Three Level Architecture

The proposal for a standardized vocabulary and architecture for database system were developed and published in 1971 by the Database Task Group appointed by the Conference on Data Systems and Languages. 

A later similar architecture and vocabulary were published in 1975 by the Standards Planning and Requirements Committee of the American National Standards Institute committee on Computers and Information Processing, ANSI/X3/SPARC.

Database can be viewed at three levels of abstraction. These levels form a three level architecture (three models). The permanent structure of the database is called intension of the database. The information stored at a given time is called an extension of the database (database instance). For describing the structure of the database we use the database model. What is database model??? What is the purpose of database model????Database model is a collection of tools that may include a type of diagram and vocabulary and model gives a description of the data, the relationship between the data, constraints on the data. Database store the definition of the structure of database. Three level architecture is used to store different schema at different levels. There are several reasons to isolate the levels,

The same data can be viewed differently by different users. 

To change the physical structure such as to change the storage devices without effecting the database structure. 

The user is not bothered with the complexity of the database storage structures. 

Three Level Architecture have three views or levels 

1: External level /view

2: Logical or Conceptual level /view 

3: Physical or internal level /view

External level

                    External level consists of many different external models of the database. This level is explicitly an end users and present data as desired by the user of the database. Each user is interested in only some of their attributes and relationships and other attributes and relationships that is not of interest of users is actually present in the database but user will unaware of them. For example one user may believe dates are stored in the form (month, day, year) while another may believe dates are represented in the form as (year, month, day)

Database management system uses the external schema to create a user interface. User sees the database through this interface. It hides the conceptual, internal and physical details and not allows the user to see. In external level some views are virtual, it means that data is not present as such but created when needed.

Conceptual or logical level:

                       Conceptual level is a middle level in Three Level Architecture. The information about the structure of database is store in logical model. The complete description of the database content information is store in conceptual schema. Conceptual model supports external level. Conceptual schema is used to create the logical record interface. Internal level details not cross the conceptual schema. Conceptual level include entities their attribute and their relationship, constraint on data, integrity and security information. It is necessary that conceptual model should be able to modify to accommodate the changes that are made in external level otherwise it effect the levels of database. The change in conceptual model takes a lot of time and effort. 

Internal or physical level

                   Physical level is used to store data on storage media. It describes the data structure and file organization used to store data on storage devices. Data is store in binary format. Data compression can be performed to consume minimum space for data. Different encryption method is applied for the security on the data. At physical level data is in character format and managed by operating system. Internal schema describe the internal level they contain record definition, indexes, representation method of data. 

                                       Diagram of three level architecture