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摘要: 本文主要基于 spring cloud + spring jpa + spring cloud alibaba fescar + mysql + seata 的结构,搭建一个分布式系统的 demo,通过 seata 的 debug 日志和源代码,从 client 端(RM、TM)的角度分析其工作流程及原理。

前言

在分布式系统中,分布式事务是一个必须要解决的问题,目前使用较多的是最终一致性方案。自年初阿里开源了Fescar(四月初更名为Seata)后,该项目受到了极大的关注,目前已接近 8000 Star。Seata以高性能和零侵入的特性为目标解决微服务领域的分布式事务难题,目前正处于快速迭代中,近期小目标是生产可用的 Mysql 版本。

本文主要基于 spring cloud + spring jpa + spring cloud alibaba fescar + mysql + seata 的结构,搭建一个分布式系统的 demo,通过 seata 的 debug 日志和源代码,从 client 端(RM、TM)的角度分析其工作流程及原理。(示例项目:https://github.com/fescar-group/fescar-samples/tree/master/springcloud-jpa-seata

为了更好地理解全文,我们来熟悉一下相关概念:

  • XID:全局事务的唯一标识,由 ip:port:sequence 组成;
  • Transaction Coordinator (TC):事务协调器,维护全局事务的运行状态,负责协调并驱动全局事务的提交或回滚;
  • Transaction Manager (TM ):控制全局事务的边界,负责开启一个全局事务,并最终发起全局提交或全局回滚的决议;
  • Resource Manager (RM):控制分支事务,负责分支注册、状态汇报,并接收事务协调器的指令,驱动分支(本地)事务的提交和回滚;
提示:文中代码是基于 fescar-0.4.1 版本,由于项目刚更名为 seata 不久,其中一些包名、类名、jar包等名称还没统一更换过来,故下文中仍使用 fescar 进行表述。

分布式框架支持

Fescar 使用 XID 表示一个分布式事务,XID 需要在一次分布式事务请求所涉的系统中进行传递,从而向 feacar-server 发送分支事务的处理情况,以及接收 feacar-server 的 commit、rollback 指令。 Fescar 官方已支持全版本的 dubbo 协议,而对于 spring cloud(spring-boot)的分布式项目社区也提供了相应的实现

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-alibaba-fescar</artifactId>
    <version>2.1.0.BUILD-SNAPSHOT</version>
</dependency>

该组件实现了基于 RestTemplate、Feign 通信时的 XID 传递功能。

业务逻辑

业务逻辑是经典的下订单、扣余额、减库存流程。 根据模块划分为三个独立的服务,且分别连接对应的数据库:

  • 订单:order-server
  • 账户:account-server
  • 库存:storage-server

另外还有发起分布式事务的业务系统:

  • 业务:business-server

项目结构如下图

正常业务:

  1. business发起购买请求
  2. storage扣减库存
  3. order创建订单
  4. account扣减余额

异常业务

  1. business发起购买请求
  2. storage扣减库存
  3. order创建订单
  4. account扣减余额异常

正常流程下 2、3、4 步的数据正常更新全局 commit,异常流程下的数据则由于第 4 步的异常报错全局回滚。

配置文件

fescar 的配置入口文件是 registry.conf, 查看代码 ConfigurationFactory 得知目前还不能指定该配置文件,所以配置文件名称只能为 registry.conf。

private static final String REGISTRY_CONF = "registry.conf";
public static final Configuration FILE_INSTANCE = new FileConfiguration(REGISTRY_CONF);

在 registry 中可以指定具体配置的形式,默认使用 file 类型,在 file.conf 中有 3 部分配置内容:

  1. transport transport 部分的配置对应 NettyServerConfig 类,用于定义 Netty 相关的参数,TM、RM 与 fescar-server 之间使用 Netty 进行通信。
  2. service

    service {
     #vgroup->rgroup
     vgroup_mapping.my_test_tx_group = "default"
     #配置Client连接TC的地址
     default.grouplist = "127.0.0.1:8091"
     #degrade current not support
     enableDegrade = false
     #disable
     是否启用seata的分布式事务
     disableGlobalTransaction = false
    }
  3. client

    client {
      #RM接收TC的commit通知后缓冲上限
      async.commit.buffer.limit = 10000
      lock {
        retry.internal = 10
        retry.times = 30
      }
    }

数据源 Proxy

除了前面的配置文件,fescar 在 AT 模式下稍微有点代码量的地方就是对数据源的代理指定,且目前只能基于DruidDataSource的代理。 (注:在最新发布的 0.4.2 版本中已支持任意数据源类型)

@Bean
@ConfigurationProperties(prefix = "spring.datasource")
public DruidDataSource druidDataSource() {
    DruidDataSource druidDataSource = new DruidDataSource();
    return druidDataSource;
}

@Primary
@Bean("dataSource")
public DataSourceProxy dataSource(DruidDataSource druidDataSource) {
    return new DataSourceProxy(druidDataSource);
}

使用 DataSourceProxy 的目的是为了引入 ConnectionProxy ,fescar 无侵入的一方面就体现在 ConnectionProxy 的实现上,即分支事务加入全局事务的切入点是在本地事务的 commit 阶段,这样设计可以保证业务数据与 undo_log 是在一个本地事务中。

undo_log 是需要在业务库上创建的一个表,fescar 依赖该表记录每笔分支事务的状态及二阶段 rollback 的回放数据。不用担心该表的数据量过大形成单点问题,在全局事务 commit 的场景下事务对应的 undo_log 会异步删除。

CREATE TABLE `undo_log` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `branch_id` bigint(20) NOT NULL,
  `xid` varchar(100) NOT NULL,
  `rollback_info` longblob NOT NULL,
  `log_status` int(11) NOT NULL,
  `log_created` datetime NOT NULL,
  `log_modified` datetime NOT NULL,
  `ext` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `ux_undo_log` (`xid`,`branch_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

启动 Server

前往https://github.com/seata/seata/releases 下载与 Client 版本对应的 fescar-server,避免由于版本的不同导致的协议不一致问题 进入解压之后的 bin 目录,执行:

./fescar-server.sh 8091 ../data

启动成功输出:

2019-04-09 20:27:24.637 INFO [main]c.a.fescar.core.rpc.netty.AbstractRpcRemotingServer.start:152 -Server started ... 

启动 Client

fescar 的加载入口类位于 GlobalTransactionAutoConfiguration,对基于 spring boot 的项目能够自动加载,当然也可以通过其他方式示例化 GlobalTransactionScanner

@Configuration
@EnableConfigurationProperties({FescarProperties.class})
public class GlobalTransactionAutoConfiguration {
    private final ApplicationContext applicationContext;
    private final FescarProperties fescarProperties;

    public GlobalTransactionAutoConfiguration(ApplicationContext applicationContext, FescarProperties fescarProperties) {
        this.applicationContext = applicationContext;
        this.fescarProperties = fescarProperties;
    }

    /**
    * 示例化GlobalTransactionScanner
    * scanner为client初始化的发起类
    */
    @Bean
    public GlobalTransactionScanner globalTransactionScanner() {
        String applicationName = this.applicationContext.getEnvironment().getProperty("spring.application.name");
        String txServiceGroup = this.fescarProperties.getTxServiceGroup();
        if (StringUtils.isEmpty(txServiceGroup)) {
            txServiceGroup = applicationName + "-fescar-service-group";
            this.fescarProperties.setTxServiceGroup(txServiceGroup);
        }

        return new GlobalTransactionScanner(applicationName, txServiceGroup);
    }
}

可以看到支持一个配置项FescarProperties,用于配置事务分组名称:

spring.cloud.alibaba.fescar.tx-service-group=my_test_tx_group

如果不指定服务组,则默认使用spring.application.name+ -fescar-service-group生成名称,所以不指定spring.application.name启动会报错。

@ConfigurationProperties("spring.cloud.alibaba.fescar")
public class FescarProperties {
    private String txServiceGroup;

    public FescarProperties() {
    }

    public String getTxServiceGroup() {
        return this.txServiceGroup;
    }

    public void setTxServiceGroup(String txServiceGroup) {
        this.txServiceGroup = txServiceGroup;
    }
}

获取 applicationId 和 txServiceGroup 后,创建 GlobalTransactionScanner 对象,主要看类中 initClient 方法。

private void initClient() {
    if (StringUtils.isNullOrEmpty(applicationId) || StringUtils.isNullOrEmpty(txServiceGroup)) {
        throw new IllegalArgumentException(
            "applicationId: " + applicationId + ", txServiceGroup: " + txServiceGroup);
    }
    //init TM
    TMClient.init(applicationId, txServiceGroup);

    //init RM
    RMClient.init(applicationId, txServiceGroup);

}

方法中可以看到初始化了 TMClient 和 RMClient,对于一个服务既可以是TM角色也可以是RM角色,至于什么时候是 TM 或者 RM 则要看在一次全局事务中 @GlobalTransactional 注解标注在哪。 Client 创建的结果是与 TC 的一个 Netty 连接,所以在启动日志中可以看到两个 Netty Channel,其中标明了 transactionRole 分别为 TMROLE 和 RMROLE

2019-04-09 13:42:57.417  INFO 93715 --- [imeoutChecker_1] c.a.f.c.rpc.netty.NettyPoolableFactory   : NettyPool create channel to {"address":"127.0.0.1:8091","message":{"applicationId":"business-service","byteBuffer":{"char":"\u0000","direct":false,"double":0.0,"float":0.0,"int":0,"long":0,"readOnly":false,"short":0},"transactionServiceGroup":"my_test_tx_group","typeCode":101,"version":"0.4.1"},"transactionRole":"TMROLE"}
2019-04-09 13:42:57.505  INFO 93715 --- [imeoutChecker_1] c.a.f.c.rpc.netty.NettyPoolableFactory   : NettyPool create channel to {"address":"127.0.0.1:8091","message":{"applicationId":"business-service","byteBuffer":{"char":"\u0000","direct":false,"double":0.0,"float":0.0,"int":0,"long":0,"readOnly":false,"short":0},"transactionServiceGroup":"my_test_tx_group","typeCode":103,"version":"0.4.1"},"transactionRole":"RMROLE"}
2019-04-09 13:42:57.629 DEBUG 93715 --- [lector_TMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:RegisterTMRequest{applicationId='business-service', transactionServiceGroup='my_test_tx_group'}
2019-04-09 13:42:57.629 DEBUG 93715 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:RegisterRMRequest{resourceIds='null', applicationId='business-service', transactionServiceGroup='my_test_tx_group'}
2019-04-09 13:42:57.699 DEBUG 93715 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null,messageId:1
2019-04-09 13:42:57.699 DEBUG 93715 --- [lector_TMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null,messageId:2
2019-04-09 13:42:57.701 DEBUG 93715 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : com.alibaba.fescar.core.rpc.netty.RmRpcClient@3b06d101 msgId:1, future :com.alibaba.fescar.core.protocol.MessageFuture@28bb1abd, body:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null
2019-04-09 13:42:57.701 DEBUG 93715 --- [lector_TMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : com.alibaba.fescar.core.rpc.netty.TmRpcClient@65fc3fb7 msgId:2, future :com.alibaba.fescar.core.protocol.MessageFuture@9a1e3df, body:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null
2019-04-09 13:42:57.710  INFO 93715 --- [imeoutChecker_1] c.a.fescar.core.rpc.netty.RmRpcClient    : register RM success. server version:0.4.1,channel:[id: 0xe6468995, L:/127.0.0.1:57397 - R:/127.0.0.1:8091]
2019-04-09 13:42:57.710  INFO 93715 --- [imeoutChecker_1] c.a.f.c.rpc.netty.NettyPoolableFactory   : register success, cost 114 ms, version:0.4.1,role:TMROLE,channel:[id: 0xd22fe0c5, L:/127.0.0.1:57398 - R:/127.0.0.1:8091]
2019-04-09 13:42:57.711  INFO 93715 --- [imeoutChecker_1] c.a.f.c.rpc.netty.NettyPoolableFactory   : register success, cost 125 ms, version:0.4.1,role:RMROLE,channel:[id: 0xe6468995, L:/127.0.0.1:57397 - R:/127.0.0.1:8091]

日志中可以看到

  1. 创建Netty连接
  2. 发送注册请求
  3. 得到响应结果
  4. RmRpcClientTmRpcClient 成功实例化

TM 处理流程

在本例中,TM 的角色是 business-service, BusinessService 的 purchase 方法标注了 @GlobalTransactional 注解:

@Service
public class BusinessService {

    @Autowired
    private StorageFeignClient storageFeignClient;
    @Autowired
    private OrderFeignClient orderFeignClient;

    @GlobalTransactional
    public void purchase(String userId, String commodityCode, int orderCount){
        storageFeignClient.deduct(commodityCode, orderCount);

        orderFeignClient.create(userId, commodityCode, orderCount);
    }
}

方法调用后将会创建一个全局事务,首先关注 @GlobalTransactional 注解的作用,在 GlobalTransactionalInterceptor 中被拦截处理。

/**
 * AOP拦截方法调用
 */
@Override
public Object invoke(final MethodInvocation methodInvocation) throws Throwable {
    Class<?> targetClass = (methodInvocation.getThis() != null ? AopUtils.getTargetClass(methodInvocation.getThis()) : null);
    Method specificMethod = ClassUtils.getMostSpecificMethod(methodInvocation.getMethod(), targetClass);
    final Method method = BridgeMethodResolver.findBridgedMethod(specificMethod);

    //获取方法GlobalTransactional注解
    final GlobalTransactional globalTransactionalAnnotation = getAnnotation(method, GlobalTransactional.class);
    final GlobalLock globalLockAnnotation = getAnnotation(method, GlobalLock.class);

    //如果方法有GlobalTransactional注解,则拦截到相应方法处理
    if (globalTransactionalAnnotation != null) {
        return handleGlobalTransaction(methodInvocation, globalTransactionalAnnotation);
    } else if (globalLockAnnotation != null) {
        return handleGlobalLock(methodInvocation);
    } else {
        return methodInvocation.proceed();
    }
}

handleGlobalTransaction 方法中对 TransactionalTemplate 的 execute 进行了调用,从类名可以看到这是一个标准的模版方法,它定义了 TM 对全局事务处理的标准步骤,注释已经比较清楚了。

public Object execute(TransactionalExecutor business) throws TransactionalExecutor.ExecutionException {
    // 1\. get or create a transaction
    GlobalTransaction tx = GlobalTransactionContext.getCurrentOrCreate();

    try {
        // 2\. begin transaction
        try {
            triggerBeforeBegin();
            tx.begin(business.timeout(), business.name());
            triggerAfterBegin();
        } catch (TransactionException txe) {
            throw new TransactionalExecutor.ExecutionException(tx, txe,
                TransactionalExecutor.Code.BeginFailure);
        }
        Object rs = null;
        try {
            // Do Your Business
            rs = business.execute();
        } catch (Throwable ex) {
            // 3\. any business exception, rollback.
            try {
                triggerBeforeRollback();
                tx.rollback();
                triggerAfterRollback();
                // 3.1 Successfully rolled back
                throw new TransactionalExecutor.ExecutionException(tx, TransactionalExecutor.Code.RollbackDone, ex);
            } catch (TransactionException txe) {
                // 3.2 Failed to rollback
                throw new TransactionalExecutor.ExecutionException(tx, txe,
                    TransactionalExecutor.Code.RollbackFailure, ex);
            }
        }
        // 4\. everything is fine, commit.
        try {
            triggerBeforeCommit();
            tx.commit();
            triggerAfterCommit();
        } catch (TransactionException txe) {
            // 4.1 Failed to commit
            throw new TransactionalExecutor.ExecutionException(tx, txe,
                TransactionalExecutor.Code.CommitFailure);
        }
        return rs;
    } finally {
        //5\. clear
        triggerAfterCompletion();
        cleanUp();
    }
}

通过 DefaultGlobalTransaction 的 begin 方法开启全局事务。

public void begin(int timeout, String name) throws TransactionException {
    if (role != GlobalTransactionRole.Launcher) {
        check();
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Ignore Begin(): just involved in global transaction [" + xid + "]");
        }
        return;
    }
    if (xid != null) {
        throw new IllegalStateException();
    }
    if (RootContext.getXID() != null) {
        throw new IllegalStateException();
    }
    //具体开启事务的方法,获取TC返回的XID
    xid = transactionManager.begin(null, null, name, timeout);
    status = GlobalStatus.Begin;
    RootContext.bind(xid);
    if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Begin a NEW global transaction [" + xid + "]");
    }
}

方法开头处if (role != GlobalTransactionRole.Launcher)对 role 的判断有关键的作用,表明当前是全局事务的发起者(Launcher)还是参与者(Participant)。如果在分布式事务的下游系统方法中也加上@GlobalTransactional注解,那么它的角色就是 Participant,会忽略后面的 begin 直接 return,而判断是 Launcher 还是 Participant 是根据当前上下文是否已存在 XID 来判断,没有 XID 的就是 Launcher,已经存在 XID的就是 Participant。由此可见,全局事务的创建只能由 Launcher 执行,而一次分布式事务中也只有一个Launcher 存在。

DefaultTransactionManager负责 TM 与 TC 通讯,发送 begin、commit、rollback 指令。

@Override
public String begin(String applicationId, String transactionServiceGroup, String name, int timeout)
    throws TransactionException {
    GlobalBeginRequest request = new GlobalBeginRequest();
    request.setTransactionName(name);
    request.setTimeout(timeout);
    GlobalBeginResponse response = (GlobalBeginResponse)syncCall(request);
    return response.getXid();
}

至此拿到 fescar-server 返回的 XID 表示一个全局事务创建成功,日志中也反应了上述流程。

2019-04-09 13:46:57.417 DEBUG 31326 --- [nio-8084-exec-1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : offer message: timeout=60000,transactionName=purchase(java.lang.String,java.lang.String,int)
2019-04-09 13:46:57.417 DEBUG 31326 --- [geSend_TMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : write message:FescarMergeMessage timeout=60000,transactionName=purchase(java.lang.String,java.lang.String,int), channel:[id: 0xa148545e, L:/127.0.0.1:56120 - R:/127.0.0.1:8091],active?true,writable?true,isopen?true
2019-04-09 13:46:57.418 DEBUG 31326 --- [lector_TMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:FescarMergeMessage timeout=60000,transactionName=purchase(java.lang.String,java.lang.String,int)
2019-04-09 13:46:57.421 DEBUG 31326 --- [lector_TMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:MergeResultMessage com.alibaba.fescar.core.protocol.transaction.GlobalBeginResponse@2dc480dc,messageId:1196
2019-04-09 13:46:57.421 DEBUG 31326 --- [nio-8084-exec-1] c.a.fescar.core.context.RootContext      : bind 192.168.224.93:8091:2008502699
2019-04-09 13:46:57.421 DEBUG 31326 --- [nio-8084-exec-1] c.a.f.tm.api.DefaultGlobalTransaction    : Begin a NEW global transaction [192.168.224.93:8091:2008502699]

全局事务创建后,就开始执行 business.execute(),即业务代码storageFeignClient.deduct(commodityCode, orderCount)进入 RM 处理流程,此处的业务逻辑为调用 storage-service 的扣减库存接口。

RM 处理流程

@GetMapping(path = "/deduct")
public Boolean deduct(String commodityCode, Integer count){
    storageService.deduct(commodityCode,count);
    return true;
}

@Transactional
public void deduct(String commodityCode, int count){
    Storage storage = storageDAO.findByCommodityCode(commodityCode);
    storage.setCount(storage.getCount()-count);

    storageDAO.save(storage);
}

storage 的接口和 service 方法并未出现 fescar 相关的代码和注解,体现了 fescar 的无侵入。那它是如何加入到这次全局事务中的呢?答案在ConnectionProxy中,这也是前面说为什么必须要使用DataSourceProxy的原因,通过 DataSourceProxy 才能在业务代码的本地事务提交时,fescar 通过该切入点,向 TC 注册分支事务并发送 RM 的处理结果。

由于业务代码本身的事务提交被ConnectionProxy代理实现,所以在提交本地事务时,实际执行的是ConnectionProxy 的 commit 方法。

public void commit() throws SQLException {
    //如果当前是全局事务,则执行全局事务的提交
    //判断是不是全局事务,就是看当前上下文是否存在XID
    if (context.inGlobalTransaction()) {
        processGlobalTransactionCommit();
    } else if (context.isGlobalLockRequire()) {
        processLocalCommitWithGlobalLocks();
    } else {
        targetConnection.commit();
    }
}

private void processGlobalTransactionCommit() throws SQLException {
    try {
        //首先是向TC注册RM,拿到TC分配的branchId
        register();
    } catch (TransactionException e) {
        recognizeLockKeyConflictException(e);
    }

    try {
        if (context.hasUndoLog()) {
            //写入undolog
            UndoLogManager.flushUndoLogs(this);
        }

        //提交本地事务,写入undo_log和业务数据在同一个本地事务中
        targetConnection.commit();
    } catch (Throwable ex) {
        //向TC发送RM的事务处理失败的通知
        report(false);
        if (ex instanceof SQLException) {
            throw new SQLException(ex);
        }
    }
    //向TC发送RM的事务处理成功的通知
    report(true);
    context.reset();
}

private void register() throws TransactionException {
    //注册RM,构建request通过netty向TC发送注册指令
    Long branchId = DefaultResourceManager.get().branchRegister(BranchType.AT, getDataSourceProxy().getResourceId(),
            null, context.getXid(), null, context.buildLockKeys());
    //将返回的branchId存在上下文中
    context.setBranchId(branchId);
}

通过日志印证一下上面的流程。

2019-04-09 21:57:48.341 DEBUG 38933 --- [nio-8081-exec-1] o.s.c.a.f.web.FescarHandlerInterceptor   : xid in RootContext null xid in RpcContext 192.168.0.2:8091:2008546211
2019-04-09 21:57:48.341 DEBUG 38933 --- [nio-8081-exec-1] c.a.fescar.core.context.RootContext      : bind 192.168.0.2:8091:2008546211
2019-04-09 21:57:48.341 DEBUG 38933 --- [nio-8081-exec-1] o.s.c.a.f.web.FescarHandlerInterceptor   : bind 192.168.0.2:8091:2008546211 to RootContext
2019-04-09 21:57:48.386  INFO 38933 --- [nio-8081-exec-1] o.h.h.i.QueryTranslatorFactoryInitiator  : HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select storage0_.id as id1_0_, storage0_.commodity_code as commodit2_0_, storage0_.count as count3_0_ from storage_tbl storage0_ where storage0_.commodity_code=?
Hibernate: update storage_tbl set count=? where id=?
2019-04-09 21:57:48.673  INFO 38933 --- [nio-8081-exec-1] c.a.fescar.core.rpc.netty.RmRpcClient    : will connect to 192.168.0.2:8091
2019-04-09 21:57:48.673  INFO 38933 --- [nio-8081-exec-1] c.a.fescar.core.rpc.netty.RmRpcClient    : RM will register :jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false
2019-04-09 21:57:48.673  INFO 38933 --- [nio-8081-exec-1] c.a.f.c.rpc.netty.NettyPoolableFactory   : NettyPool create channel to {"address":"192.168.0.2:8091","message":{"applicationId":"storage-service","byteBuffer":{"char":"\u0000","direct":false,"double":0.0,"float":0.0,"int":0,"long":0,"readOnly":false,"short":0},"resourceIds":"jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false","transactionServiceGroup":"hello-service-fescar-service-group","typeCode":103,"version":"0.4.0"},"transactionRole":"RMROLE"}
2019-04-09 21:57:48.677 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:RegisterRMRequest{resourceIds='jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false', applicationId='storage-service', transactionServiceGroup='hello-service-fescar-service-group'}
2019-04-09 21:57:48.680 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null,messageId:9
2019-04-09 21:57:48.680 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : com.alibaba.fescar.core.rpc.netty.RmRpcClient@7d61f5d4 msgId:9, future :com.alibaba.fescar.core.protocol.MessageFuture@186cd3e0, body:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null
2019-04-09 21:57:48.680  INFO 38933 --- [nio-8081-exec-1] c.a.fescar.core.rpc.netty.RmRpcClient    : register RM success. server version:0.4.1,channel:[id: 0xd40718e3, L:/192.168.0.2:62607 - R:/192.168.0.2:8091]
2019-04-09 21:57:48.680  INFO 38933 --- [nio-8081-exec-1] c.a.f.c.rpc.netty.NettyPoolableFactory   : register success, cost 3 ms, version:0.4.1,role:RMROLE,channel:[id: 0xd40718e3, L:/192.168.0.2:62607 - R:/192.168.0.2:8091]
2019-04-09 21:57:48.680 DEBUG 38933 --- [nio-8081-exec-1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : offer message: transactionId=2008546211,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,lockKey=storage_tbl:1
2019-04-09 21:57:48.681 DEBUG 38933 --- [geSend_RMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : write message:FescarMergeMessage transactionId=2008546211,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,lockKey=storage_tbl:1, channel:[id: 0xd40718e3, L:/192.168.0.2:62607 - R:/192.168.0.2:8091],active?true,writable?true,isopen?true
2019-04-09 21:57:48.681 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:FescarMergeMessage transactionId=2008546211,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,lockKey=storage_tbl:1
2019-04-09 21:57:48.687 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:MergeResultMessage BranchRegisterResponse: transactionId=2008546211,branchId=2008546212,result code =Success,getMsg =null,messageId:11
2019-04-09 21:57:48.702 DEBUG 38933 --- [nio-8081-exec-1] c.a.f.rm.datasource.undo.UndoLogManager  : Flushing UNDO LOG: {"branchId":2008546212,"sqlUndoLogs":[{"afterImage":{"rows":[{"fields":[{"keyType":"PrimaryKey","name":"id","type":4,"value":1},{"keyType":"NULL","name":"count","type":4,"value":993}]}],"tableName":"storage_tbl"},"beforeImage":{"rows":[{"fields":[{"keyType":"PrimaryKey","name":"id","type":4,"value":1},{"keyType":"NULL","name":"count","type":4,"value":994}]}],"tableName":"storage_tbl"},"sqlType":"UPDATE","tableName":"storage_tbl"}],"xid":"192.168.0.2:8091:2008546211"}
2019-04-09 21:57:48.755 DEBUG 38933 --- [nio-8081-exec-1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : offer message: transactionId=2008546211,branchId=2008546212,resourceId=null,status=PhaseOne_Done,applicationData=null
2019-04-09 21:57:48.755 DEBUG 38933 --- [geSend_RMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : write message:FescarMergeMessage transactionId=2008546211,branchId=2008546212,resourceId=null,status=PhaseOne_Done,applicationData=null, channel:[id: 0xd40718e3, L:/192.168.0.2:62607 - R:/192.168.0.2:8091],active?true,writable?true,isopen?true
2019-04-09 21:57:48.756 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:FescarMergeMessage transactionId=2008546211,branchId=2008546212,resourceId=null,status=PhaseOne_Done,applicationData=null
2019-04-09 21:57:48.758 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:MergeResultMessage com.alibaba.fescar.core.protocol.transaction.BranchReportResponse@582a08cf,messageId:13
2019-04-09 21:57:48.799 DEBUG 38933 --- [nio-8081-exec-1] c.a.fescar.core.context.RootContext      : unbind 192.168.0.2:8091:2008546211
2019-04-09 21:57:48.799 DEBUG 38933 --- [nio-8081-exec-1] o.s.c.a.f.web.FescarHandlerInterceptor   : unbind 192.168.0.2:8091:2008546211 from RootContext
  1. 获取business-service传来的XID
  2. 绑定XID到当前上下文中
  3. 执行业务逻辑sql
  4. 向TC创建本次RM的Netty连接
  5. 向TC发送分支事务的相关信息
  6. 获得TC返回的branchId
  7. 记录Undo Log数据
  8. 向TC发送本次事务PhaseOne阶段的处理结果
  9. 从当前上下文中解绑XID

其中第 1 步和第 9 步,是在FescarHandlerInterceptor中完成的,该类并不属于 fescar,是前面提到的 spring-cloud-alibaba-fescar,它实现了基于 feign、rest 通信时将 xid bind 和 unbind 到当前请求上下文中。到这里 RM 完成了 PhaseOne 阶段的工作,接着看 PhaseTwo 阶段的处理逻辑。

事务提交

各分支事务执行完成后,TC 对各 RM 的汇报结果进行汇总,给各 RM 发送 commit 或 rollback 的指令。

2019-04-09 21:57:49.813 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Receive:xid=192.168.0.2:8091:2008546211,branchId=2008546212,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,applicationData=null,messageId:1
2019-04-09 21:57:49.813 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.AbstractRpcRemoting    : com.alibaba.fescar.core.rpc.netty.RmRpcClient@7d61f5d4 msgId:1, body:xid=192.168.0.2:8091:2008546211,branchId=2008546212,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,applicationData=null
2019-04-09 21:57:49.814  INFO 38933 --- [atch_RMROLE_1_8] c.a.f.core.rpc.netty.RmMessageListener   : onMessage:xid=192.168.0.2:8091:2008546211,branchId=2008546212,branchType=AT,resourceId=jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false,applicationData=null
2019-04-09 21:57:49.816  INFO 38933 --- [atch_RMROLE_1_8] com.alibaba.fescar.rm.AbstractRMHandler  : Branch committing: 192.168.0.2:8091:2008546211 2008546212 jdbc:mysql://127.0.0.1:3306/db_storage?useSSL=false null
2019-04-09 21:57:49.816  INFO 38933 --- [atch_RMROLE_1_8] com.alibaba.fescar.rm.AbstractRMHandler  : Branch commit result: PhaseTwo_Committed
2019-04-09 21:57:49.817  INFO 38933 --- [atch_RMROLE_1_8] c.a.fescar.core.rpc.netty.RmRpcClient    : RmRpcClient sendResponse branchStatus=PhaseTwo_Committed,result code =Success,getMsg =null
2019-04-09 21:57:49.817 DEBUG 38933 --- [atch_RMROLE_1_8] c.a.f.c.rpc.netty.AbstractRpcRemoting    : send response:branchStatus=PhaseTwo_Committed,result code =Success,getMsg =null,channel:[id: 0xd40718e3, L:/192.168.0.2:62607 - R:/192.168.0.2:8091]
2019-04-09 21:57:49.817 DEBUG 38933 --- [lector_RMROLE_1] c.a.f.c.rpc.netty.MessageCodecHandler    : Send:branchStatus=PhaseTwo_Committed,result code =Success,getMsg =null

从日志中可以看到

  1. RM 收到 XID=192.168.0.2:8091:2008546211,branchId=2008546212 的 commit 通知;
  2. 执行 commit 动作;
  3. 将 commit 结果发送给 TC,branchStatus 为 PhaseTwo_Committed;

具体看下二阶段 commit 的执行过程,在AbstractRMHandler类的 doBranchCommit 方法:

/**
 * 拿到通知的xid、branchId等关键参数
 * 然后调用RM的branchCommit
 */
protected void doBranchCommit(BranchCommitRequest request, BranchCommitResponse response) throws TransactionException {
    String xid = request.getXid();
    long branchId = request.getBranchId();
    String resourceId = request.getResourceId();
    String applicationData = request.getApplicationData();
    LOGGER.info("Branch committing: " + xid + " " + branchId + " " + resourceId + " " + applicationData);
    BranchStatus status = getResourceManager().branchCommit(request.getBranchType(), xid, branchId, resourceId, applicationData);
    response.setBranchStatus(status);
    LOGGER.info("Branch commit result: " + status);
}

最终会将 branchCommit 的请求调用到AsyncWorker的 branchCommit 方法。AsyncWorker 的处理方式是fescar 架构的一个关键部分,因为大部分事务都是会正常提交的,所以在 PhaseOne 阶段就已经结束了,这样就可以将锁最快的释放。PhaseTwo 阶段接收 commit 的指令后,异步处理即可。将 PhaseTwo 的时间消耗排除在一次分布式事务之外。

private static final List<Phase2Context> ASYNC_COMMIT_BUFFER = Collections.synchronizedList( new ArrayList<Phase2Context>());

/**
 * 将需要提交的XID加入list
 */
@Override
public BranchStatus branchCommit(BranchType branchType, String xid, long branchId, String resourceId, String applicationData) throws TransactionException {
    if (ASYNC_COMMIT_BUFFER.size() < ASYNC_COMMIT_BUFFER_LIMIT) {
        ASYNC_COMMIT_BUFFER.add(new Phase2Context(branchType, xid, branchId, resourceId, applicationData));
    } else {
        LOGGER.warn("Async commit buffer is FULL. Rejected branch [" + branchId + "/" + xid + "] will be handled by housekeeping later.");
    }
    return BranchStatus.PhaseTwo_Committed;
}

/**
 * 通过定时任务消费list中的XID
 */
public synchronized void init() {
    LOGGER.info("Async Commit Buffer Limit: " + ASYNC_COMMIT_BUFFER_LIMIT);
    timerExecutor = new ScheduledThreadPoolExecutor(1,
        new NamedThreadFactory("AsyncWorker", 1, true));
    timerExecutor.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            try {
                doBranchCommits();
            } catch (Throwable e) {
                LOGGER.info("Failed at async committing ... " + e.getMessage());
            }
        }
    }, 10, 1000 * 1, TimeUnit.MILLISECONDS);
}

private void doBranchCommits() {
    if (ASYNC_COMMIT_BUFFER.size() == 0) {
        return;
    }
    Map<String, List<Phase2Context>> mappedContexts = new HashMap<>();
    Iterator<Phase2Context> iterator = ASYNC_COMMIT_BUFFER.iterator();

    //一次定时循环取出ASYNC_COMMIT_BUFFER中的所有待办数据
    //以resourceId作为key分组待commit数据,resourceId是一个数据库的连接url
    //在前面的日志中可以看到,目的是为了覆盖应用的多数据源创建
    while (iterator.hasNext()) {
        Phase2Context commitContext = iterator.next();
        List<Phase2Context> contextsGroupedByResourceId = mappedContexts.get(commitContext.resourceId);
        if (contextsGroupedByResourceId == null) {
            contextsGroupedByResourceId = new ArrayList<>();
            mappedContexts.put(commitContext.resourceId, contextsGroupedByResourceId);
        }
        contextsGroupedByResourceId.add(commitContext);

        iterator.remove();

    }

    for (Map.Entry<String, List<Phase2Context>> entry : mappedContexts.entrySet()) {
        Connection conn = null;
        try {
            try {
                //根据resourceId获取数据源以及连接
                DataSourceProxy dataSourceProxy = DataSourceManager.get().get(entry.getKey());
                conn = dataSourceProxy.getPlainConnection();
            } catch (SQLException sqle) {
                LOGGER.warn("Failed to get connection for async committing on " + entry.getKey(), sqle);
                continue;
            }
            List<Phase2Context> contextsGroupedByResourceId = entry.getValue();
            for (Phase2Context commitContext : contextsGroupedByResourceId) {
                try {
                    //执行undolog的处理,即删除xid、branchId对应的记录
                    UndoLogManager.deleteUndoLog(commitContext.xid, commitContext.branchId, conn);
                } catch (Exception ex) {
                    LOGGER.warn(
                        "Failed to delete undo log [" + commitContext.branchId + "/" + commitContext.xid + "]", ex);
                }
            }

        } finally {
            if (conn != null) {
                try {
                    conn.close();
                } catch (SQLException closeEx) {
                    LOGGER.warn("Failed to close JDBC resource while deleting undo_log ", closeEx);
                }
            }
        }
    }
}

所以对于commit动作的处理,RM只需删除xid、branchId对应的undo_log即可。

事务回滚

对于rollback场景的触发有两种情况

  1. 分支事务处理异常,即ConnectionProxyreport(false)的情况
  2. TM捕获到下游系统上抛的异常,即发起全局事务标有@GlobalTransactional注解的方法捕获到的异常。在前面TransactionalTemplate类的execute模版方法中,对business.execute()的调用进行了catch,catch后会调用rollback,由TM通知TC对应XID需要回滚事务
public void rollback() throws TransactionException {
   //只有Launcher能发起这个rollback
   if (role == GlobalTransactionRole.Participant) {
       // Participant has no responsibility of committing
       if (LOGGER.isDebugEnabled()) {
           LOGGER.debug("Ignore Rollback(): just involved in global transaction [" + xid + "]");
       }
       return;
   }
   if (xid == null) {
       throw new IllegalStateException();
   }

   status = transactionManager.rollback(xid);
   if (RootContext.getXID() != null) {
       if (xid.equals(RootContext.getXID())) {
           RootContext.unbind();
       }
   }
}

TC 汇总后向参与者发送 rollback 指令,RM 在AbstractRMHandler类的 doBranchRollback 方法中接收这个rollback 的通知。

protected void doBranchRollback(BranchRollbackRequest request, BranchRollbackResponse response) throws TransactionException {
    String xid = request.getXid();
    long branchId = request.getBranchId();
    String resourceId = request.getResourceId();
    String applicationData = request.getApplicationData();
    LOGGER.info("Branch rolling back: " + xid + " " + branchId + " " + resourceId);
    BranchStatus status = getResourceManager().branchRollback(request.getBranchType(), xid, branchId, resourceId, applicationData);
    response.setBranchStatus(status);
    LOGGER.info("Branch rollback result: " + status);
}

然后将 rollback 请求传递到DataSourceManager类的 branchRollback 方法。

public BranchStatus branchRollback(BranchType branchType, String xid, long branchId, String resourceId, String applicationData) throws TransactionException {
    //根据resourceId获取对应的数据源
    DataSourceProxy dataSourceProxy = get(resourceId);
    if (dataSourceProxy == null) {
        throw new ShouldNeverHappenException();
    }
    try {
        UndoLogManager.undo(dataSourceProxy, xid, branchId);
    } catch (TransactionException te) {
        if (te.getCode() == TransactionExceptionCode.BranchRollbackFailed_Unretriable) {
            return BranchStatus.PhaseTwo_RollbackFailed_Unretryable;
        } else {
            return BranchStatus.PhaseTwo_RollbackFailed_Retryable;
        }
    }
    return BranchStatus.PhaseTwo_Rollbacked;
}

最终会执行UndoLogManager类的 undo 方法,因为是纯 jdbc 操作代码比较长就不贴出来了,可以通过连接到github 查看源码,说一下 undo 的具体流程:

  1. 根据 xid 和 branchId 查找 PhaseOne 阶段提交的 undo_log;
  2. 如果找到了就根据 undo_log 中记录的数据生成回放 sql 并执行,即还原 PhaseOne 阶段修改的数据;
  3. 第 2 步处理完后,删除该条 undo_log 数据;
  4. 如果第 1 步没有找到对应的 undo_log,就插入一条状态为GlobalFinished的 undo_log。出现没找到的原因可能是 PhaseOne 阶段的本地事务异常了,导致没有正常写入。 因为 xid 和 branchId 是唯一索引,所以第 4步的插入,可以防止 PhaseOne 阶段恢复后的成功写入,那么 PhaseOne 阶段就会异常,这样一来业务数据也就不会提交成功,数据达到了最终回滚了的效果。

总结

本地结合分布式业务场景,分析了 fescar client 侧的主要处理流程,对 TM 和 RM 角色的主要源码进行了解析,希望能对大家理解 fescar 的工作原理有所帮助。

随着 fescar 的快速迭代以及后期 Roadmap 规划的不断完善,假以时日,相信 fescar 能够成为开源分布式事务的标杆解决方案。



本文作者:中间件小哥

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