DATABASE SYSTEM ARCHITECTURE

We know that the database system provides users with an abstract view of data, by hiding certain details of how data is stored and manipulated. Therefore, before to design a database, the data of organization is considered on  abstract level.
Database system architecture means design or construction of database system. The database system architecture provides general concept and structure of database system. The architecture of most commercial database management systems is based on the three-level architecture by DBTG.

The Three-Level Architecture
An early proposal for a standardized terminology (or vocabulary) and architecture for database system was developed and published in 1971 by DBTG (Database Task Group) appointed by CODASYL. A similar architecture and terminology were developed and published in 1975 by SPARC (Standard Planning and Requirements Committee) of American National Institute. As a result of these and later reports, databases can be viewed at three levels, know as external, conceptual and internal levels. The three-level architecture is also known as schema architecture. The purpose of the three-level architecture is to separate the user application and the physical database. The reasons of this separation are:

1. Different users and need different views of the same data.
2. Users should not have to deal directly with the physical database storage details.
3. The DBA should be able to change the database storage structure or storage device without affecting other user's views etc.

The three level architecture is divided into three view levels, External view level, Conceptual view level and Internal view level.The Figure shows the three-level architecture of the database system.