External failure-database recovery and security, Database Management System

Assignment Help:

External failure: A failure can also result because of an external cause, such as earthquakes, fire, floods, etc. The database must be duly backed up to avoid troubles occurring due to such failures.

In practice software failures are more ordinary than hardware failures.  Fortunately, recovery from software failures is much faster.

The essential unit of recovery is a transaction. But, how are the transactions can handle during recovery? Let some transactions are deadlocked, then at least one of these transactions has to be start again to break the deadlock and therefore the partial updates made by such restarted program in the database require to be undone so that the database did not go to an inconsistent state. So the transaction must roll back which makes sure that the transaction does not bring the database to an inconsistent state. This is one type of recovery. Let us take a case when a transaction has committed but the alteration made by the transaction has not been communicated to permanently store physical database. A software failure now happens and the contents of the RAM / CPU are lost. This remains the database in an inconsistent state. Such failure needs that on restarting the system the database be brought to a consistent state by using redo operation. The redo operation needs the changes made by the transaction again to bring the system to a consistent state. The database system then could be made available to the users. The main point here is that the database updates are performed in the buffer in the memory. Figure shows some cases of redo and undo. You can make more such cases.

                               511_External failure.png

 

Physical

Database

RAM

Activity

Case 1

A=6000

B=8000

A=4000

B=8000

Transaction T1 has just changed the value in RAM. Now it aborts, value in RAM is lost. No problem. But we are not sure that the physical database has been written back, so must undo.

Case 2

A=4000

B=8000

A=4000

B=8000

The value in physical database has got updated due to buffer management, now the transaction aborts. The transaction must be undone.

Case 3

A=6000

B=8000

A=4000

B=10000

Commit

The value B in physical database has not got updated due to buffer management. In case of failure now when the transaction has committed. The changes of transaction must be redone to ensure transfer of correct values to physical database.

                                                    Figure: Database Updates and Recovery


Related Discussions:- External failure-database recovery and security

Describe the most important classes of parallel machines, Question 1 Discu...

Question 1 Discuss the classification of DBMS based on location of database Question 2 Explain with examples different SQL commands used for creating and deleting relations.

How to apply the bcnf decomposition , Consider the following relational sch...

Consider the following relational scheme: R (a,b,c,d,e,f) and its corresponding set of FDs {d→ a, be → c, ac → e, b → f, f → d, a → c}. a. Is the following decomposition i) non

Describe disk manager and their work, Describe disk manager and their work?...

Describe disk manager and their work? Disk Manager: Responsible for fetching data regulated through file Manager from disk storage within Main memory and deciding what data to

Central database schema, Consider an information system designed for an onl...

Consider an information system designed for an online company which provides IT products and services. These include desktops, laptops, networking products, IT books, parts, and te

Illustrate the definition of the classes and operation, The definition of t...

The definition of the classes and operation can often be adjusted to increase the amount of inheritance". Justify the above statement.   Adjustment of Inheritance The def

Advantages to copying records from one table to another, What kind of advan...

What kind of advantages to copying records from one table to another? -) When an algorithm that modifies a table is used after that it's safer to work with the copy -) When

Ordering of tuples, In a relation, tuples are not inserted in any definite ...

In a relation, tuples are not inserted in any definite order. Ordering of tuples is not defined as a part of a relation definition. Though, records may be structured later accordin

Hangman, a method named WonGame that checks each letter in the word is matc...

a method named WonGame that checks each letter in the word is matched by its label

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

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