DBMS Architecture

DBMS Architecture

The main characteristics of database approach are (i) insulation of programs and data (ii) support of multiple user views (iii) use of a catalog to store the database description. The architecture of the DBMS is proposed to visualise these characteristics and so called the three schema architecture. It is also called ANSI/SPARC (American National standard Institute/Standards planning and requirements committee) Architecture.

Goal of the architecture is to separate the user applications and physical database. In this architecture, schemas can be defined as the following three levels.

i) The internal level

It has an internal schema which describes the physical structure of the database. It describes the complete details of data storage and access paths for the database.

ii) The conceptual level

It has a conceptual schema, which describes the structure of the whole database for a community of users. It hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations and constrains.

iii) The external or view level

It includes a number of external schema or user views. Each external schema describes the past of the database that a particular user group is interested in and hides the rest of the database from that user group.

Fig. Three schema Architecture

Three schema architecture is a tool for the user to visualize the schema levels in a database system. Most DBMS don't separate the three levels data actually exists at the physical level. User/groups refer only to its own external schema. So DBMS must transform a request from users into a request against conceptual schema and then into a request on the internal schema for processing over the stored database. If the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user's external view. The process of transforming requests and results between levels are called mappings