CHAP2 Three Level Architecture.pdf

7/26/2019 CHAP2 Three Level Architecture.pdf

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Architecture of DatabaseManagement System

ByDr. Parteek Bhatia

Assistant ProfessorDepartment of Computer Science &

EngineeringThapar University

Patiala



Early Two Level Architecture

Early proposal for a standard terminology and

general architecture for database systemswas produced in 1971 by the DBTG (Data

Base Task Group)

Appointed by the Conference on DataSystems and Languages (CODASYL, 1971).

Recommend two level approach.

Simplified Approach To DBMSBy Parteek Bhatia



Early Two Level Architecture

Contd..

Schema

Subschema




ANSI Three Level Architecture

External Level

Conceptual Level

Internal Level




Three Level Architecture




External Level or View level

It is the users’ view of the database. This

level describes that part of the database thatis relevant to each user.

,form (day, month, year), while another may

view dates as (year, month, day).




Conceptual Level or Logical level

It is the community view of the database. This

level describes what data is stored in thedatabase and the relationships among the data.

All entities, their attributes, and theirrelationships;

The constraints on the data;

Security and integrity information.




Internal Level or Storage level

It is the physical representation of the

database on the computer. This leveldescribes how the data is stored in thedatabase.




Internal Level or Storage level

The internal level is concerned with:

Storage space allocation for data and

indexes;

Record descriptions for storage;

Record placement;

Data compression and data encryptiontechniques.




Schema

The overall description of the database is

called the Database Schema.

A schema is defined as an outline or a plan

existing at the particular level.

The schema is sometimes called the

intension of the database, while an instanceis called an extension (or state) of thedatabase.




Example




Mapping between Views

External/Conceptual Mapping

A mapping between the external and

conceptual views gives the correspondence

the external and conceptual views.




Conceptual/Internal Mapping

Conceptual schema is related to the internal

schema by the conceptual/internal mapping.This enables the DBMS to find the actualrecord or combination of records in h sical

storage that constitute a logical record inconceptual schema.




Data Independence-Achievement

of Layered Architecture of DBMSTwo kinds of data independence:

Logical data independence

Ph sical data inde endence




Logical data independence

Logical data independence indicates that the

conceptual schema can be changed withoutaffecting the existing external schemas.




Physical data independence

Physical data independence indicates that

the physical storage structures or devicescould be changed without affectingconce tual schema.




Procedure for Database Access




Metadata or Data Dictionary

A metadata (also called the data

dictionary) is the data about the data. Data dictionary may be either active or

passive. An active data dictionary (also

managed automatically by the databasemanagement software.

The passive data dictionary (also called

non-integrated data dictionary) is the oneused only for documentation purposes.




Components of a DBMS




References

Simplified Approach to DBMS

By

Parteek Bhatia, Kalyani Publisher


Documents

CHAP2 Three Level Architecture.pdf