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版本:v1.7

Seata事务隔离

本文目标:帮助用户明白使用Seata AT模式时,该如何正确实现事务隔离,防止脏读脏写。

希望读者在阅读本文前,已阅读过seata官网中对AT模式的介绍,并且对数据库本地锁有所了解

(例如,两个事务同时在对同一条记录做update时,只有拿到record lock的事务才能更新成功,另一个事务在record lock未释放前只能等待,直到事务超时)

首先请看这样的一段代码,尽管看着“初级”,但持久层框架实际上帮我们做的主要事情也就这样。

@Service
public class StorageService {

@Autowired
private DataSource dataSource;

@GlobalTransactional
public void batchUpdate() throws SQLException {
Connection connection = null;
PreparedStatement preparedStatement = null;
try {
connection = dataSource.getConnection();
connection.setAutoCommit(false);
String sql = "update storage_tbl set count = ?" +
" where id = ? and commodity_code = ?";
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setInt(1, 100);
preparedStatement.setLong(2, 1);
preparedStatement.setString(3, "2001");
preparedStatement.executeUpdate();
connection.commit();
} catch (Exception e) {
throw e;
} finally {
IOutils.close(preparedStatement);
IOutils.close(connection);
}
}

}

从代理数据源说起

使用AT模式,最重要的事情便是代理数据源,那么用DataSourceProxy代理数据源有什么作用呢?

DataSourceProxy能帮助我们获得几个重要的代理对象

  • 通过DataSourceProxy.getConnection()获得ConnectionProxy

  • 通过ConnectionProxy.prepareStatement(...)获得StatementProxy

Seata的如何实现事务隔离,就藏在这2个Proxy中,我先概述下实现逻辑。

StatementProxy.executeXXX()的处理逻辑

  • 当调用io.seata.rm.datasource.StatementProxy.executeXXX()会将sql交给io.seata.rm.datasource.exec.ExecuteTemplate.execute(...)处理。

    • ExecuteTemplate.execute(...)方法中,Seata根据不同dbType和sql语句类型使用不同的Executer,调用io.seata.rm.datasource.exec.Executer类的execute(Object... args)
    • 如果选了DML类型Executer,主要做了以下事情:
      • 查询前镜像(select for update,因此此时获得本地锁)
      • 执行业务sql
      • 查询后镜像
      • 准备undoLog
    • 如果你的sql是select for update则会使用SelectForUpdateExecutor(Seata代理了select for update),代理后处理的逻辑是这样的:
      • 先执行 select for update(获取数据库本地锁)
      • 如果处于@GlobalTransactional or @GlobalLock检查是否有全局锁
      • 如果有全局锁,则未开启本地事务下会rollback本地事务,再重新争抢本地锁和全局锁,以此类推,除非拿到全局锁

ConnectionProxy.commit()的处理逻辑

  • 处于全局事务中(即,数据持久化方法带有@GlobalTransactional
    • 注册分支事务,获取全局锁
    • undoLog数据入库
    • 让数据库commit本次事务
  • 处于@GlobalLock中(即,数据持久化方法带有@GlobalLock
    • 向tc查询是否有全局锁存在,如存在,则抛出异常
    • 让数据库commit本次事务
  • 除了以上情况(else分支)
    • 让数据库commit本次事务

@GlobalTransactional的作用

标识一个全局事务

@GlobalLock + select for update的作用

如果像updateA()方法带有@GlobalLock + select for update,Seata在处理时,会先获取数据库本地锁,然后查询该记录是否有全局锁存在,若有,则抛出LockConflictException。

先举一个脏写的例子,再来看Seata如何防止脏写

假设你的业务代码是这样的:

  • updateAll()用来同时更新A和B表记录,updateA() updateB()则分别更新A、B表记录
  • updateAll()已经加上了@GlobalTransactional
class YourBussinessService {

DbServiceA serviceA;
DbServiceB serviceB;

@GlobalTransactional
public boolean updateAll(DTO dto) {
serviceA.update(dto.getA());
serviceB.update(dto.getB());
}

public boolean updateA(DTO dto) {
serviceA.update(dto.getA());
}

}
class DbServiceA {
@Transactional
public boolean update(A a) {

}
}

dirty-write |

怎么用Seata防止脏写?

办法一:updateA()也加上@GlobalTransactional,此时Seata会如何保证事务隔离?

class DbServiceA {

@GlobalTransactional
@Transactional
public boolean updateA(DTO dto) {

serviceA.update(dto.getA());

}
}
  • updateAll()先被调用(未完成),updateA()后被调用

dirty-write

办法二: @GlobalLock + select for update

class DbServiceA {

@GlobalLock
@Transactional
public boolean updateA(DTO dto) {

serviceA.selectForUpdate(dto.getA());

serviceA.update(dto.getA());

}
}
  • updateAll()先被调用(未完成),updateA()后被调用 dirty-write

  • 那如果是updateA()先被调用(未完成),updateAll()后被调用呢?
    由于2个业务都是要先获得本地锁,因此同样不会发生脏写

  • 一定有人会问,“这里为什么要加上select for update? 只用@GlobalLock能不能防止脏写?” 能。但请再回看下上面的图,select for update能带来这么几个好处:

    • 锁冲突更“温柔”些。如果只有@GlobalLock,检查到全局锁,则立刻抛出异常,也许再“坚持”那么一下,全局锁就释放了,抛出异常岂不可惜了。
    • updateA()中可以通过select for update获得最新的A,接着再做更新。

如何防止脏读?

场景: 某业务先调用updateAll()updateAll()未执行完成,另一业务后调用queryA()

dirty-write


源码展示

@Service
public class StorageService {

@Autowired
private DataSource dataSource;

@GlobalTransactional
public void update() throws SQLException {
Connection connection = null;
PreparedStatement preparedStatement = null;
try {
connection = dataSource.getConnection();
connection.setAutoCommit(false);
String sql = "update storage_tbl set count = ?" +
" where id = ? and commodity_code = ?";
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setInt(1, 100);
preparedStatement.setLong(2, 1);
preparedStatement.setString(3, "2001");
preparedStatement.execute();
connection.commit();
} catch (Exception e) {
throw e;
} finally {
IOutils.close(preparedStatement);
IOutils.close(connection);
}
}

}

这段代码虽然看着很初级,没有使用持久层框架,但如果将框架帮我们做的事情抽象出来,其实也就是上面这段代码。

简单说明接下来源码介绍的脉络(主要关注和事务隔离有关的源码)

  • 代理数据源的用途
    • DataSourceProxy的作用(返回ConnectionProxy
      • 介绍 ConnectionProxy的一个小功能(存放undolog)
    • ConnectionProxy的作用(返回StatementProxy
    • StatementProxy.execute()的处理逻辑
      • io.seata.rm.datasource.exec.UpdateExecutor的执行逻辑(查前镜像、执行sql、查后镜像、准备undoLog)
      • SelectForUpdateExecutor的执行逻辑(争本地锁,查全局锁。有全局锁,回滚,再争...)
    • ConnectionProxy.commit()的处理逻辑(注册分支事务(争全局锁),写入undoLog,数据库提交)
  • 介绍RootContext
  • GlobalTransactionalInterceptor的不同代理逻辑
    • 带有@GlobalTransactional如何处理
    • 带有@GlobalLock如何处理

DataSourceProxy的作用

DataSourceProxy帮助我们获得几个重要的代理对象

  • 通过DataSourceProxy.getConnection()获得ConnectionProxy
    package io.seata.rm.datasource;

    import java.sql.Connection;

    public class DataSourceProxy extends AbstractDataSourceProxy implements Resource {

    @Override
    public ConnectionProxy getConnection() throws SQLException {
    Connection targetConnection = targetDataSource.getConnection();
    return new ConnectionProxy(this, targetConnection);
    }
    }
    • 现在先介绍下ConnectionProxy中的ConnectionContext,它的有一个功能是存放undoLog
      package io.seata.rm.datasource;

      import io.seata.rm.datasource.undo.SQLUndoLog;

      public class ConnectionProxy extends AbstractConnectionProxy {

      private ConnectionContext context = new ConnectionContext();

      public void appendUndoLog(SQLUndoLog sqlUndoLog) {
      context.appendUndoItem(sqlUndoLog);
      }

      }
      package io.seata.rm.datasource;

      public class ConnectionContext {

      private static final Savepoint DEFAULT_SAVEPOINT = new Savepoint() {
      @Override
      public int getSavepointId() throws SQLException {
      return 0;
      }

      @Override
      public String getSavepointName() throws SQLException {
      return "DEFAULT_SEATA_SAVEPOINT";
      }
      };

      private final Map<Savepoint, List<SQLUndoLog>> sqlUndoItemsBuffer = new LinkedHashMap<>();

      private Savepoint currentSavepoint = DEFAULT_SAVEPOINT;

      void appendUndoItem(SQLUndoLog sqlUndoLog) {
      sqlUndoItemsBuffer.computeIfAbsent(currentSavepoint, k -> new ArrayList<>()).add(sqlUndoLog);
      }

      }

通过ConnectionProxy.prepareStatement(...)获得StatementProxy

package io.seata.rm.datasource;

public class ConnectionProxy extends AbstractConnectionProxy {

public ConnectionProxy(DataSourceProxy dataSourceProxy, Connection targetConnection) {
super(dataSourceProxy, targetConnection);
}

}
package io.seata.rm.datasource;

import java.sql.Connection;

public abstract class AbstractConnectionProxy implements Connection {

protected Connection targetConnection;

public AbstractConnectionProxy(DataSourceProxy dataSourceProxy, Connection targetConnection) {
this.dataSourceProxy = dataSourceProxy;
this.targetConnection = targetConnection;
}

@Override
public PreparedStatement prepareStatement(String sql) throws SQLException {
String dbType = getDbType();
// support oracle 10.2+
PreparedStatement targetPreparedStatement = null;
if (BranchType.AT == RootContext.getBranchType()) { //为什么这里会返回AT?
List<SQLRecognizer> sqlRecognizers = SQLVisitorFactory.get(sql, dbType);
if (sqlRecognizers != null && sqlRecognizers.size() == 1) {
SQLRecognizer sqlRecognizer = sqlRecognizers.get(0);
if (sqlRecognizer != null && sqlRecognizer.getSQLType() == SQLType.INSERT) {
TableMeta tableMeta = TableMetaCacheFactory.getTableMetaCache(dbType).getTableMeta(getTargetConnection(),
sqlRecognizer.getTableName(), getDataSourceProxy().getResourceId());
String[] pkNameArray = new String[tableMeta.getPrimaryKeyOnlyName().size()];
tableMeta.getPrimaryKeyOnlyName().toArray(pkNameArray);
// 如果是insert语句,这里创建的PreparedStatement需要可以返回自动生成的主键,因此使用这个prepareStatement()
targetPreparedStatement = getTargetConnection().prepareStatement(sql,pkNameArray);

}
}
}
if (targetPreparedStatement == null) {
targetPreparedStatement = getTargetConnection().prepareStatement(sql);
}
return new PreparedStatementProxy(this, targetPreparedStatement, sql);
}


public Connection getTargetConnection() {
return targetConnection;
}

}

先在这打下个疑问,后边解释。
RootContext.getBranchType()的返回值怎么会是AT?

StatementProxy.execute()的处理逻辑

  • 当调用io.seata.rm.datasource.StatementProxy.execute()会将sql交给io.seata.rm.datasource.exec.ExecuteTemplate.execute(...)处理。

    package io.seata.rm.datasource;

    public class PreparedStatementProxy extends AbstractPreparedStatementProxy
    implements PreparedStatement, ParametersHolder {

    @Override
    public boolean execute() throws SQLException {
    return ExecuteTemplate.execute(this, (statement, args) -> statement.execute());
    }

    }
    • ExecuteTemplate.execute(...)方法中,Seata根据不同dbType和sql语句类型使用不同的Executer,调用io.seata.rm.datasource.exec.Executer类的execute(Object... args)

      package io.seata.rm.datasource.exec;


      public class ExecuteTemplate {

      public static <T, S extends Statement> T execute(StatementProxy<S> statementProxy,
      StatementCallback<T, S> statementCallback,
      Object... args) throws SQLException {
      return execute(null, statementProxy, statementCallback, args);
      }

      public static <T, S extends Statement> T execute(List<SQLRecognizer> sqlRecognizers,
      StatementProxy<S> statementProxy,
      StatementCallback<T, S> statementCallback,
      Object... args) throws SQLException {
      if (!RootContext.requireGlobalLock() && BranchType.AT != RootContext.getBranchType()) {
      // Just work as original statement
      return statementCallback.execute(statementProxy.getTargetStatement(), args);
      }

      String dbType = statementProxy.getConnectionProxy().getDbType();
      if (CollectionUtils.isEmpty(sqlRecognizers)) {
      sqlRecognizers = SQLVisitorFactory.get(
      statementProxy.getTargetSQL(),
      dbType);
      }
      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 = EnhancedServiceLoader.load(InsertExecutor.class, dbType,
      new Class[]{StatementProxy.class, StatementCallback.class, SQLRecognizer.class},
      new Object[]{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;
      }

      }

      也在这打下个疑问,后边解释。
      RootContext.requireGlobalLock()怎么判断当前是否需要全局锁?

      先以io.seata.rm.datasource.exec.UpdateExecutor举例,UpdateExecutor extends AbstractDMLBaseExecutor extends BaseTransactionalExecutor。 观察execute()方法的做了什么

      package io.seata.rm.datasource.exec;

      public abstract class BaseTransactionalExecutor<T, S extends Statement> implements Executor<T> {


      protected StatementProxy<S> statementProxy;

      protected StatementCallback<T, S> statementCallback;

      protected SQLRecognizer sqlRecognizer;

      public BaseTransactionalExecutor(StatementProxy<S> statementProxy, StatementCallback<T, S> statementCallback,
      SQLRecognizer sqlRecognizer) {
      this.statementProxy = statementProxy;
      this.statementCallback = statementCallback;
      this.sqlRecognizer = sqlRecognizer;
      }

      @Override
      public T execute(Object... args) throws Throwable {
      String xid = RootContext.getXID();
      if (xid != null) {
      statementProxy.getConnectionProxy().bind(xid);
      }

      statementProxy.getConnectionProxy().setGlobalLockRequire(RootContext.requireGlobalLock());
      return doExecute(args);
      }

      }
      public abstract class AbstractDMLBaseExecutor<T, S extends Statement> extends BaseTransactionalExecutor<T, S> {

      public AbstractDMLBaseExecutor(StatementProxy<S> statementProxy, StatementCallback<T, S> statementCallback,
      SQLRecognizer sqlRecognizer) {
      super(statementProxy, statementCallback, sqlRecognizer);
      }

      @Override
      public T doExecute(Object... args) throws Throwable {
      AbstractConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
      if (connectionProxy.getAutoCommit()) {
      return executeAutoCommitTrue(args);
      } else {
      return executeAutoCommitFalse(args);
      }
      }

      protected T executeAutoCommitTrue(Object[] args) throws Throwable {
      ConnectionProxy connectionProxy = statementProxy.getConnectionProxy();
      try {
      connectionProxy.changeAutoCommit(); // 注意,你如果没开启事务,seata帮你开启
      return new LockRetryPolicy(connectionProxy).execute(() -> {
      T result = executeAutoCommitFalse(args);
      connectionProxy.commit(); // 帮你开启事务后,通过connectionProxy来提交
      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);
      }
      }

      protected T executeAutoCommitFalse(Object[] args) throws Exception {
      if (!JdbcConstants.MYSQL.equalsIgnoreCase(getDbType()) && isMultiPk()) {
      throw new NotSupportYetException("multi pk only support mysql!");
      }
      TableRecords beforeImage = beforeImage();
      T result = statementCallback.execute(statementProxy.getTargetStatement(), args);
      TableRecords afterImage = afterImage(beforeImage);
      prepareUndoLog(beforeImage, afterImage);
      return result;
      }
      }
      package io.seata.rm.datasource.exec;

      public class UpdateExecutor<T, S extends Statement> extends AbstractDMLBaseExecutor<T, S> {

      public UpdateExecutor(StatementProxy<S> statementProxy, StatementCallback<T, S> statementCallback,
      SQLRecognizer sqlRecognizer) {
      super(statementProxy, statementCallback, sqlRecognizer);
      }

      }

    • 如果选了DML类型Executer,可以在上面的executeAutoCommitFalse()中看到,主要做了以下事情:

      • 查询前镜像(select for update,因此此时获得本地锁)

        package io.seata.rm.datasource.exec;

        public class UpdateExecutor<T, S extends Statement> extends AbstractDMLBaseExecutor<T, S> {

        private static final boolean ONLY_CARE_UPDATE_COLUMNS = CONFIG.getBoolean(
        ConfigurationKeys.TRANSACTION_UNDO_ONLY_CARE_UPDATE_COLUMNS, DefaultValues.DEFAULT_ONLY_CARE_UPDATE_COLUMNS); // 默认为true

        @Override
        protected TableRecords beforeImage() throws SQLException {
        ArrayList<List<Object>> paramAppenderList = new ArrayList<>();
        TableMeta tmeta = getTableMeta();
        String selectSQL = buildBeforeImageSQL(tmeta, paramAppenderList);
        // SELECT id, count FROM storage_tbl WHERE id = ? FOR UPDATE
        return buildTableRecords(tmeta, selectSQL, paramAppenderList);
        }

        private String buildBeforeImageSQL(TableMeta tableMeta, ArrayList<List<Object>> paramAppenderList) {
        SQLUpdateRecognizer recognizer = (SQLUpdateRecognizer) sqlRecognizer;
        List<String> updateColumns = recognizer.getUpdateColumns();
        StringBuilder prefix = new StringBuilder("SELECT ");
        StringBuilder suffix = new StringBuilder(" FROM ").append(getFromTableInSQL());
        String whereCondition = buildWhereCondition(recognizer, paramAppenderList);
        if (StringUtils.isNotBlank(whereCondition)) {
        suffix.append(WHERE).append(whereCondition);
        }
        String orderBy = recognizer.getOrderBy();
        if (StringUtils.isNotBlank(orderBy)) {
        suffix.append(orderBy);
        }
        ParametersHolder parametersHolder = statementProxy instanceof ParametersHolder ? (ParametersHolder)statementProxy : null;
        String limit = recognizer.getLimit(parametersHolder, paramAppenderList);
        if (StringUtils.isNotBlank(limit)) {
        suffix.append(limit);
        }
        suffix.append(" FOR UPDATE");
        StringJoiner selectSQLJoin = new StringJoiner(", ", prefix.toString(), suffix.toString());
        if (ONLY_CARE_UPDATE_COLUMNS) {
        if (!containsPK(updateColumns)) {// 如果本次更新的行不包含主键,那select for update的时候加上主键
        selectSQLJoin.add(getColumnNamesInSQL(tableMeta.getEscapePkNameList(getDbType())));
        }
        for (String columnName : updateColumns) {
        selectSQLJoin.add(columnName);
        }
        } else {
        for (String columnName : tableMeta.getAllColumns().keySet()) {
        selectSQLJoin.add(ColumnUtils.addEscape(columnName, getDbType()));
        }
        }
        return selectSQLJoin.toString();
        }


        protected TableRecords buildTableRecords(TableMeta tableMeta, String selectSQL, ArrayList<List<Object>> paramAppenderList) throws SQLException {
        ResultSet rs = null;
        try (PreparedStatement ps = statementProxy.getConnection().prepareStatement(selectSQL)) { // 执行select for update,然后就拿到了本地锁
        if (CollectionUtils.isNotEmpty(paramAppenderList)) {
        for (int i = 0, ts = paramAppenderList.size(); i < ts; i++) {
        List<Object> paramAppender = paramAppenderList.get(i);
        for (int j = 0, ds = paramAppender.size(); j < ds; j++) {
        ps.setObject(i * ds + j + 1, paramAppender.get(j));
        }
        }
        }
        rs = ps.executeQuery();
        return TableRecords.buildRecords(tableMeta, rs);
        } finally {
        IOUtil.close(rs);
        }
        }
        }

      • 执行业务sql

      • 查询后镜像

          package io.seata.rm.datasource.exec;

        public class UpdateExecutor<T, S extends Statement> extends AbstractDMLBaseExecutor<T, S> {

        @Override
        protected TableRecords afterImage(TableRecords beforeImage) throws SQLException {
        TableMeta tmeta = getTableMeta();
        if (beforeImage == null || beforeImage.size() == 0) {
        return TableRecords.empty(getTableMeta());
        }
        String selectSQL = buildAfterImageSQL(tmeta, beforeImage);
        //SELECT id, count FROM storage_tbl WHERE (id) in ( (?) )
        ResultSet rs = null;
        try (PreparedStatement pst = statementProxy.getConnection().prepareStatement(selectSQL)) {
        SqlGenerateUtils.setParamForPk(beforeImage.pkRows(), getTableMeta().getPrimaryKeyOnlyName(), pst);
        rs = pst.executeQuery();
        return TableRecords.buildRecords(tmeta, rs);
        } finally {
        IOUtil.close(rs);
        }
        }
        }
      • 准备undoLog

        public abstract class BaseTransactionalExecutor<T, S extends Statement> implements Executor<T> {

        protected void prepareUndoLog(TableRecords beforeImage, TableRecords afterImage) throws SQLException {
        if (beforeImage.getRows().isEmpty() && afterImage.getRows().isEmpty()) {
        return;
        }
        if (SQLType.UPDATE == sqlRecognizer.getSQLType()) {
        if (beforeImage.getRows().size() != afterImage.getRows().size()) {
        throw new ShouldNeverHappenException("Before image size is not equaled to after image size, probably because you updated the primary keys.");
        }
        }
        ConnectionProxy connectionProxy = statementProxy.getConnectionProxy();

        TableRecords lockKeyRecords = sqlRecognizer.getSQLType() == SQLType.DELETE ? beforeImage : afterImage;
        String lockKeys = buildLockKey(lockKeyRecords);
        if (null != lockKeys) {
        connectionProxy.appendLockKey(lockKeys);

        SQLUndoLog sqlUndoLog = buildUndoItem(beforeImage, afterImage);
        connectionProxy.appendUndoLog(sqlUndoLog); // 把undoLog存到connectionProxy中,具体怎么回事上面有提过
        }
        }
        }
    • 如果你的sql是select for update则会使用SelectForUpdateExecutor(Seata代理了select for update),代理后处理的逻辑是这样的:

      • 先执行 select for update(获取数据库本地锁)
      • 如果处于@GlobalTransactional or @GlobalLock检查是否有全局锁
      • 如果有全局锁,则未开启本地事务下会rollback本地事务,再重新争抢本地锁和查询全局锁,直到全局锁释放
         package io.seata.rm.datasource.exec;

      public class SelectForUpdateExecutor<T, S extends Statement> extends BaseTransactionalExecutor<T, S> {
      @Override
      public T doExecute(Object... args) throws Throwable {
      Connection conn = statementProxy.getConnection();
      DatabaseMetaData dbmd = conn.getMetaData();
      T rs;
      Savepoint sp = null;
      boolean originalAutoCommit = conn.getAutoCommit();
      try {
      if (originalAutoCommit) {
      /*
      * In order to hold the local db lock during global lock checking
      * set auto commit value to false first if original auto commit was true
      */
      conn.setAutoCommit(false);
      } else if (dbmd.supportsSavepoints()) {
      /*
      * In order to release the local db lock when global lock conflict
      * create a save point if original auto commit was false, then use the save point here to release db
      * lock during global lock checking if necessary
      */
      sp = conn.setSavepoint();
      } else {
      throw new SQLException("not support savepoint. please check your db version");
      }

      LockRetryController lockRetryController = new LockRetryController();
      ArrayList<List<Object>> paramAppenderList = new ArrayList<>();
      String selectPKSQL = buildSelectSQL(paramAppenderList);
      while (true) {
      try {
      // #870
      // execute return Boolean
      // executeQuery return ResultSet
      rs = statementCallback.execute(statementProxy.getTargetStatement(), args); //执行 select for update(获取数据库本地锁)

      // Try to get global lock of those rows selected
      TableRecords selectPKRows = buildTableRecords(getTableMeta(), selectPKSQL, paramAppenderList);
      String lockKeys = buildLockKey(selectPKRows);
      if (StringUtils.isNullOrEmpty(lockKeys)) {
      break;
      }

      if (RootContext.inGlobalTransaction() || RootContext.requireGlobalLock()) {
      // Do the same thing under either @GlobalTransactional or @GlobalLock,
      // that only check the global lock here.
      statementProxy.getConnectionProxy().checkLock(lockKeys);
      } else {
      throw new RuntimeException("Unknown situation!");
      }
      break;
      } catch (LockConflictException lce) {
      if (sp != null) {
      conn.rollback(sp);
      } else {
      conn.rollback();// 回滚,释放本地锁
      }
      // trigger retry
      lockRetryController.sleep(lce);
      }
      }
      } finally {
      if (sp != null) {
      try {
      if (!JdbcConstants.ORACLE.equalsIgnoreCase(getDbType())) {
      conn.releaseSavepoint(sp);
      }
      } catch (SQLException e) {
      LOGGER.error("{} release save point error.", getDbType(), e);
      }
      }
      if (originalAutoCommit) {
      conn.setAutoCommit(true);
      }
      }
      return rs;
      }



      }

ConnectionProxy.commit()的处理逻辑

public class ConnectionProxy extends AbstractConnectionProxy {

private final static LockRetryPolicy LOCK_RETRY_POLICY = new LockRetryPolicy();

private ConnectionContext context = new ConnectionContext();

@Override
public void commit() throws SQLException {
try {
LOCK_RETRY_POLICY.execute(() -> {
doCommit();
return null;
});
} catch (SQLException e) {
if (targetConnection != null && !getAutoCommit() && !getContext().isAutoCommitChanged()) {
rollback();
}
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();
}
}

}

也在这打下个疑问,后边解释。
ConnectionProxy里的ConnectionContext是如何判断inGlobalTransaction() or isGlobalLockRequire()的呢?

  • 处于全局事务中(即,数据持久化方法带有@GlobalTransactional

    • 注册分支事务,获取全局锁
    • undoLog数据入库
    • 让数据库commit本次事务
        public class ConnectionProxy extends AbstractConnectionProxy {

    private final static LockRetryPolicy LOCK_RETRY_POLICY = new LockRetryPolicy();

    private ConnectionContext context = new ConnectionContext();

    private void processGlobalTransactionCommit() throws SQLException {
    try {
    register(); // 注册分支,争全局锁
    } catch (TransactionException e) {
    recognizeLockKeyConflictException(e, context.buildLockKeys());
    }
    try {
    UndoLogManagerFactory.getUndoLogManager(this.getDbType()).flushUndoLogs(this); // undolog入库
    targetConnection.commit(); // 分支事务提交
    } 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();
    }

    private void register() throws TransactionException {
    if (!context.hasUndoLog() || !context.hasLockKey()) {
    return;
    }
    Long branchId = DefaultResourceManager.get().branchRegister(BranchType.AT, getDataSourceProxy().getResourceId(),
    null, context.getXid(), null, context.buildLockKeys());
    context.setBranchId(branchId);
    }


    }
  • 处于@GlobalLock中(即,数据持久化方法带有@GlobalLock

    • 向tc查询是否有全局锁存在
    • 让数据库commit本次事务
       public class ConnectionProxy extends AbstractConnectionProxy {

    private final static LockRetryPolicy LOCK_RETRY_POLICY = new LockRetryPolicy();

    private ConnectionContext context = new ConnectionContext();

    private void processLocalCommitWithGlobalLocks() throws SQLException {
    checkLock(context.buildLockKeys());
    try {
    targetConnection.commit();
    } catch (Throwable ex) {
    throw new SQLException(ex);
    }
    context.reset();
    }

    public void checkLock(String lockKeys) throws SQLException {
    if (StringUtils.isBlank(lockKeys)) {
    return;
    }
    // Just check lock without requiring lock by now.
    try {
    boolean lockable = DefaultResourceManager.get().lockQuery(BranchType.AT,
    getDataSourceProxy().getResourceId(), context.getXid(), lockKeys);
    if (!lockable) {
    throw new LockConflictException();
    }
    } catch (TransactionException e) {
    recognizeLockKeyConflictException(e, lockKeys);
    }
    }


    }
  • 除了以上情况(else分支)

    • 让数据库commit本次事务

介绍RootContext

我们在上面留下了3个“扣儿”,现在到了结合RootContext源码来解答的时候。

  1. RootContext.getBranchType()的返回值怎么会是AT?
    该方法的判断逻辑是:只要判断出当前处于全局事务中(即,只要有地方调用过RootContext.bind(xid)), 就会返回默认BranchType.AT

    public class RootContext {

    public static final String KEY_XID = "TX_XID";

    private static ContextCore CONTEXT_HOLDER = ContextCoreLoader.load();

    private static BranchType DEFAULT_BRANCH_TYPE;

    @Nullable
    public static BranchType getBranchType() {
    if (inGlobalTransaction()) {
    BranchType branchType = (BranchType) CONTEXT_HOLDER.get(KEY_BRANCH_TYPE);
    if (branchType != null) {
    return branchType;
    }
    //Returns the default branch type.
    return DEFAULT_BRANCH_TYPE != null ? DEFAULT_BRANCH_TYPE : BranchType.AT;
    }
    return null;
    }

    public static boolean inGlobalTransaction() {
    return CONTEXT_HOLDER.get(KEY_XID) != null;
    }

    public static void bind(@Nonnull String xid) {
    if (StringUtils.isBlank(xid)) {
    if (LOGGER.isDebugEnabled()) {
    LOGGER.debug("xid is blank, switch to unbind operation!");
    }
    unbind();
    } else {
    MDC.put(MDC_KEY_XID, xid);
    if (LOGGER.isDebugEnabled()) {
    LOGGER.debug("bind {}", xid);
    }
    CONTEXT_HOLDER.put(KEY_XID, xid);
    }
    }

    }
  2. RootContext.requireGlobalLock()怎么判断当前是否需要全局锁?
    需要有地方调用RootContext.bindGlobalLockFlag()

    public class RootContext {

    public static final String KEY_GLOBAL_LOCK_FLAG = "TX_LOCK";
    public static final Boolean VALUE_GLOBAL_LOCK_FLAG = true;

    private static ContextCore CONTEXT_HOLDER = ContextCoreLoader.load();

    public static boolean requireGlobalLock() {
    return CONTEXT_HOLDER.get(KEY_GLOBAL_LOCK_FLAG) != null;
    }

    public static void bindGlobalLockFlag() {
    if (LOGGER.isDebugEnabled()) {
    LOGGER.debug("Local Transaction Global Lock support enabled");
    }

    //just put something not null
    CONTEXT_HOLDER.put(KEY_GLOBAL_LOCK_FLAG, VALUE_GLOBAL_LOCK_FLAG);
    }

    }
  3. ConnectionProxy.commit()会根据context的不同状态区分处理,那ConnectionContext是如何判断inGlobalTransaction() or isGlobalLockRequire()的呢?

     public class ConnectionProxy extends AbstractConnectionProxy {

    private ConnectionContext context = new ConnectionContext();

    private void doCommit() throws SQLException {
    if (context.inGlobalTransaction()) {
    processGlobalTransactionCommit();
    } else if (context.isGlobalLockRequire()) {
    processLocalCommitWithGlobalLocks();
    } else {
    targetConnection.commit();
    }
    }

    }
    • 如何判断inGlobalTransaction()?(注意下,这里和上面提到的RootContext不是一个东西)
      public class ConnectionContext {

      private String xid;

      void setXid(String xid) {
      this.xid = xid;
      }

      public boolean inGlobalTransaction() {
      return xid != null;
      }

      void bind(String xid) {
      if (xid == null) {
      throw new IllegalArgumentException("xid should not be null");
      }
      if (!inGlobalTransaction()) {
      setXid(xid);
      } else {
      if (!this.xid.equals(xid)) {
      throw new ShouldNeverHappenException();
      }
      }
      }

      }
      哪里调用的ConnectionContext.bind(xid)?
      package io.seata.rm.datasource.exec;

      public abstract class BaseTransactionalExecutor<T, S extends Statement> implements Executor<T> {

      @Override
      public T execute(Object... args) throws Throwable {
      // 那么,这里的XID哪来的呢?往后看就知道,是来自开启全局事务的时候获得的,和@GlobalTransactional有关
      String xid = RootContext.getXID();
      if (xid != null) {
      statementProxy.getConnectionProxy().bind(xid);
      }

      // 这里就是设置 isGlobalLockRequire的位置,和 @GlobalLock有关
      statementProxy.getConnectionProxy().setGlobalLockRequire(RootContext.requireGlobalLock());
      return doExecute(args);
      }

      }
      public class ConnectionProxy extends AbstractConnectionProxy {

      private ConnectionContext context = new ConnectionContext();

      public void bind(String xid) {
      context.bind(xid);
      }

      public void setGlobalLockRequire(boolean isLock) {
      context.setGlobalLockRequire(isLock);
      }

      }
    • 如何判断isGlobalLockRequire()
      public class ConnectionContext {

      private boolean isGlobalLockRequire;

      boolean isGlobalLockRequire() {
      return isGlobalLockRequire;
      }

      void setGlobalLockRequire(boolean isGlobalLockRequire) {
      this.isGlobalLockRequire = isGlobalLockRequire;
      }

      }

    在看过代码后,我们知道,只要有地方在RootContext中设置了xid,或bindGlobalLockFlag(),就会识别成不同的状态。 那么哪儿调用的呢?答案就在下方的GlobalTransactionalInterceptor中。

GlobalTransactionalInterceptor处理带有@GlobalTransactional@GlobalLock的方法

带有@GlobalTransactional@GlobalLock的方法会被代理,交给GlobalTransactionalInterceptor处理

public class GlobalTransactionalInterceptor implements ConfigurationChangeListener, 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);
if (specificMethod != null && !specificMethod.getDeclaringClass().equals(Object.class)) {
final Method method = BridgeMethodResolver.findBridgedMethod(specificMethod);
final GlobalTransactional globalTransactionalAnnotation =
getAnnotation(method, targetClass, GlobalTransactional.class);
final GlobalLock globalLockAnnotation = getAnnotation(method, targetClass, GlobalLock.class);
boolean localDisable = disable || (degradeCheck && degradeNum >= degradeCheckAllowTimes);
if (!localDisable) {
if (globalTransactionalAnnotation != null) {
return handleGlobalTransaction(methodInvocation, globalTransactionalAnnotation);// 处理 @GlobalTransactional
} else if (globalLockAnnotation != null) {
return handleGlobalLock(methodInvocation, globalLockAnnotation); // 处理 @GlobalLock
}
}
}
return methodInvocation.proceed();
}

}

先看处理@GlobalTransactional

public class GlobalTransactionalInterceptor implements ConfigurationChangeListener, MethodInterceptor {

private final TransactionalTemplate transactionalTemplate = new TransactionalTemplate();

Object handleGlobalTransaction(final MethodInvocation methodInvocation,
final GlobalTransactional globalTrxAnno) throws Throwable {

//...
try {
return transactionalTemplate.execute(...);
} catch (TransactionalExecutor.ExecutionException e) {
// ...
} finally {
//...
}
}
}

来到了经典的seata事务模板方法,我们要关注开启事务的部分

public class TransactionalTemplate {

public Object execute(TransactionalExecutor business) throws Throwable {
// 1. Get transactionInfo
//...
// 1.1 Get current transaction, if not null, the tx role is 'GlobalTransactionRole.Participant'.
GlobalTransaction tx = GlobalTransactionContext.getCurrent();

// 1.2 Handle the transaction propagation.
// ...

// 1.3 If null, create new transaction with role 'GlobalTransactionRole.Launcher'.
if (tx == null) {
tx = GlobalTransactionContext.createNew();
}

//...

try {
// 2. If the tx role is 'GlobalTransactionRole.Launcher', send the request of beginTransaction to TC,
// else do nothing. Of course, the hooks will still be triggered.
beginTransaction(txInfo, tx);

Object rs;
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
//...
}
} finally {
// If the transaction is suspended, resume it.
// ...
}
}


private void beginTransaction(TransactionInfo txInfo, GlobalTransaction tx) throws TransactionalExecutor.ExecutionException {
try {
triggerBeforeBegin();
tx.begin(txInfo.getTimeOut(), txInfo.getName());
triggerAfterBegin();
} catch (TransactionException txe) {
throw new TransactionalExecutor.ExecutionException(tx, txe,
TransactionalExecutor.Code.BeginFailure);

}
}


}

public class DefaultGlobalTransaction implements GlobalTransaction {

@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();
String currentXid = RootContext.getXID();
if (currentXid != null) {
throw new IllegalStateException("Global transaction already exists," +
" can't begin a new global transaction, currentXid = " + currentXid);
}
xid = transactionManager.begin(null, null, name, timeout);
status = GlobalStatus.Begin;
RootContext.bind(xid); // 绑定xid
if (LOGGER.isInfoEnabled()) {
LOGGER.info("Begin new global transaction [{}]", xid);
}
}
}

看到了吗?RootContext.bind(xid);

接着看处理@GlobalLock

public class GlobalTransactionalInterceptor implements ConfigurationChangeListener, MethodInterceptor {

private final GlobalLockTemplate globalLockTemplate = new GlobalLockTemplate();

Object handleGlobalLock(final MethodInvocation methodInvocation,
final GlobalLock globalLockAnno) throws Throwable {

return globalLockTemplate.execute(new GlobalLockExecutor() {...});
}
}

也使用了模板方法来处理GlobalLock

public class GlobalLockTemplate {

public Object execute(GlobalLockExecutor executor) throws Throwable {
boolean alreadyInGlobalLock = RootContext.requireGlobalLock();
if (!alreadyInGlobalLock) {
RootContext.bindGlobalLockFlag();
}

// set my config to config holder so that it can be access in further execution
// for example, LockRetryController can access it with config holder
GlobalLockConfig myConfig = executor.getGlobalLockConfig();
GlobalLockConfig previousConfig = GlobalLockConfigHolder.setAndReturnPrevious(myConfig);

try {
return executor.execute();
} finally {
// only unbind when this is the root caller.
// otherwise, the outer caller would lose global lock flag
if (!alreadyInGlobalLock) {
RootContext.unbindGlobalLockFlag();
}

// if previous config is not null, we need to set it back
// so that the outer logic can still use their config
if (previousConfig != null) {
GlobalLockConfigHolder.setAndReturnPrevious(previousConfig);
} else {
GlobalLockConfigHolder.remove();
}
}
}
}

看到吗,一进模板方法就RootContext.bindGlobalLockFlag();