Mybatis plus的多数据源@DS切换为什么不起作用了，谁的锅，@Transactional

由于使用了微服务，会有多个数据库的情况，有时业务需要，需要切换数据源，所以使用了Mybatis plus的@DS来切换多数据源

yml数据库配置如下：

spring:
  datasource:
    dynamic:
      primary: master
      datasource:
        master:
          driverClassName: com.mysql.cj.jdbc.Driver
          url: jdbc:mysql://user
          username: root
          password: root
        common:
          driverClassName: com.mysql.cj.jdbc.Driver
          url: jdbc:mysql://book
          username: root
          password: root

service如下，默认是master数据源

@Service
@Slf4j
public class MasterService {
    @Autowired
    UserService userService;
    @Autowired
    BookService bookService;

    /**必须master库方法先执行，才能回滚，达到事务效果*/
    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto reqDto){
        userService.save(reqDto);
        bookService.save(reqDto);
    }
}

userService

@Service
@Log4j2
public class UserService extends ServiceImpl<UserMapper, User> {
    @Resource
    private UserMapper userMapper;
    
    public void save(ReqDto reqDto) {
        userMapper.save(reqDto);
    }
}

bookService

@Service
@Log4j2
@DS("common")
public class BookService extends ServiceImpl<BookMapper, Book> {
    @Resource
    private BookMapper bookMapper;
    
    public void save(ReqDto reqDto) {
        bookMapper.save(reqDto);
    }
}

但是神奇的事发生的，bookService的数据库应该是common，但是却是master的，也就是说@DS切换数据源没有起作用
于是开始排查

• 去除MasterService.upload上面的@Transactional，数据源切换正常，但是事务无效
• BookService的save上面加@Transactional，数据源没有切换
• BookService的save上面加@Transactional(propagation = Propagation.REQUIRES_NEW)，数据源切换，且事务有效

原因：

• 开启事务的同时，会从数据库连接池获取数据库连接；
• 如果内层的service使用@DS切换数据源，只是又做了一层拦截，但是并没有改变整个事务的连接;
• 在这个事务内的所有数据库操作，都是在事务连接建立之后，所以会产生数据源没有切换的问题;
• 为了使@DS起作用，必须替换数据库连接，也就是改变事务的传播机智，产生新的事务，获取新的数据库连接;
• 所以bookService的save方法上除了加@Transactional外，还需要设置propagation = Propagation.REQUIRES_NEW
  使得代码走以下逻辑：

private TransactionStatus handleExistingTransaction(
        .....省略.....
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			SuspendedResourcesHolder suspendedResources = suspend(transaction);
			try {
				return startTransaction(definition, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error beginEx) {
				resumeAfterBeginException(transaction, suspendedResources, beginEx);
				throw beginEx;
			}
		}
		.....省略.....)

在走startTransaction，再走doBegin，重新创建新事务，获取新的数据库连接，从而得到@DS的数据源

startTransaction(definition, transaction, debugEnabled, suspendedResources);

protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			if (!txObject.hasConnectionHolder() ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				Connection newCon = obtainDataSource().getConnection();//获取数据库连接
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}

			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
			con = txObject.getConnectionHolder().getConnection();

			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);
			txObject.setReadOnly(definition.isReadOnly());

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				con.setAutoCommit(false);
			}
			.....省略.....)

最终代码如下，只需要修改的是bookService
bookService

@Service
@Log4j2
@DS("common")
public class BookService extends ServiceImpl<BookMapper, Book> {
    @Resource
    private BookMapper bookMapper;
    
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void save(ReqDto reqDto) {
        bookMapper.save(reqDto);
    }
}

1. @DS数据源切换生效
2. @Transaction事务生效

需要注意：
master：userService
common：bookService

common数据库的操作，需要在master之后，这样当bookService.save失败，会使得userService回滚；
如果common的操作先，那当userService失败，无法使bookService回滚

会回滚

    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto respDto){
        userService.save(respDto);
        bookService.save(respDto);
    }

不会回滚

    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto respDto){
        bookService.save(respDto);
        userService.save(respDto);
    }