Seata的分布式事务实现原理
Seata分布式事务方案
简介
Seata是阿里开源的分布式事务解决方案中间件，对业务侵入小，在应用中Seata整体事务逻辑基于两阶段提交的模型，核心概念包含三个角色：
- TM：事务发起者。用来告诉TC全局事务的开始，提交，回滚。
- RM：事务资源，每一个RM都会作为一个分支事务注册在TC。
- TC：事务协调者，即独立运行的seata-server，用于接收事务注册，提交和回滚。
Seata的运行分AT和MT两种模式。还有其他的模式如SAGA，还未研究。
AT(Auto Transaction)模式
这个模式需要模块为Java语言，并且数据库支持本地事务。一个典型的分布式事务过程：
- TM 向 TC 申请开启一个全局事务，全局事务创建并生成一个全局唯一的XID。
- XID 在微服务调用链路的上下文中传播。
- RM 向 TC 注册分支事务，将其纳入 XID 对应全局事务的管辖。
- TM 向 TC 发起针对 XID 的全局提交或回滚决议。
- TC 调度 XID 下管辖的全部分支事务完成提交或回滚请求。
MT(Manual Transaction)模式
这个模式适合其他的场景，因为底层存储可能没有事务支持，需要自己实现 prepare、commit和rollback的逻辑
源码分析
参考 https://juejin.im/post/6844904148089962510
初始化
全局的两阶段提交，实际上是通过对数据源的代理实现的，Seata中的代理数据源对druid数据源做了一层代理
两阶段提交
在需要加全局事务的方法上，加上GlobalTransactional注解，Seata中拦截全局事务的拦截器是GlobalTransactionalInterceptor
@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);
final GlobalTransactional globalTransactionalAnnotation =
getAnnotation(method, targetClass, GlobalTransactional.class);
final GlobalLock globalLockAnnotation = getAnnotation(method, targetClass, GlobalLock.class);
if (!disable && globalTransactionalAnnotation != null) {
return handleGlobalTransaction(methodInvocation, globalTransactionalAnnotation);
} else if (!disable && globalLockAnnotation != null) {
return handleGlobalLock(methodInvocation);
} else {
return methodInvocation.proceed();
}
}
调用handleGlobalTransaction方法开启全局事务；否则按普通方法执行。handleGlobalTransaction方法
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();
}
public String name() {
String name = globalTrxAnno.name();
if (!StringUtils.isNullOrEmpty(name)) {
return name;
}
return formatMethod(methodInvocation.getMethod());
}
@Override
public TransactionInfo getTransactionInfo() {
TransactionInfo transactionInfo = new TransactionInfo();
transactionInfo.setTimeOut(globalTrxAnno.timeoutMills());
transactionInfo.setName(name());
transactionInfo.setPropagation(globalTrxAnno.propagation());
Set<RollbackRule> rollbackRules = new LinkedHashSet<>();
for (Class<?> rbRule : globalTrxAnno.rollbackFor()) {
rollbackRules.add(new RollbackRule(rbRule));
}
for (String rbRule : globalTrxAnno.rollbackForClassName()) {
rollbackRules.add(new RollbackRule(rbRule));
}
for (Class<?> rbRule : globalTrxAnno.noRollbackFor()) {
rollbackRules.add(new NoRollbackRule(rbRule));
}
for (String rbRule : globalTrxAnno.noRollbackForClassName()) {
rollbackRules.add(new NoRollbackRule(rbRule));
}
transactionInfo.setRollbackRules(rollbackRules);
return transactionInfo;
}
});
} 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();
case RollbackRetrying:
failureHandler.onRollbackRetrying(e.getTransaction(), e.getCause());
throw e.getCause();
default:
throw new ShouldNeverHappenException(String.format("Unknown TransactionalExecutor.Code: %s", code));
}
}
}
方法中调用了TransactionalTemplate的execute方法
public Object execute(TransactionalExecutor business) throws Throwable {
// 1 get transactionInfo
TransactionInfo txInfo = business.getTransactionInfo();
if (txInfo == null) {
throw new ShouldNeverHappenException("transactionInfo does not exist");
}
// 1.1 get or create a transaction
GlobalTransaction tx = GlobalTransactionContext.getCurrentOrCreate();
// 1.2 Handle the Transaction propatation and the branchType
Propagation propagation = txInfo.getPropagation();
SuspendedResourcesHolder suspendedResourcesHolder = null;
try {
switch (propagation) {
case NOT_SUPPORTED:
suspendedResourcesHolder = tx.suspend(true);
return business.execute();
case REQUIRES_NEW:
suspendedResourcesHolder = tx.suspend(true);
break;
case SUPPORTS:
if (!existingTransaction()) {
return business.execute();
}
break;
case REQUIRED:
break;
case NEVER:
if (existingTransaction()) {
throw new TransactionException(
String.format("Existing transaction found for transaction marked with propagation 'never',xid = %s"
,RootContext.getXID()));
} else {
return business.execute();
}
case MANDATORY:
if (!existingTransaction()) {
throw new TransactionException("No existing transaction found for transaction marked with propagation 'mandatory'");
}
break;
default:
throw new TransactionException("Not Supported Propagation:" + propagation);
}
try {
// 2. begin transaction
beginTransaction(txInfo, tx);
Object rs = null;
try {
// Do Your Business
rs = business.execute();
} catch (Throwable ex) {
// 3.the needed business exception to rollback.
completeTransactionAfterThrowing(txInfo, tx, ex);
throw ex;
}
// 4. everything is fine, commit.
commitTransaction(tx);
return rs;
} finally {
//5. clear
triggerAfterCompletion();
cleanUp();
}
} finally {
tx.resume(suspendedResourcesHolder);
}
}
该方法中主要有以下几个步骤：
- 获取事务信息，
- 开启事务
- 执行业务方法
- 提交事务（没有抛出异常）
- 回滚操作（抛出异常）
beginTransaction最终调用了DefaultGlobalTransaction的begin方法
@Override
public void begin(int timeout, String name) throws TransactionException {
if (role != GlobalTransactionRole.Launcher) {
assertXIDNotNull();
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Ignore Begin(): just involved in global transaction [{}]", xid);
}
return;
}
assertXIDNull();
if (RootContext.getXID() != null) {
throw new IllegalStateException();
}
xid = transactionManager.begin(null, null, name, timeout);
status = GlobalStatus.Begin;
RootContext.bind(xid);
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Begin new global transaction [{}]", xid);
}
}
通过transactionManager.begin()方法通过TmRpcClient与server通信并生成一个xid，再将将xid绑定到Root上下文中。全局事务拦截成功后还是会执行原业务方法，但是由于seata代理了数据源，sql解析undolog是在代理数据源中完成的。seata不止会代理数据源，还会对Connection，Statement做代理封装。对sql解析发生在StatementProxy中
@Override
public ResultSet executeQuery(String sql) throws SQLException {
this.targetSQL = sql;
return ExecuteTemplate.execute(this, (statement, args) -> statement.executeQuery((String) args[0]), sql);
}
最终执行了ExecuteTemplate类的execute方法：
public static <T, S extends Statement> T execute(
List<SQLRecognizer> sqlRecognizers, StatementProxy<S> statementProxy,
StatementCallback<T, S> statementCallback, Object... args) throws SQLException {
if (!shouldExecuteInATMode()) {
// Just work as original statement
return statementCallback.execute(statementProxy.getTargetStatement(), args);
}
if (sqlRecognizers == null) {
sqlRecognizers = SQLVisitorFactory.get(
statementProxy.getTargetSQL(),
statementProxy.getConnectionProxy().getDbType());
}
Executor<T> executor;
if (CollectionUtils.isEmpty(sqlRecognizers)) {
executor = new PlainExecutor<>(statementProxy, statementCallback);
} else {
if (sqlRecognizers.size() == 1) {
SQLRecognizer sqlRecognizer = sqlRecognizers.get(0);
switch (sqlRecognizer.getSQLType()) {
case INSERT:
executor = new InsertExecutor<>(statementProxy, statementCallback, sqlRecognizer);
break;
case UPDATE:
executor = new UpdateExecutor<>(statementProxy, statementCallback, sqlRecognizer);
break;
case DELETE:
executor = new DeleteExecutor<>(statementProxy, statementCallback, sqlRecognizer);
break;
case SELECT_FOR_UPDATE:
executor = new SelectForUpdateExecutor<>(statementProxy, statementCallback, sqlRecognizer);
break;
default:
executor = new PlainExecutor<>(statementProxy, statementCallback);
break;
}
} else {
executor = new MultiExecutor<>(statementProxy, statementCallback, sqlRecognizers);
}
}
T rs;
try {
rs = executor.execute(args);
} catch (Throwable ex) {
if (!(ex instanceof SQLException)) {
// Turn other exception into SQLException
ex = new SQLException(ex);
}
throw (SQLException) ex;
}
return rs;
}
先判断是否存在全局事务，不在全局事务中按普通方法执行，如果在全局事务中则开始解析sql，对不同的DML语句做响应的处理，再调用执行方法。具体流程为：
- 先判断是否开启了全局事务，如果没有，不走代理，不解析sql。
- 调用SQLVisitorFactory对目标sql进行解析。
- 针对特定类型sql操作(INSERT,UPDATE,DELETE,SELECT_FOR_UPDATE)等进行特殊解析。
- 执行sql并返回结果。
关键点在于特定类型执行器里面的execute方法(下面以InsertExecutor类的execute方法举例), 调用了父类BaseTransactionalExecutor的execute方法，
@Override
public T execute(Object... args) throws Throwable {
if (RootContext.inGlobalTransaction()) {
String xid = RootContext.getXID();
statementProxy.getConnectionProxy().bind(xid);
}
statementProxy.getConnectionProxy().setGlobalLockRequire(RootContext.requireGlobalLock());
return doExecute(args);
}
将XID绑定到connectionProxy中并调用了doExecute方法，这里又调用了它的子类的AbstractDMLBaseExecutor的doExecute方法
@Override
public T doExecute(Object... args) throws Throwable {
AbstractConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
if (connectionProxy.getAutoCommit()) {
return executeAutoCommitTrue(args);
} else {
return executeAutoCommitFalse(args);
}
}
executeAutoCommitTrue方法中也会将AutoCommit属性设置为false，对sql进行解析生成undolog，防止在undolog生成之前入库。
protected T executeAutoCommitTrue(Object[] args) throws Throwable {
ConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
try {
connectionProxy.setAutoCommit(false);
return new LockRetryPolicy(connectionProxy).execute(() -> {
T result = executeAutoCommitFalse(args);
connectionProxy.commit();
return result;
});
} catch (Exception e) {
// when exception occur in finally,this exception will lost, so just print it here
LOGGER.error("execute executeAutoCommitTrue error:{}", e.getMessage(), e);
if (!LockRetryPolicy.isLockRetryPolicyBranchRollbackOnConflict()) {
connectionProxy.getTargetConnection().rollback();
}
throw e;
} finally {
connectionProxy.getContext().reset();
connectionProxy.setAutoCommit(true);
}
}
在将AutoCommit设置为false后会继续执行AbstractDMLBaseExecutor中的executeAutoCommitFalse(args)
protected T executeAutoCommitFalse(Object[] args) throws Exception {
TableRecords beforeImage = beforeImage();
T result = statementCallback.execute(statementProxy.getTargetStatement(), args);
TableRecords afterImage = afterImage(beforeImage);
prepareUndoLog(beforeImage, afterImage);
return result;
}
注意这是一个很关键的方法，executeAutoCommitFalse中主要分四步执行：
- 获取sql执行前记录快照beforeImage；
- 执行sql；
- 获取sql执行后记录快照afterimage；
- 根据beforeImage，afterImage生成undolog记录并添加到connectionProxy的上下文中
生成undolog的方法，就是记录lockKey后，将beforeImage和afterImage都记录下来
protected void prepareUndoLog(TableRecords beforeImage, TableRecords afterImage) throws SQLException {
if (!beforeImage.getRows().isEmpty() || !afterImage.getRows().isEmpty()) {
ConnectionProxy connectionProxy = this.statementProxy.getConnectionProxy();
TableRecords lockKeyRecords = this.sqlRecognizer.getSQLType() == SQLType.DELETE ? beforeImage : afterImage;
String lockKeys = this.buildLockKey(lockKeyRecords);
connectionProxy.appendLockKey(lockKeys);
SQLUndoLog sqlUndoLog = this.buildUndoItem(beforeImage, afterImage);
connectionProxy.appendUndoLog(sqlUndoLog);
}
}
protected SQLUndoLog buildUndoItem(TableRecords beforeImage, TableRecords afterImage) {
SQLType sqlType = this.sqlRecognizer.getSQLType();
String tableName = this.sqlRecognizer.getTableName();
SQLUndoLog sqlUndoLog = new SQLUndoLog();
sqlUndoLog.setSqlType(sqlType);
sqlUndoLog.setTableName(tableName);
sqlUndoLog.setBeforeImage(beforeImage);
sqlUndoLog.setAfterImage(afterImage);
return sqlUndoLog;
}
最终会通过UndoLogManager，对undolog记录进行undo或delete操作
try {
// put serializer name to local
setCurrentSerializer(parser.getName());
List<SQLUndoLog> sqlUndoLogs = branchUndoLog.getSqlUndoLogs();
if (sqlUndoLogs.size() > 1) {
Collections.reverse(sqlUndoLogs);
}
for (SQLUndoLog sqlUndoLog : sqlUndoLogs) {
TableMeta tableMeta = TableMetaCacheFactory.getTableMetaCache(dataSourceProxy.getDbType()).getTableMeta(
conn, sqlUndoLog.getTableName(), dataSourceProxy.getResourceId());
sqlUndoLog.setTableMeta(tableMeta);
AbstractUndoExecutor undoExecutor = UndoExecutorFactory.getUndoExecutor(
dataSourceProxy.getDbType(), sqlUndoLog);
undoExecutor.executeOn(conn);
}
} finally {
// remove serializer name
removeCurrentSerializer();
}
但是在这之前，会对当前数据库中的记录和afterImage中的记录进行对比，需要相同才会继续进行
/**
* Data validation.
*
* @param conn the conn
* @return return true if data validation is ok and need continue undo, and return false if no need continue undo.
* @throws SQLException the sql exception such as has dirty data
*/
protected boolean dataValidationAndGoOn(Connection conn) throws SQLException {
TableRecords beforeRecords = sqlUndoLog.getBeforeImage();
TableRecords afterRecords = sqlUndoLog.getAfterImage();
// Compare current data with before data
// No need undo if the before data snapshot is equivalent to the after data snapshot.
Result<Boolean> beforeEqualsAfterResult = DataCompareUtils.isRecordsEquals(beforeRecords, afterRecords);
if (beforeEqualsAfterResult.getResult()) {
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Stop rollback because there is no data change " +
"between the before data snapshot and the after data snapshot.");
}
// no need continue undo.
return false;
}
// Validate if data is dirty.
TableRecords currentRecords = queryCurrentRecords(conn);
// compare with current data and after image.
Result<Boolean> afterEqualsCurrentResult = DataCompareUtils.isRecordsEquals(afterRecords, currentRecords);
if (!afterEqualsCurrentResult.getResult()) {
// If current data is not equivalent to the after data, then compare the current data with the before
// data, too. No need continue to undo if current data is equivalent to the before data snapshot
Result<Boolean> beforeEqualsCurrentResult = DataCompareUtils.isRecordsEquals(beforeRecords, currentRecords);
if (beforeEqualsCurrentResult.getResult()) {
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Stop rollback because there is no data change " +
"between the before data snapshot and the current data snapshot.");
}
// no need continue undo.
return false;
} else {
if (LOGGER.isInfoEnabled()) {
if (StringUtils.isNotBlank(afterEqualsCurrentResult.getErrMsg())) {
LOGGER.info(afterEqualsCurrentResult.getErrMsg(), afterEqualsCurrentResult.getErrMsgParams());
}
}
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("check dirty datas failed, old and new data are not equal," +
"tableName:[" + sqlUndoLog.getTableName() + "]," +
"oldRows:[" + JSON.toJSONString(afterRecords.getRows()) + "]," +
"newRows:[" + JSON.toJSONString(currentRecords.getRows()) + "].");
}
throw new SQLException("Has dirty records when undo.");
}
}
return true;
}
从这个可以说明，Seata的分布式事务实际上全局是串行的，对于存在热点资源的情况下，会导致性能问题。
分支事务注册与事务提交
业务sql和undolog执行完成后会在代理连接ConnectionProxy中执行commit操作
@Override
public void commit() throws SQLException {
try {
LOCK_RETRY_POLICY.execute(() -> {
doCommit();
return null;
});
} catch (SQLException e) {
throw e;
} catch (Exception e) {
throw new SQLException(e);
}
}
private void doCommit() throws SQLException {
if (context.inGlobalTransaction()) {
processGlobalTransactionCommit();
} else if (context.isGlobalLockRequire()) {
processLocalCommitWithGlobalLocks();
} else {
targetConnection.commit();
}
}
如果处于全局事务中则调用processGlobalTransactionCommit处理全局事务提交；
如果加了全局锁注释调用 processLocalCommitWithGlobalLocks()加全局锁并提交；
其他情况直接进行事务提交。
private void processGlobalTransactionCommit() throws SQLException {
try {
register();
} catch (TransactionException e) {
recognizeLockKeyConflictException(e, context.buildLockKeys());
}
try {
UndoLogManagerFactory.getUndoLogManager(this.getDbType()).flushUndoLogs(this);
targetConnection.commit();
} catch (Throwable ex) {
LOGGER.error("process connectionProxy commit error: {}", ex.getMessage(), ex);
report(false);
throw new SQLException(ex);
}
if (IS_REPORT_SUCCESS_ENABLE) {
report(true);
}
context.reset();
}
processGlobalTransactionCommit方法有以下几个操作步骤：
- 注册分支事务，将branchId分支绑定在上下文中。
- 如果包含undolog，则将之前绑定到上下文中的undolog进行入库；
- 提交本地事务；
- 如果操作失败，report()中通过RM提交第一阶段失败消息，如果成功，report()提交第一阶段成功消息
private void report(boolean commitDone) throws SQLException {
if (context.getBranchId() == null) {
return;
}
int retry = REPORT_RETRY_COUNT;
while (retry > 0) {
try {
DefaultResourceManager.get().branchReport(BranchType.AT, context.getXid(), context.getBranchId(),
commitDone ? BranchStatus.PhaseOne_Done : BranchStatus.PhaseOne_Failed, null);
return;
} catch (Throwable ex) {
LOGGER.error("Failed to report [" + context.getBranchId() + "/" + context.getXid() + "] commit done ["
+ commitDone + "] Retry Countdown: " + retry);
retry--;
if (retry == 0) {
throw new SQLException("Failed to report branch status " + commitDone, ex);
}
}
}
}
由于undolog入库和业务sql的执行调用了同一个connection，处于同一个事务中，这就保证了业务sql和undolog肯定是成对存在。
总结
Seata的AT模式实现的是一个传统意义的分布式事务，以自动生成undolog的形式实现了各资源节点的两段式提交。
这个方案的好处在于对现有基于MySQL, PostgreSQL和Oracle的应用可以快速实现分布式事务并且对现有代码无需大量改造，但是缺点在于整体是串行的，并且因为undolog的处理会带来额外损耗，不能解决热点资源的性能问题。
Seata在每个子模块中增加undolog表, 利用节点数据库的单机事务保证子事务和补偿信息的原子性, 可以在分布式事务设计中借鉴
其他: 分布式事务的模式有2PC, TCC, SAGA等, 其中SAGA有集中编排和自由编排两种形式, 分布式事务框架除了Seata, 还有Axon, ServiceComb等.
