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[doc perf]优化事务传播行为中TransactionDefinition.PROPAGATION_NESTED的解释说明 #2610

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[doc perf]优化事务传播行为中TransactionDefinition.PROPAGATION_NESTED的解释说明 #2610

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7 changes: 6 additions & 1 deletion docs/system-design/framework/spring/spring-transaction.md
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Expand Up @@ -419,11 +419,16 @@ Class B {

**3.`TransactionDefinition.PROPAGATION_NESTED`**:

如果当前存在事务，就在嵌套事务内执行；如果当前没有事务，就执行与`TransactionDefinition.PROPAGATION_REQUIRED`类似的操作。也就是说：
如果当前存在事务，则创建一个事务作为当前事务的嵌套事务执行； 如果当前没有事务，就执行与`TransactionDefinition.PROPAGATION_REQUIRED`类似的操作。也就是说：

- 在外部方法开启事务的情况下，在内部开启一个新的事务，作为嵌套事务存在。
- 如果外部方法无事务，则单独开启一个事务，与 `PROPAGATION_REQUIRED` 类似。

`TransactionDefinition.PROPAGATION_NESTED`代表的嵌套事务以父子关系呈现，其核心理念是子事务不会独立提交，依赖于父事务，在父事务中运行；当父事务提交时，子事务也会随着提交，理所当然的，当父事务回滚时，子事务也会回滚；
> 与`TransactionDefinition.PROPAGATION_REQUIRES_NEW`区别于：`PROPAGATION_REQUIRES_NEW`是独立事务，不依赖于外部事务，以平级关系呈现，执行完就会立即提交，与外部事务无关；

子事务也有自己的特性，可以独立进行回滚，不会引发父事务的回滚，但是前提是需要处理子事务的异常，避免异常被父事务感知导致外部事务回滚；

举个例子：
- 如果 `aMethod()` 回滚的话，作为嵌套事务的`bMethod()`会回滚。
- 如果 `bMethod()` 回滚的话，`aMethod()`是否回滚，要看`bMethod()`的异常是否被处理：
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