What is recovery?, Database Management System

What Is Recovery?

 During the life of a transaction, i.e., a after the start of a transaction but before the transaction commits, various changes may be made in a database state. The database through such a state is in an inconsistent state. What occurs when a failure occurs at this stage? Let us define this with the help of an example:

Suppose that a transaction transfers Rs.2000/- from A's account to B's account. For simplicity we are not showing any error checking in the transaction. The transaction may be written as:

Transaction T1:

 READ A

A = A - 2000

WRITE A

      →     Failure

READ B

B = B + 2000

WRITE B COMMIT

What would occur if the transaction fails after account A has been written back to database? As far as the holder of account A is concerned s/he has transferred the money but that has never been taken by account holder B.

Why did this trouble occur? Because although a transaction is considered to be atomic, yet it has a life cycle during which the database gets into an inconsistent state and failure has happened at that stage.

What is the solution? In this situation where the transaction has not yet committed the changes made by it, the partial updates require to be undone.

How can we do that? By remembering information about a transaction such as when did it begin, what items it updated etc. All such details are occurred in a log file.

 

Posted Date: 3/12/2013 3:32:21 AM | Location : United States







Related Discussions:- What is recovery?, Assignment Help, Ask Question on What is recovery?, Get Answer, Expert's Help, What is recovery? Discussions

Write discussion on What is recovery?
Your posts are moderated
Related Questions
explain in detail about time stamp orderting/

Create centralized relational database using Top down method- Construction of Logical Model: Translation EERD into Relational Schema: Use the 8 steps algorithm of converting EERD

You must implement the entity relationship model provided in the model solution to Assignment 1. The model solution will be released on the course website approximately TWO (2) we

What are the Constituents of File? Also elaborates all the possible file operations? A file is organised logically as sequence of record. These records are mapped onto disk blo

A customer places orders for parts and belongs to a particular industry group. An individual order is taken by a particular sales representative on a specific date and the sales re

Increases efficiency : Data is stored close to most often used site, therefore retrieval would be faster. Also, data that is not required by local applications is not stored, there

What is Collation Collation is a kind of sort order. There are majorly 3 types of sort orders, namely: i.) Dictionary case insensitive ii.)Dictionary - case sensitive

What are temporary tables? Temporary tables exist solely for a particular session or whose data persists for the duration of the transaction.

Consider the relations EMP(ENO,ENAME,AGE,BASIC_SALARY) WORK_IN(ENO,DNO) DEPT(DNO,DNAME,CITY) Express the following queries in SQL (i) Find names of employees who work in a dep

The figure below depicts the conceptual diagram of a banking system. (a) Describe one example relevant to the system in figure, for each of the 4 types of enterprise appl