《MySQL Undo Log：事务回滚的底层原理》（449）

# **MySQL Undo Log：事务回滚的底层原理** 🔄 

在MySQL中，**Undo Log（回滚日志）** 是实现事务**原子性（Atomicity）** 和**多版本并发控制（MVCC）** 的核心机制之一。它记录了事务修改前的数据状态，确保事务失败时可以回滚到之前的状态，保障数据一致性。 

## **Undo Log 的作用** 📝 
1. **事务回滚** 🔙 
  当事务执行失败或显式调用`ROLLBACK`时，MySQL会利用Undo Log中的记录，将数据恢复到修改前的状态。 
 
2. **MVCC 支持** 🔍 
  在InnoDB的MVCC机制中，Undo Log存储了数据的多个版本，使不同事务可以读取不同时间点的数据快照，避免读写冲突。 

## **Undo Log 的工作原理** ⚙️ 
- **写入阶段** ✍️ 
 当事务修改数据时，InnoDB会先将被修改数据的原始值（Before Image）存入Undo Log，再更新内存中的数据页。 
- **回滚阶段** ↩️ 
 如果事务回滚，InnoDB会根据Undo Log逆向执行操作，恢复数据到原始状态。 
- **清理阶段** 🗑️ 
 当事务提交且没有其他事务依赖该Undo Log时，系统会逐步清理无用的日志记录，释放存储空间。 

## **Undo Log 的存储方式** 💾 
Undo Log存储在**系统表空间**或**独立的Undo表空间**中，采用**段（Segment）** 结构管理，支持高效写入和回收。 

## **总结** 🎯 
Undo Log是MySQL事务回滚和MVCC的基石，通过记录数据变更前的状态，确保事务的原子性和一致性。理解其原理，有助于优化事务设计和排查数据一致性问题。 🚀
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