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?

Architectural standard, Architectural standard: Designation : Archit...

Architectural standard: Designation : Architect , Consultant Job Profile : As a part of Oracle Fusion Middleware at Satyam, I am involved with Presales, Delivery, A

Logical entity relationship diagrams, How can i turn a conceptual entity re...

How can i turn a conceptual entity relationship diagram into a logical entity relationship diagram? what do i have to do with "mandetory or" and "optional or" relationships?

Explain hash file organization, Explain hash file organization? Hash fu...

Explain hash file organization? Hash functions a) The distribution is uniform Hash functions assign each bucket the similar number of search -key values from the set of a

Define various ways in which select statement, Define various ways in which...

Define various ways in which SELECT statement can be used for record selection Technology?  SELECT gives you control over a number of aspects for record retrieval: 1) Which

Project, the of super market system done in database

the of super market system done in database

Define a view and a trigger, Define a view and a trigger. Construct a view ...

Define a view and a trigger. Construct a view for the above relations that have the information about suppliers and the elements they supply. The view holds the S#, SNAME, P# , PNA

Constraints violation during insertion, If the insertion violates one or mo...

If the insertion violates one or more constraints, then two alternatives are available: Default option: - Insertion can be discarded and the reason of refusal can also be ex

What is the main goal of raid technology, What is the main goal of RAID tec...

What is the main goal of RAID technology? Describe the levels 1 through 5. RAID stands for Redundant Array of Inexpensive (or sometimes "Independent")Disks. RAID is a met

What is object oriented model, What is Object Oriented model? This mod...

What is Object Oriented model? This model is based on collection of objects. An object has values stored in instance variables with in the object. An object also having bodies

ER diagram, Construct a clean ans concise ER diagram for the database indic...

Construct a clean ans concise ER diagram for the database indicating the cardinality mapping for Employee management syatem

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