【COMP207 LEC13，LEC14】

LEC13、14

LOGGING in DBMS

>> UNDO without REDO

Ensure Atomicity, so to ensure durability, it can use FORCE: 

1. Force the writing of updates to disk before committing

2. Force is expensive in disk operations

>> REDO without UNDO

Ensure durability, so to ensure atomicity, it can use No Steal:

1. No Steal means that uncommitted data may not overwrite committed data on disk

2. No Steal is expensive to ensure

>> Without Force / No Steal, it is hard and expensive to ensure Atomicity of UNDO and durability of REDO

>> CHECKPOINT

一段一段地checkpoint the log，这样就不用undo在Checkpoint之前的transactions了

 Time 0-10（？？我觉得应该是到10吧，但是ppt确实是标到8） 就是上图里logfile的部分，是在写changes to disk所以不用undo

Time 11-19 是checkpoint的部分， 写在Time 19 下面，用来检查数据是否从缓存传输到了disk，因为本来write()就只是把数据传输到了buffer，到了output()才传输到disk，所以checkpoint检查是否传输到了disk(), Transaction 3 直到 checkpoint 完成后才开始进行

 >> ARIES checkpoints

>> Dirty reads

当事务B读取了事务A更新的内容，但这次数据的更改并没有commit并且可能在之后再次更改，于是事务B就读取了错误的值且发生了脏读

>> Cascading Rollback 级联回滚

单个事务(Ti)失败会导致一系列事务回滚（以递归的方式），如果这些事务读取了Ti所写的数据

>> Cascadeless Schedules

如果事务Ti的读写和commit都在事务Tj之前，那么就不会发生cascading rollback以及dirty read的情况

当然的事务Ti的读写和commit都在事务Tj之前，那么这个schedule肯定也是recoverable的：Cascadeless schedules are recoverable

 >> Recoverable schedule

如果一个事务T1读取了事务T2的数据，那么事务T2应该先于事务T1commit

>> Strict schedules

A schedule is strict if each transaction in it reads and writes only values that were written by transactions that have already committed.

>> Strict two-phase locking 

必须同时满足：Conflict-serialisability and Strict schdules

Strict locking condition (in addition to 2PL condition):

           A transaction T must not release any lock (that allows T to write data 其实就是除了exclusive locks 就是在x-lock之后可以unlock，其他时候都需要满足以下条件了再unlock) until :

                   T has committed or aborted, and 

                   the commit/abort log record has been written to disk

 x-lock之后可以unlock()：

例子：认真分清每一个概念

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