External View Level

The external view level is closest to the users. It is concerned with the way the data is viewed by individual users. You can say that external level is the individual user level. A user can either be an application programmer or an end-user, but DBA is an important special case.
The external level consist of many different external views of database. Each external view describes that a particular user group is interested in and hides the rest of the database from that user group. In addition, different views may have different representations of the same data. For example, one user may view date in the form (day, month, year).. while another may view date as (year, month, day). Some users may view calculated data, which is not actually stored in database. The calculated data is created temporarily when needed. For example, marks of subject "C++" of students are stored in the database and average marks of this subject are calculated by system  when the user refers to it. Similarly, if a date-of-birth of a student is stored in the database then you can find the age of the student.
The external views are defined by means of external schemes, which are written in the data definition language (DDL). Usually, the DBA writes an external schema to create a user view. Each user's view. An external record is a record seen by a particular user (a part of his external view). Actually, an external view is a collection of external records.
The external schemes are compiled by the DBMS and store in its data dictionary. The DBMS uses the external schema created for a specific user, to create a user interface to access the database. Thus user-interface created through external schema accepts and displays information in the format the user expects. It also acts as barrier to hide the information of database from the user that are not permitted to him.

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.

Data Processing On Internet and Object Oriented Database Systems

• Data Processing On Internet

The Internet was introduced in 1969 by Advanced Research Projects Agency (ARPA) of USA. Today, most the database systems are online. It means that databases, DBMS software and database application are stored on the Web server. The database technology is used in conjunctions with Internet technology to access data on the Web server. The database application are developed using Internet technology to access the database on the Web server. The database on the Internet uses the hypertext transfer protocol (HTTP), dynamic, hypertext markup language (DHTML), and extensible markup language (XML), to communicate information between database application and database stored on the Web server.

• Object Oriented Database Systems

In the middle of 1980s, it had become clear that there were several fields where relational databases were not applicable, due to the types of data involved. These included medicine, multimedia and high-energy physics, all of which needed more flexibility in how their data was represented and accessed. This let to research being started in object-oriented databases where users could define their own methods of access to data and how it was represented and manipulated. In the start of 1990s, two systems had appeared: the Object-Oriented DBMS (OODMBS) and the object-relational database management system (ORDBMS). However, unlike previous models, the actual composition of these models are not clear. This evolution represents third generation database management system.

Client-Server Database Application and Entity Relationship Model

Client-Server Database Application


Earlier multi-user architectures used mainframe computer to process the database. The mainframe computer provides all the functions to the connected user directly. The mainframe contains the DBMS software, application programs, and user-interfaces. The users connected to the mainframe through their terminals. The remote users were connected to mainframe in a communication network.
In the mid 1980s, most users began to share data through local area network (LAN). The microcomputers were linked in a LAN so that data and resources such as printers, storage devices etc could be shared. The LAN enabled the users to send data to one another through computers, The first application of LAN enabled the users to share the resources such as printers, storage devices (e.g., large capacity disk) and to communication via electronic mail. The end-users also wanted to share their database, which led to the development of multi-user database application on local area networks. After this, the Client-Server architecture was introduced to share data on the computer network.

Entity-Relationship Model
The relational model also has limited modeling, capabilities. In 1976, Peter Chen presented the Entity-Relationship model for database design. The E-R data model is a detailed. logical representation of the data for organization. The E-R model is expressed in terms of entities, relationships between entities and the attributes (or properties) of both the entities and their relationships. The E-R model is normally expressed as an E-R diagram, which is a graphical representation of an E-R  model

Entity-Relationship Model