High Level Architecture
The following graphic presents a high level (and highly simplified) view of the ANSI/SPARC architecture.
ANSI/SPARC specifically names three levels of the architecture.
The External level is composed of all the different views (not specifically in the sense of views in relational systems) of the underlying physical database.
The Conceptual level is concerned with transparently mapping External level views to the physical storage of the actual data in the database. In terms of a relational system, this level is composed of the database management system and the file system.
The Internal level describes the physical storage of the data on the storage media.
The following diagram portrays the ANSI/X3/SPARC architecture in greater detail and explicitly relates the various levels to their manifestations in a relational database management system. Note the following points:
- Depending on the situation and object privileges, end users communicate with the database in one of two possible ways:
- Indirectly through an external view
- Communications through views must be converted, or mapped, in both directions.
- Communications between the database and the disk subsystem are made through the file system.
The higher level within the External level is represented by the SQL language for relational systems. The SQL language elements can be embedded within a client application, within a stored procedure, or presented to the database in the form of an ad hoc, interactive query made through a query manager, or through a natural language SQL code generator application.
The lower level with the External level is represented by a relational view. Views not only mask the underlying database storage, but also the conceptual structure it supports, acting as a virtual tabular interface on the physical base tables.
Note that not all users communicate with the database through views: some, particularly administrative users, communicate with the database directly.
The Conceptual level of the architecture is represented by the relational database management system and the file system. The role of the file system is more nebulous than the diagram portrays and is actually intermediate between the Conceptual and Internal levels, but for purposes of this description of the architecture, it is regarded as a component of the Conceptual level.
The Internal level of the architecture is represented by the disk subsystem. Depending on the configuration, the Internal level could also include storage media such as tape and optical disk. From the perspective of the architecture, only the file system has direct access to the disk subsystem, and it must map data requests and responses in a manner appropriate to the level or component it communicates with.
The following table explains the abbreviations used in this detailed architecture flow diagram:
|EVA||External View A|
|EVB||External View B|