事务隔离

事务隔离

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) {

    }
}

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

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
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()`

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

        public static &lt;T, S extends Statement> T execute(List&lt;SQLRecognizer> sqlRecognizers,
                                             StatementProxy&lt;S> statementProxy,
                                             StatementCallback&lt;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&lt;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&lt;T, S extends Statement> implements Executor&lt;T> {

        protected StatementProxy&lt;S> statementProxy;

        protected StatementCallback&lt;T, S> statementCallback;

        protected SQLRecognizer sqlRecognizer;

        public BaseTransactionalExecutor(StatementProxy&lt;S> statementProxy, StatementCallback&lt;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&lt;T, S extends Statement> extends BaseTransactionalExecutor&lt;T, S> {

        public AbstractDMLBaseExecutor(StatementProxy&lt;S> statementProxy, StatementCallback&lt;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&lt;T, S extends Statement> extends AbstractDMLBaseExecutor&lt;T, S> {

        public UpdateExecutor(StatementProxy&lt;S> statementProxy, StatementCallback&lt;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&lt;T, S extends Statement> extends AbstractDMLBaseExecutor&lt;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&lt;List&lt;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&lt;List&lt;Object>> paramAppenderList) {
                SQLUpdateRecognizer recognizer = (SQLUpdateRecognizer) sqlRecognizer;
                List&lt;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&lt;List&lt;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&lt;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&lt;T, S extends Statement> extends AbstractDMLBaseExecutor&lt;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&lt;T, S extends Statement> implements Executor&lt;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&lt;T, S extends Statement> extends BaseTransactionalExecutor&lt;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&lt;List&lt;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 implements Executor {

@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);

（本文超出站点单文档体积上限，已按小节截断）

事务隔离

事务隔离

Seata事务隔离

从代理数据源说起

StatementProxy.executeXXX()的处理逻辑

ConnectionProxy.commit()的处理逻辑

@GlobalTransactional的作用

@GlobalLock + select for update的作用

先举一个脏写的例子，再来看Seata如何防止脏写

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？

办法二： @GlobalLock + select for update

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()

源码展示

DataSourceProxy的作用

通过ConnectionProxy.prepareStatement(...)获得StatementProxy

StatementProxy.execute()的处理逻辑

ConnectionProxy.commit()的处理逻辑

`StatementProxy.executeXXX()`的处理逻辑

`ConnectionProxy.commit()`的处理逻辑

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

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

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

`StatementProxy.execute()`的处理逻辑

`ConnectionProxy.commit()`的处理逻辑