three level architecture of dbms

Architecture of DBMS

A database managements system is composed of many interrelated components. These components are organized so that they work in tandem and achieve the goal of the system is referred. These systems are also designed in different types of architecture. A DBMS system that provides three levels of data is three level architects that are external, conceptual and internal levels.

External Level or Sub-schema

The external level is at the top level of database system and it is very first level of architect .this level basically are the users who use database .each view of this level is represent by the means of schema known as external schema or sub-schema. It consists logical records and relationship‘s definition in the external view.  The external schema consists of methods that derived objects from the conceptual view to the external view. The object includes entities, attributes, and relationship.

Conceptual Level or Conceptual schema

Conceptual level is second and middle level of architect. The whole database is represented by the single conceptual view and all of the database entities and the relationships among them are included in the conceptual level. All of the records and relational ship included in conceptual view are described by it in the database. The schema of this level defines the conceptual view therefore there is only single schema per database. The conceptual schema consists of methods that derived objects from the internal view to the conceptual view.

Internal level or physical schema

Internal level is third and the last level of architect. This is closest to physical storage memory in database. The database uses the access methods and data structure that are described by the internal view. The means of stored records and the method in which data fields are represented are defined in it.

Mapping between three levels

There are only two types of mapping that are needed in the database associated with different levels. Mapping between first and second level defines and provide inter-exchange of records and relationship among the external and conceptual levels.

                           The mapping between conceptual   level and internal level is done in second phase. There are possibilities that the internal records are divided into two or more physical records. These records are stored on secondary devices that are used for storage.

All of the mappings are supplied by data base managements systems