableMetaCacheFactory.getTableMetaCache(dbType).getTableMeta(getTargetConnection(),
                            sqlRecognizer.getTableName(), getDataSourceProxy().getResourceId());
                    //得到表的主键列名
                    String[] pkNameArray = new String[tableMeta.getPrimaryKeyOnlyName().size()];
                    tableMeta.getPrimaryKeyOnlyName().toArray(pkNameArray);
                    targetPreparedStatement = getTargetConnection().prepareStatement(sql,pkNameArray);
                
            
        
        if (targetPreparedStatement == null) 
            targetPreparedStatement = getTargetConnection().prepareStatement(sql);
        
        //创建PreparedStatementProxy代理
        return new PreparedStatementProxy(this, targetPreparedStatement, sql);
    

在这两个代理对象中,都用到了以下几个方法:

@Override
public ResultSet executeQuery(String sql) throws SQLException 
    this.targetSQL = sql;
    return ExecuteTemplate.execute(this, (statement, args) -> statement.executeQuery((String) args[0]), sql);


@Override
public int executeUpdate(String sql) throws SQLException 
    this.targetSQL = sql;
    return ExecuteTemplate.execute(this, (statement, args) -> statement.executeUpdate((String) args[0]), sql);


@Override
public boolean execute(String sql) throws SQLException 
    this.targetSQL = sql;
    return ExecuteTemplate.execute(this, (statement, args) -> statement.execute((String) args[0]), sql);

在这些方法中都调用了 ExecuteTemplate.execute(),所以我们就看一下在 ExecuteTemplate类中具体是做了什么操作:

public class ExecuteTemplate 

    public static <T, S extends Statement> T execute(List<SQLRecognizer> sqlRecognizers,
                                                     StatementProxy<S> statementProxy,
                                                     StatementCallback<T, S> statementCallback,
                                                     Object... args) throws SQLException 
        //如果没有全局锁,并且不是AT模式,直接执行SQL
        if (!RootContext.requireGlobalLock() && BranchType.AT != RootContext.getBranchType()) 
            // Just work as original statement
            return statementCallback.execute(statementProxy.getTargetStatement(), args);
        

        //得到数据库类型- mysql/oracle
        String dbType = statementProxy.getConnectionProxy().getDbType();
        if (CollectionUtils.isEmpty(sqlRecognizers)) 
            //sqlRecognizers 为SQL语句的解析器,获取执行的SQL,通过它可以获得SQL语句表名、相关的列名、类型等信息,最后解析出对应的SQL表达式
            sqlRecognizers = SQLVisitorFactory.get(
                    statementProxy.getTargetSQL(),
                    dbType);
        
        Executor<T> executor;
        if (CollectionUtils.isEmpty(sqlRecognizers)) 
            //如果seata没有找到合适的SQL语句解析器,那么便创建简单执行器PlainExecutor
            //PlainExecutor直接使用原生的Statment对象执行SQL
            executor = new PlainExecutor<>(statementProxy, statementCallback);
         else 
            if (sqlRecognizers.size() == 1) 
                SQLRecognizer sqlRecognizer = sqlRecognizers.get(0);
                switch (sqlRecognizer.getSQLType()) 
                    //新增
                    case INSERT:
                        executor = EnhancedServiceLoader.load(InsertExecutor.class, dbType,
                                    new Class[]StatementProxy.class, StatementCallback.class, SQLRecognizer.class,
                                    new Object[]statementProxy, statementCallback, sqlRecognizer);
                        break;
                        //修改
                    case UPDATE:
                        executor = new UpdateExecutor<>(statementProxy, statementCallback, sqlRecognizer);
                        break;
                        //删除
                    case DELETE:
                        executor = new DeleteExecutor<>(statementProxy, statementCallback, sqlRecognizer);
                        break;
                        //加锁
                    case SELECT_FOR_UPDATE:
                        executor = new SelectForUpdateExecutor<>(statementProxy, statementCallback, sqlRecognizer);
                        break;
                        //插入加锁
                    case INSERT_ON_DUPLICATE_UPDATE:
                        switch (dbType) 
                            case JdbcConstants.MYSQL:
                            case JdbcConstants.MARIADB:
                                executor =
                                    new MySQLInsertOrUpdateExecutor(statementProxy, statementCallback, sqlRecognizer);
                                break;
                            default:
                                throw new NotSupportYetException(dbType + " not support to INSERT_ON_DUPLICATE_UPDATE");
                        
                        break;
                        //原生
                    default:
                        executor = new PlainExecutor<>(statementProxy, statementCallback);
                        break;
                
             else 
                //批量处理SQL语句
                executor = new MultiExecutor<>(statementProxy, statementCallback, sqlRecognizers);
            
        
        T rs;
        try 
            //执行
            rs = executor.execute(args);
         catch (Throwable ex) 
            if (!(ex instanceof SQLException)) 
                // Turn other exception into SQLException
                ex = new SQLException(ex);
            
            throw (SQLException) ex;
        
        return rs;
    

ExecuteTemplate就一个 execute(),Seata将SQL执行委托给不同的执行器(模板),Seata提供了6种执行器也就是我们代码 case 中(INSERTUPDATEDELETESELECT_FOR_UPDATE,INSERT_ON_DUPLICATE_UPDATE),这些执行器的父类都是AbstractDMLBaseExecutor

  • UpdateExecutor: 执行update语句
  • InsertExecutor: 执行insert语句
  • DeleteExecutor: 执行delete语句
  • SelectForUpdateExecutor: 执行select for update语句
  • PlainExecutor: 执行普通查询语句
  • MultiExecutor: 复合执行器,在一条SQL语句中执行多条语句

关系图如下:

然后我们找到rs = executor.execute(args); 最终执行的方法,找到最顶级的父类BaseTransactionalExecutor.execute()

    @Override
    public T execute(Object... args) throws Throwable 
        String xid = RootContext.getXID();
        if (xid != null) 
            //获取XID
            statementProxy.getConnectionProxy().bind(xid);
        
        //设置全局锁
        statementProxy.getConnectionProxy().setGlobalLockRequire(RootContext.requireGlobalLock());
        return doExecute(args);
    

在根据doExecute(args);找到其中的重写方法 AbstractDMLBaseExecutor.doExecute()

    @Override
    public T doExecute(Object... args) throws Throwable 
        AbstractConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
        //是否自动提交
        if (connectionProxy.getAutoCommit()) 
            return executeAutoCommitTrue(args);
         else 
            return executeAutoCommitFalse(args);
        
    

对于数据库而言,本身都是自动提交的,所以我们进入executeAutoCommitTrue()

    protected T executeAutoCommitTrue(Object[] args) throws Throwable 
        ConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
        try 
            //设置为手动提交
            connectionProxy.changeAutoCommit();
            return new LockRetryPolicy(connectionProxy).execute(() -> 
                //调用手动提交方法,得到分支执行的最终结果
                T result = executeAutoCommitFalse(args);
                //执行提交
                connectionProxy.commit();
                return result;
            );
         catch (Exception e) 
            // when exception occur in finally,this exception will lost, so just print it here
            LOGGER.error("execute executeAutoCommitTrue error:", e.getMessage(), e);
            if (!LockRetryPolicy.isLockRetryPolicyBranchRollbackOnConflict()) 
                connectionProxy.getTargetConnection().rollback();
            
            throw e;
         finally 
            connectionProxy.getContext().reset();
            connectionProxy.setAutoCommit(true);
        
    

connectionProxy.changeAutoCommit()方法,修改为手动提交后,我们看来最关键的代码executeAutoCommitFalse()

    protected T executeAutoCommitFalse(Object[] args) throws Exception 
        if (!JdbcConstants.MYSQL.equalsIgnoreCase(getDbType()) && isMultiPk()) 
            throw new NotSupportYetException("multi pk only support mysql!");
        
        //获取前镜像
        TableRecords beforeImage = beforeImage();
        //执行具体业务
        T result = statementCallback.execute(statementProxy.getTargetStatement(), args);
        //获取执行数量
        int updateCount = statementProxy.getUpdateCount();
        //判断如果执行数量大于0
        if (updateCount > 0) 
            //获取后镜像
            TableRecords afterImage = afterImage(beforeImage);
            //暂存到undolog中,在Commit的时候保存到数据库
            prepareUndoLog(beforeImage, afterImage);
        
        return result;
    

我们再回到executeAutoCommitTrue中,去看看提交做了哪些操作connectionProxy.commit();

    @Override
    public void commit() throws SQLException 
        try 
            lockRetryPolicy.execute(() -> 
                //具体执行
                doCommit();
                return null;
            );
         catch (SQLException e) 
            if (targetConnection != null && !getAutoCommit() && !getContext().isAutoCommitChanged()) 
                rollback();
            
            throw e;
         catch (Exception e) 
            throw new SQLException(e);
        
    

进入到doCommit()

    private void doCommit() throws SQLException 
        //判断是否存在全局事务
        if (context.inGlobalTransaction()) 
            processGlobalTransactionCommit();
         else if (context.isGlobalLockRequire()) 
            processLocalCommitWithGlobalLocks();
         else 
            targetConnection.commit();
        
    

作为分布式事务,一定是存在全局事务的,所以我们进入 processGlobalTransactionCommit()

  private void processGlobalTransactionCommit() throws SQLException 
        try 
            //注册分支事务
            register();
         catch (TransactionException e) 
            recognizeLockKeyConflictException(e, context.buildLockKeys());
        
        try 
            //写入数据库undolog
            UndoLogManagerFactory.getUndoLogManager(this.getDbType()).flushUndoLogs(this);
            //执行原生提交 一阶段提交
            targetConnection.commit();
         catch (Throwable ex) 
            LOGGER.error("process connectionProxy commit error: ", ex.getMessage(), ex);
            report(false);
            throw new SQLException(ex);
        
        if (IS_REPORT_SUCCESS_ENABLE) 
            report(true);
        
        context.reset();
    

其中register()方法就是注册分支事务的方法,同时还会将undoLog写入数据库和执行提交等操作


    //注册分支事务,生成分支事务ID
    private void register() throws TransactionException 
        if (!context.hasUndoLog() || !context.hasLockKey()) 
            return;
        
        //注册分支事务
        Long branchId = DefaultResourceManager.get().branchRegister(BranchType.AT, getDataSourceProxy().getResourceId(),
            null, context.getXid(), context.getApplicationData(), context.buildLockKeys());
        context.setBranchId(branchId);
    

然后我们在回到processGlobalTransactionCommit中,看看写入数据库中的flushUndoLogs()

 @Override
    public void flushUndoLogs(ConnectionProxy cp) throws SQLException 
        ConnectionContext connectionContext = cp.getContext();
        if (!connectionContext.hasUndoLog()) 
            return;
        
        //获取XID
        String xid = connectionContext.getXid();
        //获取分支ID
        long branchId = connectionContext.getBranchId();

        BranchUndoLog branchUndoLog = new BranchUndoLog();
        branchUndoLog.setXid(xid);
        branchUndoLog.setBranchId(branchId);
        branchUndoLog.setSqlUndoLogs(connectionContext.getUndoItems());

        UndoLogParser parser = UndoLogParserFactory.getInstance();
        byte[] undoLogContent = parser.encode(branchUndoLog);

        if (LOGGER.isDebugEnabled()) 
            LOGGER.debug("Flushing UNDO LOG: ", new String(undoLogContent, Constants.DEFAULT_CHARSET));
        

        CompressorType compressorType = CompressorType.NONE;
        if (needCompress(undoLogContent)) 
            compressorType = ROLLBACK_INFO_COMPRESS_TYPE;
            undoLogContent = CompressorFactory.getCompressor(compressorType.getCode()).compress(undoLogContent);
        
        //写入数据库具体位置
        insertUndoLogWithNormal(xid, branchId, buildContext(parser.getName(), compressorType), undoLogContent, cp.getTargetConnection());
    

具体写入方法,此时我们使用的是MySql,所以执行的是MySql实现类MySQLUndoLogManager.insertUndoLogWithNormal()

    @Override
    protected void insertUndoLogWithNormal(String xid, long branchId, String rollbackCtx, byte[] undoLogContent,
                                           Connection conn) throws SQLException 
        insertUndoLog(xid, branchId, rollbackCtx, undoLogContent, State.Normal, conn);
    

        //具体写入操作
    private void insertUndoLog(String xid, long branchId, String rollbackCtx, byte[] undoLogContent,
                               State state, Connection conn) throws SQLException 
        try (PreparedStatement pst = conn.prepareStatement(INSERT_UNDO_LOG_SQL)) 
            pst.setLong(1, branchId);
            pst.setString(2, xid);
            pst.setString(3, rollbackCtx);
            pst.setBytes(4, undoLogContent);
            pst.setInt(5, state.getValue());
            pst.executeUpdate();
         catch (Exception e) 
            if (!(e instanceof SQLException)) 
                e = new SQLException(e);
            
            throw (SQLException) e;
        
    

具体流程如下所示:

Seata 服务端

我们找到Server.java 这里就是启动入口,在这个入口中找到协调者,因为TC整体的操作就是协调整体的全局事务

  //默认协调者
        DefaultCoordinator coordinator = DefaultCoordinator.getInstance(nettyRemotingServer);

DefaultCoordinator类中我们找到 一个doGlobalBegin 这个就是处理全局事务开始的方法,以及全局提交 doGlobalCommit 和全局回滚 doGlobalRollback

    //处理全局事务
    @Override
    protected void doGlobalBegin(GlobalBeginRequest request, GlobalBeginResponse response, RpcContext rpcContext)
            throws TransactionException 
        //响应客户端xid
        response.setXid(core.begin(rpcContext.getApplicationId(), rpcContext.getTransactionServiceGroup(),
                request.getTransactionName(), request.getTimeout()));
        if (LOGGER.isInfoEnabled()) 
            LOGGER.info("Begin new global transaction applicationId: ,transactionServiceGroup: , transactionName: ,timeout:,xid:",
                    rpcContext.getApplicationId(), rpcContext.getTransactionServiceGroup(), request.getTransactionName(), request.getTimeout(), response.getXid());
        
    

        //处理全局提交
    @Override
    protected void doGlobalCommit(GlobalCommitRequest request, GlobalCommitResponse response, RpcContext rpcContext)
            throws TransactionException 
        MDC.put(RootContext.MDC_KEY_XID, request.getXid());
        response.setGlobalStatus(core.commit(request.getXid()));
    

    //处理全局回滚
    @Override
    protected void doGlobalRollback(GlobalRollbackRequest request, GlobalRollbackResponse response,
                                    RpcContext rpcContext) throws TransactionException 
        MDC.put(RootContext.MDC_KEY_XID, request.getXid());
        response.setGlobalStatus(core.rollback(request.getXid()));
    

在这里我们首先关注 doGlobalBegincore.begin()

    @Override
    public String begin(String applicationId, String transactionServiceGroup, String name, int timeout)
        throws TransactionException 
        //创建全局事务Session
        GlobalSession session = GlobalSession.createGlobalSession(applicationId, transactionServiceGroup, name,
            timeout);
        MDC.put(RootContext.MDC_KEY_XID, session.getXid());

        //为Session重添加回调监听,SessionHolder.getRootSessionManager() 获取一个全局Session管理器DataBaseSessionManager
        //观察者设计模式,创

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