THREE LEVEL ARCHITECTURE OF DBMS
The logical architecture describes how data in the database is perceived by users. It is not concerned with how the data is handled and processed by the DBMS, but only with how it looks. The method of data storage on the underlying file system is not revealed, and the users can manipulate the data without worrying about where it is located or how it is actually stored. This results in the database having different levels of abstraction.
The majority of commercial Database Management System available today are based on the ANSI/SPARC generalized DBMS architecture, as proposed by the ANSI/SPARC Study Group on Data Base Management Systems. Hence this is also called as the ANSI/SPARC model. It divides the system into three levels of abstraction : the internal or physical level, the conceptual level, and the external or view level.
The External or View Level:
The external or view level is the highest level of abstraction of database. It provides a window on the conceptual view, which allows the user to see only the data of interest to them. The user can be either an application program or an end user. There can be many external views as any number of external schema can be defined and they can overlap each other. It consist of the definition of logical records and relationships in the external view. It also contains the method of deriving the objects such as entities, attributes and relationships in the external view from the conceptual view.
The Conceptual Level or Global Level:
The conceptual level presents a logical view of the entire database as a unified whole. It allows the user to bring all the data in the database together and see it in a consistent manner. Hence , there is only one conceptual schema per database. The first stage in the design of a database is to define the conceptual view, and a DBMS provides a data definition language for this purpose. it describes all the records and relationships included in the database.
The data definition language used to create the conceptual level must not specify any physical storage considerations that should be handled by the physical level. It does not provide any storage or acess details, but defines the information content only.
The Internal or Physical Level:
The collection of files permanently stored on secondary storage devices is known as the physical database. The physical or internal level is the one closest to the physical storage, ans it provide a low level description of the physical database, and an interface between the operating system file system and the record structures used in higher level of abstraction. It is at this level that record types and methods of storage are defined, as well as how stored fields are represented, what physical sequence the stored records are in, and what other physical structures exist.