解惑 spring 嵌套事务

反过来说，所有嵌套事务都执行成功，外部事务也执行成功。那么发部事务要发送commit请求给各个嵌套事务，以同时提交，是这个意思吗？

那么对于数据库来说，它相当于在瞬间处理多个事务的commit，在中间的任何一步也是有可能出错的。那这个时候整个事务的回滚如何进行呢？

PROPAGATION_NESTED on the other hand starts a "nested" transaction, which is a true subtransaction of the existing one. What will happen is that a savepoint will be taken at the start of the nested transaction. íf the nested transaction fails, we will roll back to that savepoint. The nested transaction is part of of the outer transaction, so it will only be committed at the end of of the outer transaction.

Nested transactions essentially allow to try some execution subpaths as subtransactions: rolling back to the state at the beginning of the failed subpath, continuing with another subpath or with the main execution path there - all within one isolated transaction, and not losing any previous work done within the outer transaction.

For example, consider parsing a very large input file consisting of account transfer blocks: The entire file should essentially be parsed within one transaction, with one single commit at the end. But if a block fails, its transfers need to be rolled back, writing a failure marker somewhere. You could either start over the entire transaction every time a block fails, remembering which blocks to skip - or you mark each block as a nested transaction, only rolling back that specific set of operations, keeping the previous work of the outer transaction. The latter is of course much more efficient, in particular when a block at the end of the file fails.

Rolling back the entire transaction is the choice of the demarcation code/config that started the outer transaction.

So if an inner transaction throws an exception and is supposed to be rolled back (according to the rollback rules), the transaction will get rolled back to the savepoint taken at the start of the inner transaction. The immediate calling code can then decide to catch the exception and proceed down some other path within the outer transaction.

If the code that called the inner transaction lets the exception propagate up the call chain, the exception will eventually reach the demarcation code of the outer transaction. At that point, the rollback rules of the outer transaction decide whether to trigger a rollback. That would be a rollback of the entire outer transaction then.

So essentially, it depends on your exception handling. If you catch the exception thrown by the inner transaction, you can proceed down some other path within the outer transaction. If you let the exception propagate up the call chain, it's eventually gonna cause a rollback of the entire outer transaction.

ServiceA {   
       
    /**  
     * 事务属性配置为 PROPAGATION_NESTED  
     */  
    void methodA() {   
        // do A ...

        ServiceB.methodB(); 
  
        // do A ...
    }   
  
}

ServiceA {   
       
    /**  
     * 事务属性配置为 PROPAGATION_NESTED  
     */  
    void methodA() {   
        // do A ...
        try {   
            ServiceB.methodB();   
        } catch (Exception ex) {   
            throw ex;
        }   
        // do A ...
    }   
  
}

在我所见过的误解中, 最常见的是下面这种:


引用

假如有两个业务接口 ServiceA 和 ServiceB, 其中 ServiceA 中有一个方法实现如下

/**
* 事务属性配置为 PROPAGATION_REQUIRED
*/
void methodA() {
// 调用 ServiceB 的方法
ServiceB.methodB();
}

那么如果 ServiceB 的 methodB 如果配置了事务, 就必须配置为 PROPAGATION_NESTED



这种想法可能害了不少人, 认为 Service 之间应该避免互相调用, 其实根本不用担心这点，PROPAGATION_REQUIRED 已经说得很明白,
如果当前线程中已经存在事务, 方法调用会加入此事务, 果当前没有事务，就新建一个事务, 所以 ServiceB#methodB() 的事务只要遵循最普通的规则配置为 PROPAGATION_REQUIRED 即可, 如果 ServiceB#methodB (我们称之为内部事务, 为下文打下基础) 抛了异常, 那么 ServiceA#methodA(我们称之为外部事务) 如果没有特殊配置此异常时事务提交 (即 +MyCheckedException的用法), 那么整个事务是一定要 rollback 的, 什么 Service 只能调 Dao 之类的言论纯属无稽之谈, spring 只负责配置了事务属性方法的拦截, 它怎么知道你这个方法是在 Service 还是 Dao 里 ?