Download Presentation

Why 3-level architecture?

Each user should be able to access the same data, but with its own customized view.

User need not have to deal with physical structure of database directly.

DBA should be able to make changes to physical structure without affecting the user view.

The database internal structure should be unaffected by changes made to physical storage.

3- level architecture

EXTERNAL LEVEL

User’s view of the database. This level shows the view that is relevant to the specific user i.e. hiding the unrelated
details. Only those entities , attributes & relationships are shown which are relevant to the specific user. Different views with different representation of same data

CONCEPTUAL LEVEL

Community view of the database i.e. whole database for community of users. It contains the logical structure of database as viewed by the DBA.

Entities, Attributes & their Relationships

Data Constraints

Security & Integrity Information

Conceptual level supports each user view. This level hides the physical storage structure & focuses on data entities, attributes & relationships.

INTERNAL LEVEL

Low level physical and actual representation of database. This level tells about how the data is stored in secondary storage devices like disks etc. Storage Space Allocations for data. Data Compression & Encryption. Data Structures and File Organization used to store data.

int empid;
char Name[20];
char Department[15];
float salary

Database Schema

Overall design & description of database is called database schema.

It is a skelton structure which represents logical view of entire database.

EXTERNAL SCHEMA, CONCEPTUAL SCHEMA, INTERNAL SCHEMA

DATABASE INSTANCE

Status of a database at particular instant of time. DBA might create multiple instances of the same database for different purposes.

Mapping between schema

Conceptual / Internal Mapping

The conceptual schema of database is related (mapped) with internal schema with the help of conceptual / internal mapping. This helps the DBMS to find actual records in physical storage that can be represented
as logical record in conceptual schema.

External / Conceptual Mapping

The external schema of database is related (mapped) with conceptual schema with the help of external / conceptual mapping. This helps the DBMS to take relevent information from conceptual schema and show
to the particular user.

Data Independence

This is one of the major objective of 3-level architecture. Data Independence means that the upper levels in DBMS are unaffected by the changes made at the lower levels.

Logical Data Independence : It means that the external schemas are not affected by the changes made to the conceptual schema.

Changes to the conceptual schema, such as the addition or removal of new entities, attributes, or relationships, should be possible without having to change existing external schemas or having to rewrite application programs.

Physical Data Independence : It means that the conceptual schema is not affected by the changes made to the internal schema.

Changes to the internal schema, such as using different file organizations or storage struc- tures, using different storage devices, modifying indexes, or hashing algorithms, should be possible without having to change the conceptual or external schemas.