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)
What is Primary key? Primary Key - Primary key is one of the candidate keys which uniquely identifies each row in the relation.
State the Component Diagram A component diagram represents a set of component and relationships among them. In a dynamic model, component diagram is used to model physical com
What is a Data Flow Diagram? Data Flow Diagram: A data flow diagram is the graph that shows flow of data values from their sources in objects in the course of processes whic
What is an object ID? Each class-derived table has an ID for primary key, one or more object IDs form primary key for association derived tables. An object ID is equivalent dat
Explain what is DDL? DDL - Data Definition Language: statements used to define the database structure or schema. Some examples: CREATE - to create objects in the database ALT
a project based on normalization with first, second and third normalization. There need to be a diagram also
Create a Student database and Retrieve the names of all students majoring in ‘CS' Consider the following relational database: STUDENT (name, student#, class, major) COURSE (co
Explain the conventions for displaying an ER schema as an ER diagram? An E-R Diagram scheme might describe certain constraints to that the contents of a database must conform.
What is a recursive relationship? Give an example not used in the text. A recursive relationship is a relationship among an entity and itself. For example, given the entity PER
Nested Queries Let us see some more complex queries in this section. Sub-queries: Some of the basic concerns of sub-queries are: A sub-query is a SELECT statemen
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