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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。
    • 在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或@GlobalLock的方法​

带有@GlobalTransactional和@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();