Three-Level Architecture
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Database Task Group (DBTG) developed and published a proposal for a standard vocabulary and architecture for database systems in 1971. It was appointed by Conference on Data Systems and Languages (CODASYL). The Standards Planning and Requirements Committee of the American National Standards Institute (ANSI) Committee on Computers and Information Processing developed and published a similar vocabulary and architecture in 1975.
The result of these reports was the three-level architecture. Three-level architecture is the basis of modern database architecture. The database can be viewed at three levels. The three levels are depicted by three models known as three-level schema. The models refer to the structure of the database, not the data stored in it. The permanent structure of the database is known as the intension of the database or database schema. The data stored at a given time is known as an extension of a database or database instance.
The intention of a database should not be changed once it has been defined. This is because a small change in the intention of the database may require many changes to the data stored in a database. The extension of the database is performed after the intention of the database has been finalized. It means that data is stored in a database when the database structure has been defined. The extension of the database is performed according to the rules defined in the intention of the database.
The schemas are used to store definitions of the structures of the database. It can be anything like a single entity or the whole organization. Three-level architecture defines different schemas stored at different levels to isolate the details of different levels from one another.
