RDBMS : Architecture of a Database Management System

The architecture of a database management system can be broadly divided into three levels :

a. External level
b. Conceptual level
c. Internal level



The External Level
This is the highest level, one that is closest to the user. It is also called the user view. The user view is different from the way data is stored in the database. This view describes only a part of the actual database. Because each user is not concerned with the entire database, only the part that is relevant to the user is visible. For example, end users and application programmers get different external views.

For example, an instructor will view the database as a collection of students and courses offered by the university. An administrator will view the database as a collection of records on the stock of course material provided by the university. The instructor is concerned with only a portion of database that is relevant to the instructor and the administrator is concerned with only the portion of database that is relevant to the administrator. These portions of database, which are viewed, by the instructor and administrator are reffered as their user’s view or external view.

Each user uses a language to carry out database operations. The application programmer uses either a conventional third-generation language, such as COBOL or C, or a fourth-generation language specific to the DBMS, such as visual FoxPro or MS Access.

The end user uses a query language to access data from the database. A query language is a combination of three subordinate language :
 Data Definition Language (DDL)
 Data Manipulation Language (DML)
 Data Control Language (DCL)

The data definition language defines and declares the database object, while the data manipulation language performs operations on these objects. The data control language is used to control the user’s access to database objects.

The Conceptual Level
This level comes between the external and the internal levels. The conceptual level represents the entire database as a whole, and is used by the DBA. This level is the view of the data “as it really is”. The user’s view of the data is constrained by the language that they are using. At the conceptual level, the data is viewed without any of these constraints.

The Internal Level
This level deals with the physical storage of data, and is the lowest level of the architecture. The internal level describes the physical sequence of the stored records.
Following is an example of the three levels :

External
Cout << “Emp#” << Employee_Code;
Cout << “Dept#” << DepartmentEmployee_Code;
Cout << “Salary#” << Salary;

Conceptual
Employee
Employee_Code Character 6
Department_Code Character 4
Salary Numeric 5

Internal
Stored Employee Length=18
Prefix Type=Byte(6), offset=0
Emp# type=byte(6), offset=6, index empx
Dept# type=byte(6), offset=12,
Salary type=byte(6), offset=6

Mappings
Mapping determines the correspondence between one level an another. There are two levels of mapping involved in this architecture. One is between the external and the conceptual levels, while the other is between the conceptual and the internal levels. The external-conceptual mapping determines the correspondence between the conceptual anad the user views. This specifies how a user views the conceptual data. The conceptual-internal mapping determines the correspondence between the conceptual and internal views. It specifies how the conceptual data is stored.

The first step in designing a databse is to define the conceptual level. The conceptual level is then mapped to the external level. Each user view and the requirement is taken into consideration. Next, the conceptual-internal mapping is done. The way data is stored is derived from the conceptual level. Ths three-level architecture of a DBMS helps achieve data independence.