Architecture of-dbms-and-data-independence

Architecture of DBMS

Mrs. Maninder Kaur
professormaninder@gmail.com

1 Mrs. Maninder Kaur www.eazynotes.com

Introduction to DBMS
Architecture
DBMS is a collection of programs that enable the users to
create and maintain the database .
Purpose of DBMS is to provide the user with the abstract
view of database i.e. system does not provide all the details
of the data, rather it hides the details how data is stored &
maintained.
In order to use the system, data must be retrieved efficiently.
This leads to the design of complex data structures in the
database.


DBMS Architecture


Three levels of the architecture
 External – individual user view
 Conceptual – community user view
 Internal – physical or storage view

The three level database architecture allows a clear separation of
the information meaning (conceptual view) from the external data
representation and from the physical data structure layout. A database
system that is able to separate the three different views of data is likely
to be flexible and adaptable. This flexibility and adaptability is data
independence

Mrs. Maninder Kaur www.eazynotes.com

External View / Level / Schema /
User View / Global View
 Highest or Top level of data abstraction ( No knowledge of
DBMS S/W and H/W or physical storage).
 This level is concerned with the user.
 Each external schema describes the part of the database that a
particular user is interested in and hides the rest of the database
from user.
 There can be n number of external views for database where n is
the number of users.
 For example, a accounts department may only be interested in
the student fee details. It would not be expected to have any
interest in the personal information about students.


Conceptual View
This level is in between the user level and physical storage
view.
There is only one conceptual view for single database.
It hides the details of physical storage structures and
concentrates on describing entities, data types, relationships,
user operations, and constraints.


Internal View / Physical View
 It is the lowest level of data abstraction. (it has the knowledge about s/w and
h/w)

 At this level, it keeps the information about the actual representation of the
entire database i.e. the actual storage of the data on the disk in the form of
records or blocks.
 It is close to the physical storage method.

 The internal view is the view that tells us what data is stored in the database and
how. At least the following aspects are considered at this level: Storage
allocation, Access paths etc.

 The internal view does not deal with the physical devices directly. Instead it
views a physical device as a collection of physical pages and allocates space in
terms of logical pages.


TYPES OF DATA INDEPENDENCE

LOGICAL DATA INDEPENDENCE

PHYSICAL DATA INDEPENDENCE


LOGICAL DATA INDEPENDENCE
Whenever, there is a change or modification at the
conceptual level without affecting the user level or
external level, it is known as logical data independence.

E.g. The name field in conceptual view is stored as first
name, middle name and last name whereas in external
view, it remains to be as a single name field.


PHYSICAL DATA INDEPENDENCE
Whenever, the changes are made at the internal level
without affecting the above layers, it is known as physical
data independence.

E.g. The location of the database, if changed from C
drive to D drive will not affect the conceptual view or
external view as the commands are independent of the
location of the database.


Difference between Logical Data Independence &
Physical Data Independence
Logical Data Independence Physical Data Independence
It is concerned with the structure of the data or It is concerned with storage of the data.
changing the data definition.

It is very difficult as the retrieving of data are It is easy to retrieve.
heavily dependent on logical structure of data.

Application program need not be changed if new
fields are added or deleted from the database. Physical database is concerned with the change
of the storage device.
It is concerned with the conceptual schema.

It is concerned with the internal schema.


Architecture of-dbms-and-data-independence

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