Three-Schema Architecture
This article presents the three-schema architecture, which aims to provide data independence. The three schemas are external, conceptual, and internal. Each provides a level of independence: application independence from the external schema, external schema independence from the conceptual schema, conceptual schema independence from the internal schema, and internal schema independence and thus application independence from the physical data itself.
Knowing about the conceptual and the derived logical scheme, this page explains two additional schemes - the external scheme and the internal scheme - which help to understand the DBMS architecture.
- External Scheme:
- An external data scheme describes the information about the user view of specific users (single users and user groups) and the specific methods and constraints connected with this information.
- Internal Scheme:
- The internal data scheme describes the content of the data and the required service functionality which is used for the operation of the DBMS.
Therefore, the internal scheme describes the data from a view very close to the computer or system in general. It completes the logical scheme with data technical aspects like storage methods or help functions for more efficiency.
The right-hand side of the representation above is also called the three-schemes architecture: internal, logical, and external scheme.
While the internal scheme describes the physical grouping of the data and the use of the storage space,
the logical scheme (derived from the conceptual scheme) describes the basic construction of the data structure. The external scheme of a specific application, generally, only highlights that part of the logical scheme which is relevant for its application.
Therefore, a database has exactly one internal and one logical scheme but may have several external schemes for several applications using this database.
The aim of the three-schemes architecture is the separation of the user applications from the physical database, the stored data. Physically the data is only existent on the internal level while other forms of representation are calculated or derived respectively if needed. The DBMS has the task to realize this representation between
each of these levels.
Source: Geographic Information Technology Training Alliance, http://www.gitta.info/DBSysConcept/en/html/DBMSArchitec_learningObject1.html
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