Already have an account? Get multiple benefits of using own account!
Login in your account..!
Remember me
Don't have an account? Create your account in less than a minutes,
Forgot password? how can I recover my password now!
Enter right registered email to receive password!
Log based recovery:Let us first describe the term transaction log in the context of DBMS. A transaction log is a record in DBMS that remains track of all the transactions of a database system that update any data values in the database. A log haves the following information about a transaction:
In a database system various transactions run simultaneously. When a transaction commits, the data buffers used by it require not be written back to the physical database stored on the secondary storage as these buffers may be used by various other transactions that have not yet committed. On the other hand, some of the data buffers that may have updates by various uncommitted transactions may be forced back to the physical database, as they is no longer being used by the database. So the transaction log helps in remembering which transaction did which alters. Therefore the system knows exactly how to divide the changes made by transactions that have already committed from those alters that is made by the transactions that did not yet commit. Any operation such as start transaction, insert /delete/update and end transaction (commit), adds information to the log having the transaction identifier and enough information to redo or undo the changes.
But how do we recover using log? Let us show this with the help of an example having three concurrent transactions that are active on ACCOUNTS table as:
Transaction T1
Transaction T2
Transaction T3
Read X
Read A
Read Z
Subtract 100
Add 200
Subtract 500
Write X
Write A
Write Z
Read Y
Add 100
Write Y
Figure: The sample transactions
Suppose that these transactions have the following log file (hypothetical) at a point:
Transaction Begin Marker
Transaction
Id
Operation on ACCOUNTS table
UNDO values (assumed)
REDO
values
Transaction Commit Marker
Y
T1
Sub on X Add on Y
500
800
400
Not done yet
N
T2
Add on A
1000
1200
T3
Sub on Z
900
Figure: A sample (hypothetical) Transaction log
Now suppose at this point of time a failure occurs, then how the recovery of the database will be completed on restart.
Values
Initial
Just before the failure
Operation
Required for recovery
Recovered Database Values
X
400 (assuming update has been done in physical database also)
UNDO
A
1000 (assuming update has not been done in physical database)
Z
900 (assuming update has not been done in physical database)
The advantages of a database management system (DBMS) include :- Data integrity and elimination of duplication.
Your friend owns a computer store in Yuen Long, selling Desktop and Notebook PCs and other computer peripherals. Having been rather successful with his business there, he decided t
It is mandatory that for every table in the LDB hierarchy there should exist one GET statement in the report. False. It is not essential.
What is OLAP? OnLine Analytical Processing (OLAP) is a Business Intelligence (BI) reporting system. OLAP gives the user with the capability to sum, count, average and do other
When are they useful? Justify with an example. The temporary tables are commonly used to support specialized rollups or exact application processing needs. Unlike a permanent t
Two Phase Locking (2PL) The two-phase locking protocol having of two phases: Phase 1 : The lock acquisition phase : If a transaction T needs to read an object, it wants to
why shadow copy technique is used?
Views A view is like a window by which data from tables can be changed or viewed. The table on which a view is based is known as Base table. The view is stored as a SELECT stat
What is Conceptual Schema? Conceptual Schema - Conceptual schema elaborates the structure of the overall database for a community of users. It hides the details of physical sto
This report identifies and outlines my CIS 499 project named Traffic Data Management Systems (TDMS) . My goal is to design a replica of all traffic issues to be implemented i
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!
whatsapp: +91-977-207-8620
Phone: +91-977-207-8620
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd