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

Relational algebra and calculus, Q.  What is relational algebra and calcul...

Q.  What is relational algebra and calculus? Explain the relational algebra operators :- Selection and predicate, set operation, joins and division. Sol. Relational Algebra

Explain the term - adjustment of inheritance, Explain the term - Adjustment...

Explain the term - Adjustment of inheritance The inheritance can be enlarged as object design progresses by changing class structure. The designer should do: i) Adjust or r

Computing the closure of a set of functional dependency, What is meant by c...

What is meant by computing the closure of a set of functional dependency? The closure of F denoted by F + is the set of functional dependencies logically implied by F.

Queries, Find the names of all Albums that have more than 30 tracks. Result...

Find the names of all Albums that have more than 30 tracks. Result: (name: varchar(255))

Define view? how it is related to data independence, A View may be defi...

A View may be defined of as a virtual table, that is, a table that does not really exist in its own right but is instead inferred from one or more underlying base data table. T

B-tree, construct a b-tree for the following set of key values 2,3,5,7,11,1...

construct a b-tree for the following set of key values 2,3,5,7,11,17,19,23,29,31.assume that the tree is initial empty and the values are added in ascending order

What is a database graph, What is a database graph? The partial orderin...

What is a database graph? The partial ordering implies that the set D may now be viewed as a directed acyclic graph, known as a database graph.

Compare the efficiency of the two mining processes, Consider the following ...

Consider the following database containing five transactions. Let min_sup = 60%. Mine all frequent itemsets using Apriori and FP-growth algorithm. Compare the efficiency of the t

Write a query to find name from the relational algebra, Consider the relati...

Consider the relations City (city_name, state) Hotel (name, address) City_hotel (hotel_name, city_name, owner) Answer the following queries in relational algebra (i) Find

Unix System Administration, What Command You Use What Day of the week were ...

What Command You Use What Day of the week were you born

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