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度，目前已接近8000Star。Seata以高性能和零侵入的方式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。关于Seata的总体介绍，可以查看官方WIKI获得更多更全面的内容介绍。
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）的角度分析说明其工作流程及原理。
文中代码基于fescar-0.4.1，由于项目刚更名为seata不久，例如一些包名、类名、jar包名称还都是fescar的命名，故下文中仍使用fescar进行表述。
示例项目：https://github.com/fescar-group/fescar-samples/tree/master/springcloud-jpa-seata
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
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
- business发起购买请求
- storage扣减库存
- order创建订单
- account扣减余额
异常业务
- business发起购买请求
- storage扣减库存
- order创建订单
- 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
- 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
- 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
}
- 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/apache/incubator-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
- 创建Netty连接
- 发送注册请求
- 得到响应结果
RmRpcClient、TmRpcClient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
- 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
- RM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
- 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查看源码，说一下undo的具体流程
- 根据xid和branchId查找PhaseOne阶段提交的undo_log
- 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
- 第2步处理完后，删除该条undo_log数据
- 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的Roadmap规划，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。
TMClient 和 RMClient 又会根据自身的 poolConfig 配置与 NettyPoolableFactory implements KeyedPoolableObjectFactory<NettyPoolKey, Channel> 进行 channel 连接的交互，channel 连接池根据角色 key+ip 作为连接池的 key 来定位各个连接池
，连接池对 channel 进行统一的管理。TMClient 和 RMClient 在发送过程中对于每个 ip 只会使用一个长连接，但连接不可用时，会从连接池中快速取出已经创建好并可用的连接，减少服务的不可用时间。
