DATA BASE MANAGEMENT SYSTEM
ARCHITECTURE
INTRODUCTION: Database
Architecture provides a framework that is useful for describing general
database concepts and for explaining the structure of specific database system.
The material of this chapter is fundamental to a full appreciation of the
structure and the capabilities of the modern database systems.
DBMS ARCHITECTURE:
There are many different framework have been suggested for the DBMS over the
last several year. The generalized architecture of a database system is called
ANSI/SPARC (American National Standards Institute/Standards Planning and
Requirements Committee) model. In 1972, a final report about database is
submitted by ANSI ( American National Standard Institute ) and SPARC ( Standard
Planning And Requirement Committee ). According to this approach, three levels
of a database system was suggested and they are:
- External view (Individual user view)
- Conceptual View (Global or community user view)
- Internal level (physical or storage view)
External/View level
The
highest level of abstraction where only those parts of the entire database are
included which are of concern to a user. Despite the use of simpler structures
at the logical level, some complexity remains, because of the large size of the
database. Many users of the database system will not be concerned with all this
information. Instead, such users need to access only a part of the database. So
that their interaction with the system is simplified, the view level of
abstraction is defined. The system may provide many views for the same
database.
Databases
change over time as information is inserted and deleted. The collection of
information stored in the database at a particular moment is called an instance
of the database. The overall design of the database is called the database
schema. Schemas are changed infrequently, if at all.
Database
systems have several schemas, partitioned according to the levels of
abstraction that we discussed. At the lowest level is the physical schema; at
the intermediate level is the logical schema; and at the highest level is a
subschema.
Features
External/View level
¨ The
external or user view is at the highest level of database architecture.
¨ Here
only one portion of database will be given to user.
¨ One
portion may have many views.
¨ Many
users and program can use the interested part of database.
¨ By
creating separate view of database, we can maintain security.
¨ Only
limited access ( read only, write only etc ) can be provided in this view.
Conceptual/Logical level
Database
administrators, who must decide what information is to be kept in the database,
use this level of abstraction. One conceptual view represents the entire
database. There is only one conceptual view per database.
The description of data at this level is in a
format independent of its physical representation. It also includes features
that specify the checks to retain data consistence and integrity.
Features
of Conceptual/Logical level
¨ The
conceptual or logical view describes the structure of many users.
¨ Only
DBA can be defined it.
¨ It
is the global view seen by many users.
¨ It
is represented at middle level out of three level architecture.
¨ It
is defined by defining the name, types, length of each data item. The create table commands of Oracle creates
this view.
¨ It
is independent of all hardware and software.
Internal/Physical level
The
lowest level of abstraction describes how the data are stored in the database,
and what relationships exist among those data. The entire database is thus
described in terms of a small number of relatively simple structures, although
implementation of the simple structures at the logical level may involve
complex physical-level structures, the user of the logical level does not need
to be aware of this complexity.
Features
of Internal/Physical level
¨ It
stores the data on hardware so that can be stored in optimal time and accessed
in optimal time.
¨ It
is the third level in three level architecture.
¨ It
stores the concepts like
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