Explain the layered architecture of a DBMS?

Answer 1

The ANSI/SPARC architecture is divided into three levels, known as the internal, conceptual, and external levels.

The internal level is the one closest to physical storage, i.e., it is the one concerned with the way the data is physically stored.

The external level is the one closest to the users i.e., it is the one concerned with the way the data is viewed by individual users.

The conceptual level is a "level of indirection" between the other two.

If the external level is concerned with individual user views, then the conceptual level is concerned with a community user view. In other words, there will be many distinct external views, each consisting of a more or less of a abstract representation of some portion of the total database, and there will be precisely one conceptual view, consisting of a similarly abstract representation of the database in it's entirely. Likewise, there will be precisely one internal view, representing the total database as physically stored. When we describe some representation as abstract here, we merely mean that it involves user-oriented constructs such as logical records and fields instead of machine-oriented constructs such as bits and bytes.

The external level: The external level is the individual user level. A given user can be either an application programmer or an end user of any degree of sophistication. The DBA is an important special case. Unlike other users, however, the DBA will need to be interested in the conceptual and internal levels also.

The conceptual level: The conceptual view is a representation of the entire information content of the database, again (as with an external view) in a form that is somewhat abstract in comparison with the way in which the data is physically stored. It will also be quite different, in general, in the way from which any particular user views the data. Broadly speaking, the conceptual view is intended to be the view of the data "as it really is", rather than as users are forced to see it by the constraints of the particular language or hardware they might be using.

The internal level: The third level of the architecture is the internal level. The internal view is a low level representation of the entire database; it consists of many occurrences of each of many types of internal record. "Internal record" is the ANSI/SPARC term for the construct that we have been calling a stored record.