《MySQL Undo Log：事务回滚的底层原理》（039）

# 📜 MySQL Undo Log：事务回滚的底层原理揭秘 

在MySQL的InnoDB存储引擎中，**Undo Log（回滚日志）**是实现事务**原子性（Atomicity）**的关键组件🔑。它记录了事务修改前的数据状态，为事务回滚（Rollback）和多版本并发控制（MVCC）提供了基础支持。 

## 🔄 Undo Log的工作原理 

1. **修改前的备份**📝 
  当事务对某行数据做修改时（如`UPDATE`），InnoDB会先将原始数据拷贝到Undo Log中，形成一条**回滚记录**。例如： 
  ```sql
  UPDATE users SET balance = 100 WHERE id = 1; -- 修改前balance=50会被存入Undo Log
  ``` 

2. **事务回滚机制**↩️ 
  如果事务执行失败或主动调用`ROLLBACK`，InnoDB会根据Undo Log中的记录，将数据**恢复**到修改前的状态，确保数据一致性。 

3. **MVCC支持**👥 
  Undo Log还用于实现**多版本并发控制**。当其他事务需要读取某行数据时，若该行被锁定，InnoDB会从Undo Log中获取事务开始前的数据版本，避免读写冲突。 

## 🏗️ Undo Log的存储结构 

- **逻辑存储**：Undo Log存储在系统表空间的**回滚段（Rollback Segment）**中，每个回滚段包含多个Undo Slot。 
- **物理存储**：默认存放在`ibdata1`文件中（MySQL 8.0+支持独立Undo表空间）。 
- **清理机制**🚮：当事务提交后，对应的Undo Log不会立即删除，而是由后台线程`purge`在确保无其他事务引用后清理。 

## 💡 总结 

Undo Log是MySQL事务机制的基石之一： 
✅ **保障原子性**：通过记录旧数据实现回滚 
✅ **支持MVCC**：提供多版本数据读取 
✅ **提高并发性**：减少锁争用 

理解Undo Log，能帮助我们更深入地掌握MySQL的事务实现原理！🧠
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