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)
3. (10 points) Assume that you have been presented with the following relation for the Baxter Aviation database: Charters (Pilot#, Pilot name, Aircraft ID#, #seats, Village, Fligh
Define Direct file organization? The direct file organization is employed to enhance the performance in accessing the records than sequential file organization. In direct file
Ask question #MinimThe Georgia Tech Library (GTL) has approximately 16,000 members, 100,000 titles, and 250,000 volumes (an average of 2.5 copies per book). About 10 percent of the
entity relationship diagram about finance
What is average response time? The average response time is that the average time for a transaction to be done after it has been submitted.
All edges of a cube are expanding at a rate of 6 centimeters per second. How fast is the surface area changing when each edge is (a) 2 centimeters and (b) 10 centimeters?
Scenario: You are a freelance database designer who has just been contacted by a small software development company named Altair Software. The president of the company has asked
Explain the Cartesian product: The Cartesian product operation does not need relations to union-compatible. The meaning of this is that the involved relations may have dissim
Explain concurrency control? Concurrency control Oracles multiversion concurrency control varies from the concurrency mechanism used by some other data base vendors. Read
What is foreign key? A relation schema r1 derived from an ER schema may contain among its attributes the primary key of another relation schema r2.this attribute is known as a
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