数据库期末复习 数据恢复

数据库期末复习 数据恢复

• 一、事务
• 1.1 基本概念
• 1.2 ACID特性
• 1.3 事务状态
• 二、Schedule
• 2.1 Serial
• 2.2 conflict Serializable
• 三、恢复
• 3.1 故障
• 3.2 Redo
• 3.3 Undo
• 3.4 Redo/Undo
• 四、锁
• 4.1 状态
• 4.2 共用
• 4.3 2PL Lock

一、事务

1.1 基本概念

1. 事务是用户定义的一个数据库操作序列，要么全做，要么都不做
2. 操作：
   BEGIN TRANSACTION 事务开始
   COMMIT/ROLLBACK 事务结束

1.2 ACID特性

1. 原子性

2. 一致性

3. 隔离性

4. 持续性

1.3 事务状态

二、Schedule

2.1 Serial

一个事务接一个事务，无交叉

2.2 conflict Serializable

A schedule is conflict serializable if and only if its precedence graph is acyclic(无环的).

If a schedule S can be transformed into a schedule S´ by a series of swaps of non-conflicting instructions, we say that S and S´ are conflict equivalent.

We say that a schedule S is conflict serializable if it is conflict equivalent to a serial schedule.

1. li = read(Q), lj = read(Q). li and lj don’t conflict.
2. li = read(Q), lj = write(Q). They conflict.
3. li = write(Q), lj = read(Q). They conflict
4. li = write(Q), lj = write(Q). They conflict

三、恢复

利用日志文件
日志文件基础操作：

3.1 故障

1. 事务内部的故障 用UNDO
2. 系统故障 用REDO
3. 介质故障
4. 计算机病毒

3.2 Redo

3.3 Undo

4. checkpoint
   在日志中加入< CKPT >
   标记的事务表示已提交或完成

3.4 Redo/Undo

四、锁

4.1 状态

1. Unlocked
2. Shared (Read)
3. Exclusive (Write)

4.2 共用

s-读锁
x-写锁

4.3 2PL Lock

1. two phase lock
   
2. Strict 2PL
   All locks held by a transaction are released only when the transaction completes.