What is recovery?, Database Management System

Assignment Help:

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.

 


Related Discussions:- What is recovery?

Include the create database command, Submit the SQL Statements necessary to...

Submit the SQL Statements necessary to create your database according to the following guidelines: Your SQL statements should exactly mirror your ERD. All primary keys,

Seeking hr searcher, Project Description: The work is about founding the...

Project Description: The work is about founding the best skilled person for the job. I will give you the job description. Skills required are Data Entry, Database Administrat

Computer, what is object-identity

what is object-identity

What are the two integrity rules, What are the two integrity rules?  An...

What are the two integrity rules?  Ans:  The two integrity rules are: Entity Integrity Rule & Referential Integrity Rule.   Entity Integrity Rule - If the attribute A of rel

Define deferred update technique of recovery, Discuss deferred update techn...

Discuss deferred update technique of recovery.  What are the advantages?  Ans:  The deferred update methods do not physically update the database on disk till after a transact

E r diagram, how to make an e r diagram?

how to make an e r diagram?

Describe drop table command or cascade of sql, Describe DROP TABLE command ...

Describe DROP TABLE command of SQL along with both the options - CASCADE and RESTRICT. DROP TABLE command - This command drops the specified table. Dropping a table also dro

Design a gennalization and specialization hierarchy for an a, Design a genn...

Design a gennalization and specialization hierarchy for an automobile sales company

Objectives of data management, The traffic data mgmt. systems infrastructur...

The traffic data mgmt. systems infrastructure plan describes the software, hardware, data network, and other elements that will support the TDMS. The infrastructure plan is based o

The first normal form (1nf), The First Normal Form (1NF) Let us first d...

The First Normal Form (1NF) Let us first describe 1NF: Definition: A relation (table) is in 1NF if 1.      There are no duplicate tuples or rows in the relation. 2.

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd