数据库基于日志的恢复技术_数据库管理系统中基于日志的恢复

柔情只为你懂 2023-03-05 12:49 21阅读 0赞

数据库 基于日志的恢复技术

**Introduction**

**介绍**

A **log** is the most widely used recording database modification technique. The log is a structure used for recording database modification. It is a sequence of log record recording all the update activities in the database. We can use this log to recover from failure and to bring back the database to the consistent state. An update log record contains various fields:

**日志**是最广泛使用的记录数据库修改技术。 日志是用于记录数据库修改的结构。 它是一系列日志记录，记录了数据库中的所有更新活动。 我们可以使用此日志从故障中恢复并使数据库恢复到一致状态。 更新日志记录包含多个字段：

*  **Transaction identifier:** It is the identifier which uniquely identifies the transaction.
    
    **交易标识符：**是唯一标识交易的标识符。
 *  **Data item identifier:** It is the identifier which uniquely identifies the data to be used.
    
    **数据项标识符：**是唯一标识要使用的数据的标识符。
 *  **Old value:** It is the value of the data item before the write operation.
    
    **旧值：**这是写操作之前数据项的值。
 *  **New value:** It is the value of the data item after performing the write operation.
    
    **新值：**是执行写操作后数据项的值。
    
    There are many types of log record we denote the various types of log records:
    
    日志记录有很多类型，我们表示各种类型的日志记录：
    
    1.  <Ti start> transaction Ti has started.
    2.  <Ti, Xi, V1, V2> transaction Ti has performed a write on data item Xi, Xj has value v1 before the write and will have value V2 after the write operation.
    3.  <Ti commit> transaction Ti has committed.
    4.  <Ti abort> transaction Ti ha aborted.

Whenever a transaction performs write operation database is modified only when log record for that write is created. And logs may or may not contain the record of reading of data item. It is so because reading the data item does not affect the consistency of the database and is nowhere helpful in recovery mechanism.

每当事务执行写操作时，仅在创建该写的日志记录时才修改数据库。 并且日志可能包含或可能不包含读取数据项的记录。 之所以如此，是因为读取数据项不会影响数据库的一致性，并且对恢复机制无济于事。

In the **recovery method**, we use two operations:

在**恢复方法中** ，我们使用两个操作：

1.  **Undo(Ti):** Restores the values of all data item updated by transaction Ti to the old values.
    
    **撤消(Ti)：**将事务Ti更新的所有数据项的值恢复为旧值。
2.  **Redo(Ti):** Sets the value of all data item updated by transaction Ti to the new values. We need to undo a transaction T only when log contains the record <T start> only when log contains the record <start> and <T commit> both.
    
    **重做(Ti)：**将事务Ti更新的所有数据项的值设置为新值。 仅当日志包含记录<T start>时，才需要撤消事务T；仅当日志包含记录<start>和<T commit>时，才需要撤消事务T。

## 交易修改技术 **(**Transaction modification technique**)** ##

There are mainly two techniques to ensure the transaction atomicity despite failure are as follow:

确保存在故障时事务原子性的技术主要有以下两种：

**1) Immediate database modification**

**1)立即修改数据库**

The immediate update technique allows database modification to output the database while the transaction is still in the active state. Data modification written by an active transaction is called uncommitted modification.

立即更新技术允许在事务仍处于活动状态时修改数据库以输出数据库。 由活动事务写入的数据修改称为未提交的修改。

When a system crashes or a transaction fails, the old value of the data item should be used for bringing the database into the consistent state. This can be done by undoing operation. Before a transaction, Ti starts its execution the record <Ti start> is written to the log. During its execution and write(X) operation by Ti is preceded by the writing of the appropriate new update record to the log. When Ti partially commits, the record <Ti commit> is written to the log.

当系统崩溃或事务失败时，应使用数据项的旧值使数据库进入一致状态。 这可以通过撤消操作来完成。 在事务之前，Ti开始执行，将记录<Ti start>写入日志。 在执行期间，Ti的write(X)操作之前是将适当的新更新记录写入日志。 当Ti部分提交时，记录<Ti commit>被写入日志。

**Example**

**例**

We consider an example of banking system taken earlier for transaction To and T1 such that To is followed by T1. If the system crash occurs just after the log record and during recovery we do redo (To) and undo (T1) as we have both < To start > and <To commit> in the log record. But we do not have <T1 commit> with <T1 start> in log record. Undo(T1) should be done first then redo (To) should be done.

我们考虑一个较早的交易To和T1所采用的银行系统的示例，以便To之后是T1。 如果系统崩溃是在日志记录之后和恢复期间发生的，我们将重做(To)和撤消(T1)，因为我们在日志记录中同时具有<开始>和<提交>。 但是我们在日志记录中没有<T1 commit>和<T1 start>。 首先应执行Undo(T1)，然后应完成重做(To)。

**2) Deferred modification technique**

**2)延期修改技术**

Deferred database modification technique ensures transaction atomicity by recording all database modification in the log. In this technique, all the write statements of the transaction are applied to the database only when the transaction is partially committed. A transaction is said to be partially committed once the final action of the transaction has been executed.

延迟数据库修改技术通过在日志中记录所有数据库修改来确保事务的原子性。 在这种技术中，仅当事务被部分提交时，事务的所有写语句才应用于数据库。 一旦执行了交易的最终操作，就说该交易是部分提交的。

When a transaction partially commits, then the information on the log associated with the transaction is used in executing the deferred writes. If the system crashes before the transaction completes its execution or if the transaction abort then the information on the log is ignored. Using the log the system can handle that result in the loss of information on volatile storage. The recovery scheme uses the procedure.

当事务部分提交时，与事务相关联的日志上的信息将用于执行延迟的写入。 如果系统在事务完成执行之前崩溃，或者事务中止，则日志中的信息将被忽略。 使用该日志，系统可以处理导致丢失易失性存储信息的情况。 恢复方案使用该过程。

Redo(Ti) sets the value of all data item updated by transaction Ti to the new values. The set of data item updated by Ti and their respective new values can be found in the log. The redo transaction must be idempotent. That an executing it several times must be equivalent to executing it once.

重做(Ti)将事务Ti更新的所有数据项的值设置为新值。 在日志中可以找到由Ti更新的数据项集及其各自的新值。 重做事务必须是幂等的。 多次执行必须等同于执行一次。

After a failure, the recovery subsystem consults the log to determine which transaction need to to be redone. Transaction Ti is redone if and only if the log record contains both **<Ti start>** and **<Ti commit>** statements.

发生故障后，恢复子系统将查阅日志以确定需要重做的事务。 仅当日志记录同时包含**<Ti start>**和**<Ti commit>**语句时，才重做事务Ti。

**Example**

**例**

(A)             (B)
    <To start>     <To start>
    <To A, 950>    <To A , 950>
    <To B, 205>    <To B, 2050>
                    <To commit>
                    <T1 start>
                    <T1 C, 600>

If a system fails just after the log record for the step write(B) of transaction To. The during recovery no redo operation will be done as we have only <To start> in log record but not <To commit>.

如果系统仅在事务To的步骤write(B)的日志记录之后发生故障。 恢复期间不会执行重做操作，因为日志记录中只有<To start>，而没有<To commit>。

If a system crash occurs just after the log record write C. the during recovery only redo (To) is done as we have only <To start> and <To commit> in log disk. At the same time we have <T1 start> in log disk but not <T1 commit> so redo (T1) will not be done.

如果在日志记录写入C之后立即发生系统崩溃，则仅在恢复过程中完成重做(To)，因为日志磁盘中只有<To start>和<To commit>。 同时，在日志磁盘中有<T1 start>，但没有<T1 commit>，因此重做(T1)将不会完成。

(C)   
        <To start>
        <To A , 950>
        <To B, 2050>
        <To commit>
        <T1 start>
        <T1 C, 600>
        <T1 commit>

If a system crash occurs just after the log record <T1 commit> the during the recovery we will perform both redo (T0) and redo (TI) as we have both <To start> <To commit> and <T1 start>, <T1 commit> in log disk.

如果在日志记录<T1 commit>之后立即发生系统崩溃，则在恢复期间我们将同时执行重做(T0)和重做(TI)，因为我们同时具有<To start> <To commit>和<T1 start>，< T1 commit>在日志磁盘中。

> 翻译自: [https://www.includehelp.com/dbms/log-based-recovery.aspx][https_www.includehelp.com_dbms_log-based-recovery.aspx]

数据库 基于日志的恢复技术

[https_www.includehelp.com_dbms_log-based-recovery.aspx]: https://www.includehelp.com/dbms/log-based-recovery.aspx