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?

Relation instance or relation state, A relation instance indicates as r is ...

A relation instance indicates as r is a collection of tuples for a given relational schema at a exact point of time. A relation state r of the relation schema R (A 1 ,A 2 ,.....

Define transitive dependencies, What do you understand by transitive depend...

What do you understand by transitive dependencies? Define with an example any two problems that can take place in the database if transitive dependencies are available in the datab

Memory components of sga, Problem 1. Describe the Oracle Architecture w...

Problem 1. Describe the Oracle Architecture with a neat labeled diagram, Also explain the individual components of the Architecture. 2. Write about the following Oracle 9i o

Organic compounds, #question.explain diversity and magnitude of organic com...

#question.explain diversity and magnitude of organic compounds and general properties?

What is bankers algorithm, What is banker's algorithm?  Banker's algori...

What is banker's algorithm?  Banker's algorithm is a deadlock avoidance algorithm that is applicable to a resource-allocation system with multiple instances of each resource ty

Write a query that uses the union operator, Write a Query that uses the UNI...

Write a Query that uses the UNION Operator 1. Use the Software Experts database to:   a. Display the first and the last name of all consultants who worked on any projects f

Data warehouse, 1) Define a job scheduling strategy that will meet business...

1) Define a job scheduling strategy that will meet business requirement of reporting availability by 6am CST for the following cubes? Show the job scheduling dependencies in a pict

Define the concept of generalization, Define the concept of generalization ...

Define the concept of generalization A generalization is relationship between specific kind of thing and general thing. It is also known as "is-a-kind-of" relationship. Inher

What is the use of sub queries, What is the use of sub queries? A sub q...

What is the use of sub queries? A sub query is a select-from-where expression that is nested with in another query. A common use of sub queries is to do tests for set membershi

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