Three Schema Architecture
Architecture of Three Schema
What is Three Schema Architecture?
It is commonly known as the three-tier architecture or ANSI/SPARC architecture. It describes how the data is represented or viewed by the user in the database. This architecture hides the details of physical storage from the user.
DBMS 3-tier architecture divides the complete system into three interrelated but independent modules as shown below.
External or View level
External or view level describes the actual view of data that is relevant to the particular user. This level also provides different views of the same database for a specific user or a group of users. An external view provides a powerful and flexible security mechanism by hiding the parts of the database from a particular user.
Conceptual or Logical level
The conceptual level describes the structure of the whole database. This level acts as a middle layer between the physical storage and user view. It explains what data is to be stored in the database, what relationship exists among those data, and what the datatypes are. There is only one conceptual schema per database.
Database administrators and programmers work at this level. This level does not provide any access or storage details but concentrates on the relational model of the database.
Internal or Physical level
This is the lowest level of database abstraction. It describes how the data is actually stored in the database and provides methods to access data from the database. It allows viewing the physical representation of the database on the computer system. The interface between the conceptual schema and the internal schema identifies how an element in the conceptual schema is stored and how it may be accessed. The changes in physical storage devices such as hard disks, and the files organized on storage devices, are transparent to application programs and users.