背景
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。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。
一、分布式事务简介
分布式事务有各种实现方案，不过大体可分为两类，一种不需要关注事务分支与全局事务的交互过程。另一种是将逻辑拆分成三个部分准备、提交、回滚，分支事务加入全局事务。这两种在Seata里前者称为AT模式，后者称为MT模式。
二、内容
1.分布式事务数据库操作型
MySQL XA方案 是其中一种，这种的话是直接作用于数据库。
其中RM执行本地事务提交与回滚；TM是分布式事务核心管理。
缺点的话，一是不适用于微服务，二很明显由于每次操作不提交，最后导致数据未提交越来越多时候，性能就不是很好了。那么像Seata这种业务层面的解决就相对而已来说性能强大了很多。
2.Seata分布式事务详解
Seata分布式事务是业务层民的解决方案。
而且只依赖于单台数据的事务能力。
Seata包括三个role：
1> TC 事务协调，负责协调并驱动全局事务的提交与回滚
2> TM 控制全局事务边界，负责开启全局事务，决定全局事务的提交与回滚
3> RM 分支事务，负责分支注册，接收TC指令，驱动分支事务回滚与提交
自己理解后画了个菜鸟图，如下：
3.XA vs Seata AT
AT不需要XA协议，适配于微服务，XA事务性资源的锁都要保持到第二阶段 完成才释放，AT分本地锁和全局锁，本地锁由本地事务管理，全局锁由全局事务管理，在决议第二阶段全局提交时，全局锁马上可以释放。
相关概念
- XID：一个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 ?： 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。
原理
seata涉及到三个角色之间的交互，本文通过流程图将AT模式下的基本交互流程梳理一下，为我们以后的解析打下基础。
假设有三个微服务，分别是服务A、B、C，其中服务A中调用了服务B和服务C，TM、TC、RM三者之间的交互流程如下图：
- 1、服务A启动时，GlobalTransactionScanner会对有@GlobalTransaction注解的方法进行AOP增强，并生成代理，增强的代码位于GlobalTransactionalInterceptor类中，当调用@GlobalTransaction注解的方法时，增强代码首先向TC注册全局事务，表示全局事务的开始，同时TC生成XID，并返回给TM；
- 2、服务A中调用服务B时，将XID传递给服务B；
- 3、服务B得到XID后，访问TC，注册分支事务，并从TC获得分支事务ID，TC根据XID将分支事务与全局事务关联；
- 4、接下来服务B开始执行SQL语句，在执行前将表中对应的数据保存一份，执行后在保存一份，将这两份记录作为回滚记录写入到数据库中，如果执行过程中没有异常，服务B最后将事务提交，并通知TC分支事务成功，服务B也会清除本地事务数据；
- 5、服务A访问完服务B后，访问服务C；
- 6、服务C与TC之间的交互与服务B完全一致；
- 7、服务B和服务C都成功后，服务A通过TM通知TC全局事务成功，如果失败了，服务A也会通知TC全局事务失败；
- 8、TC记录了全局事务下的每个分支事务，TC收到全局事务的结果后，如果结果成功，则通知RM成功，RM收到通知后清理之前在数据库中保存的回滚记录，如果失败了，则RM要查询出之前在数据库保存的回滚记录，对之前的SQL操作进行回滚。
因为TM、RM、TC之间的交互都是通过网络完成的，很容易出现网络断开的情况，因此TC提供了四个定时线程池，定时检测系统中是否有超时事务、异步提交事务、回滚重试事务、重试提交事务，如果发现了有这四类事务，则从全局事务中获取所有的分支事务，分别调用各个分支事务完成对应的操作，依次来确保事务的一致性。
需要考虑的问题：
通过上面流程的分析可以发现，每次SQL操作（查询除外）时，都会增加额外了三次数据库操作；每次全局事务和分支事务开启时，都涉及到TM、RM与TC的交互；全局事务期间还要承担数据短时不一致的情况，这些都是我们在使用AT模式需要考虑的情况。
项目依赖
seata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。所以在分布式系统中使用seata要解决XID的传递问题。seata目前支持全版本的dubbo，对于spring cloud的分布式项目社区也提供了相应的实现
org.springframework.cloud
spring-cloud-alibaba-fescar
该组件实现了基于RestTemplate、Feign通讯时的XID传递功能，详细说明见集成源码深度剖析：Fescar x Spring Cloud
spring-cloud-alibaba-fescar内已包含了fescar-spring的依赖，所以可以不另外引入，查看完整的pom.xml.
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
配置文件
seata的配置文件入口为registry.conf查看代码ConfigurationFactory得知目前还不能指定该配置文件，所以名称只能为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，这里使用默认的file形式。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的相关参与，client与server的通信使用的Netty
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
} //部分代码
public class GlobalTransactionScanner{
private final boolean disableGlobalTransaction =
ConfigurationFactory.getInstance().getBoolean("service.disableGlobalTransaction", false);
public void afterPropertiesSet() {
if (disableGlobalTransaction) {
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Global transaction is disabled.");
}
return;
}
initClient();
}
}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
}
}启动Server
前往
https://github.com/seata/seata/releases 下载最新版本的 Fescar Server
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
对于Spring boot项目，启动运行xxxApplication的main方法即可，seata的加载入口类位于
GlobalTransactionAutoConfiguration
@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;
}
@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如果不指定则用spring.application.name+ -fescar-service-group生成一个名称，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;
}
}有了applicationId和txServiceGroup则创建GlobalTransactionScanner对象，主要看其中的initClient方法
private void initClient() {
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Initializing Global Transaction Clients ... ");
}
if (StringUtils.isNullOrEmpty(applicationId) || StringUtils.isNullOrEmpty(txServiceGroup)) {
throw new IllegalArgumentException(
"applicationId: " + applicationId + ", txServiceGroup: " + txServiceGroup);
}
//init TM
TMClient.init(applicationId, txServiceGroup);
if (LOGGER.isInfoEnabled()) {
LOGGER.info(
"Transaction Manager Client is initialized. applicationId[" + applicationId + "] txServiceGroup["
+ txServiceGroup + "]");
}
//init RM
RMClient.init(applicationId, txServiceGroup);
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Resource Manager is initialized. applicationId[" + applicationId + "] txServiceGroup[" + txServiceGroup + "]");
}
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Global Transaction Clients are initialized. ");
}
}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，其中也标明了transactionRole分别为TMROLE和RMROLE
NettyPool create channel to {"address":"127.0.0.1:8091","message":{"applicationId":"order-service","byteBuffer":{"char":"\u0000","direct":false,"double":0.0,"float":0.0,"int":0,"long":0,"readOnly":false,"short":0},"transactionServiceGroup":"hello-service-fescar-service-group","typeCode":101,"version":"0.4.0"},"transactionRole":"TMROLE"}
NettyPool create channel to {"address":"127.0.0.1:8091","message":{"applicationId":"order-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_order?useSSL=false","transactionServiceGroup":"hello-service-fescar-service-group","typeCode":103,"version":"0.4.0"},"transactionRole":"RMROLE"}
Send:RegisterTMRequest{applicationId='order-service', transactionServiceGroup='hello-service-fescar-service-group'}
Send:RegisterRMRequest{resourceIds='jdbc:mysql://127.0.0.1:3306/db_order?useSSL=false', applicationId='order-service', transactionServiceGroup='hello-service-fescar-service-group'}
Receive:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null,messageId:2
Receive:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null,messageId:1
com.alibaba.fescar.core.rpc.netty.RmRpcClient@7904cd7c msgId:2, future :com.alibaba.fescar.core.protocol.MessageFuture@4107849f, body:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null
com.alibaba.fescar.core.rpc.netty.TmRpcClient@68609034 msgId:1, future :com.alibaba.fescar.core.protocol.MessageFuture@527cc144, body:version=0.4.1,extraData=null,identified=true,resultCode=null,msg=null
register success, cost 28 ms, version:0.4.1,role:TMROLE,channel:[id: 0xf45059d4, L:/127.0.0.1:63533 - R:/127.0.0.1:8091]
register RM success. server version:0.4.1,channel:[id: 0xb7674b6a, L:/127.0.0.1:63534 - R:/127.0.0.1:8091]
register success, cost 37 ms, version:0.4.1,role:RMROLE,channel:[id: 0xb7674b6a, L:/127.0.0.1:63534 - R:/127.0.0.1:8091]日志中可以看到创建连接后，发送了注册请求，然后得到了结果相应，RmRpcClient、TmRpcClient成功实例化。
TM处理流程
在本例中，TM的角色是business-service,因为BusinessService的purchase方法标注了@GlobalTransactional
@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);
}
}GET请求
127.0.0.1:8084/purchaseuserId=1001&commodityCode=2001&orderCount=1看看会发生什么
全局事务开启
首先需要关注的是@GlobalTransactional注解的作用，它是在
GlobalTransactionalInterceptor中被拦截处理
//部分代码
public class GlobalTransactionalInterceptor implements MethodInterceptor {
@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，则进行相应处理
if (globalTransactionalAnnotation != null) {
return handleGlobalTransaction(methodInvocation, globalTransactionalAnnotation);
} else if (globalLockAnnotation != null) {
return handleGlobalLock(methodInvocation);
} else {
return methodInvocation.proceed();
}
}
//调用了TransactionalTemplate
private Object handleGlobalTransaction(final MethodInvocation methodInvocation,
final GlobalTransactional globalTrxAnno) throws Throwable {
try {
return transactionalTemplate.execute(new TransactionalExecutor() {
@Override
public Object execute() throws Throwable {
return methodInvocation.proceed();
}
@Override
public int timeout() {
return globalTrxAnno.timeoutMills();
}
@Override
public String name() {
String name = globalTrxAnno.name();
if (!StringUtils.isNullOrEmpty(name)) {
return name;
}
return formatMethod(methodInvocation.getMethod());
}
});
} catch (TransactionalExecutor.ExecutionException e) {
TransactionalExecutor.Code code = e.getCode();
switch (code) {
case RollbackDone:
throw e.getOriginalException();
case BeginFailure:
failureHandler.onBeginFailure(e.getTransaction(), e.getCause());
throw e.getCause();
case CommitFailure:
failureHandler.onCommitFailure(e.getTransaction(), e.getCause());
throw e.getCause();
case RollbackFailure:
failureHandler.onRollbackFailure(e.getTransaction(), e.getCause());
throw e.getCause();
default:
throw new ShouldNeverHappenException("Unknown TransactionalExecutor.Code: " + code);
}
}
}
}TransactionalTemplate定义了TM对全局事务处理的标准步骤，注释写的比较清楚了
public class TransactionalTemplate {
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();
}
}
}其中DefaultGlobalTransaction的begin方法就是开启全局事务
@Override
public void begin(int timeout, String name) throws TransactionException {
//此处的角色判断有关键的作用
//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，即会忽略后面的begin就退出了
//而判断是发起者（Launcher）还是参与者（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
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 + "]");
}
} 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TC返回的XID一个全局事务就开启了，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
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);
if (count == 5){
throw new RuntimeException("storage branch exception");
}
}
storage的接口和service方法并未出现seata相关的代码和注解，那么它是如何加入到这次全局事务中的呢，答案是ConnectionProxy中，这也是前面说为什么必须要使用DataSourceProxy的原因，通过DataSourceProxy才能在业务代码的事务提交时，seata通过这个切入点，来给TC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，所以在提交本地事务时，实际执行的是ConnectionProxy的commit方法
//部分代码
public class ConnectionProxy extends AbstractConnectionProxy {
@Override
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，可以看到写入undolog和业务数据是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();
}
//注册RM，构建request通过netty向TC发送指令
//将返回的branchId存在上下文中
private void register() throws TransactionException {
Long branchId = DefaultResourceManager.get().branchRegister(BranchType.AT, getDataSourceProxy().getResourceId(),
null, context.getXid(), null, context.buildLockKeys());
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- 获取business-service传来的XID
- 绑定XID到当前上下文中
- 执行业务逻辑sql
- 向TC创建本次RM的Netty连接
- 向TC发送分支事务的相关信息
- 获得TC返回的branchId
- 记录Undo Log数据
- 向TC发送本次事务PhaseOne阶段的处理结果
- 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seata，而是
spring-cloud-alibaba-fescar中对feign、rest支持的实现。bind和unbind XID到上下文中。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
由于这次请求是正常流程无异常的，所以分支事务会正常commit。
在storage-service启动时创建了与TC通讯的Netty连接，TC在获取各RM的汇报结果后，就会给各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
- 收到XID=192.168.0.2:8091:2008546211,branchId=2008546212的commit通知
- 执行commit动作
- 将commit结果发送给TC，branchStatus为PhaseTwo_Committed
具体看下执行commit的过程，在AbstractRMHandler类的doBranchCommit方法之前是接收TC消息包装处理路由的过程
//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branceCommit的请求调用到AsyncWorker的branchCommit方法。AsyncWorker的处理方式是seata架构的一个关键部分，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。
//部分代码
public class AsyncWorker implements ResourceManagerInbound {
private static final List ASYNC_COMMIT_BUFFER = Collections.synchronizedList(
new ArrayList());
//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;
}
//通过一个定时任务消费list中的待提交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> mappedContexts = new HashMap<>();
Iterator iterator = ASYNC_COMMIT_BUFFER.iterator();
//一次定时任务取出ASYNC_COMMIT_BUFFER中的所有待办数据
//以resourceId作为key分组待办数据，resourceId就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问题
while (iterator.hasNext()) {
Phase2Context commitContext = iterator.next();
List contextsGroupedByResourceId = mappedContexts.get(commitContext.resourceId);
if (contextsGroupedByResourceId == null) {
contextsGroupedByResourceId = new ArrayList<>();
mappedContexts.put(commitContext.resourceId, contextsGroupedByResourceId);
}
contextsGroupedByResourceId.add(commitContext);
iterator.remove();
}
for (Map.Entry> 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 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undolog既可
事务回滚
对于rollback场景的触发有两种情况，分支事务处理异常，即ConnectionProxy中report(false)的情况
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，说一下undo的具体流程
根据xid和branchId查找PhaseOne阶段提交的undolog
如果找到了就根据undolog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
第2步处理完后，删除该条undolog数据
如果第1步没有找到对应的undolog，就插入一条状态为GlobalFinished的undolog.
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阶段后续又写入成功，那么PhaseOne阶段就会异常，这样业务数据也是没有提交成功的，数据最终是回滚了的效果
最后希望大家能从文章中得到帮助获得收获，也可以评论出你想看哪方面的技术。文章会持续更新，希望能帮助到大家，哪怕是让你灵光一现。喜欢的朋友可以点点赞和关注，也可以分享出去让更多的人看见，一起努力一起进步！