ARCHITECTURE OF DATABASE

A database follows three level of architecture.

§  External Level

§  Conceptual Level

§  Internal Level

External Level:

§  The user’s view of the database.

§  Consists of a number of different external views of the DB.

§  Describes part of the DB for particular group of users.

§  Provides a powerful and flexible security mechanism by hiding parts of the DB from certain users. The user is not aware of the existence of any attributes that are missing from the view.

§  It permits users to access data in a way that is customized to their needs, so that the same data can be seen by different users in different ways, at the same time.

Conceptual Level:

§  The logical structure of the entire database as seen by DBA.

§  What data is stored in the database?

§  The relationships among the data.

§  Complete view of the data requirements of the organization, independent of any storage consideration.

§  Represents:

·         Entities, attributes, relations

·         Constraints on data

·         Semantic information on data

·         Security, integrity information

§  Supports each external view: any data available to a user must be contained in, or derivable from the conceptual level.

Internal Level:

§  Physical representation of the DB on the computer

§  How the data is stored in the database.

§  Physical implementation of the DB to achieve optimal run– time performance and storage space utilization.

·         Storage space allocation for data and indexes

·         Record description for storage

·         Record placement

·         Data compression, encryption

§  Supports each external view: any data available to a user must be contained in or derivable from the conceptual level.

Physical Level:

§  Managed by the OS under the direction of the DBMS.

SCHEMAS, MAPPINGS, INSTANCES :

§  DB schema: overall description of the DB.

§  Three different schemas according to the level of abstraction.

§  DBMS: mapping between schemas, consistency of schemas.

Physical Level:

§  Managed by the OS under the direction of the DBMS.


Database Independence:

The ability to modify a scheme definition in one level without affecting a scheme definition in a higher level is called data independence.

There are two kinds of Data Independence

§  Logical Database Independence

§  Physical Database Independence

Logical Database Independence

§  The ability to modify the conceptual scheme without causing application programs to be rewritten.

§  Immunity of external schemas to changes in the conceptual schema.

§  Usually done when logical structure of database is altered.

Physical Database Independence

§  The ability to modify the internal scheme without having to change the conceptual or external schemas.

§  Modifications at this level are usually to improve performance.

§  Usually done when logical structure of database is altered.

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