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

relationship with the owner entity, A database for the Service and Mainten...

A database for the Service and Maintenance (SM) section of a Computer sales and service Company has to be developed. SM gives after sales service to customers. SM branch has a numb

Using column aliases-data manipulation language, Using Column aliases E...

Using Column aliases Example: To print column names as NAME and ANNUAL SALARY SELECT ENAME "NAME", SAL*12 "ANNUAL SALARY" FROM EMP;

Dml precompiler in database management system, DML Precompiler All th...

DML Precompiler All the DBMS have two basic sets of Languages - Data Definition Language (DDL) that have the set of commands needed to define the format of the data that is b

Information infrasture, what benefit can a company gain by managing its own...

what benefit can a company gain by managing its own infrasture and services

When it is better to use files than a dbms, When It is better to use files ...

When It is better to use files than a DBMS? It is better to use files than a DBMS when Multiple users wish to access the data.

Define what Immediate database modification technique uses, Define what Imm...

Define what Immediate database modification technique uses?  Ans: Both undo and redo.

I need help on the following 5 questions, 1. Define and discuss Work Breakd...

1. Define and discuss Work Breakdown Structure, Critical Path Analysis, PERT charts and GANTT charts. Describe the role of each in determining scope, budgets, and schedules? What

Structure of client server systems, Structure of Client Server Systems: In ...

Structure of Client Server Systems: In client/server architecture, clients show users who require services while servers give services. Both client and server are a combination of

What are the disadvantages of relational data, What are the disadvantages o...

What are the disadvantages of relational data ? Disadvantages • Performance - If the number of tables among that relationships to be established are large and the tables t

Explain the cascadeless schedule, Explain the Cascadeless Schedule A ca...

Explain the Cascadeless Schedule A cascadeless schedule is one in which for every pair of transactions Ti and Tj like that Tj reads a data item before written by Ti, the commit

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