Architecture of DBMS:
A commonly utilized views of data approach is the three-level architecture suggested by ANSI/SPARC (American National Standards Institute/Standards Planning and Requirements Committee). ANSI/SPARC produced a provisional report in 1972 followed by a final report in 1977. The reports projected an architectural framework for databases.
Under this manner a database is considered as containing data about an enterprise. The three stages of the architecture are three different views of the data:
External - individual user viewConceptual - community user viewInternal - physical or storage view
The three level database architecture permits a clear separation of the information meaning (conceptual view) from the external data representation and from the physical data structure layout. A database system that can separate the three different views of data is likely to be flexible and adaptable. This flexibility also adaptability is data independence that we have discussed earlier.
We now in brief discuss the three different views.
The external level is the vision that the individual user of the database has. This vision is often a restricted view of the database and the same database may provide a number of different views for different classes of users. In general the end users as well as even the application programmers are merely interested in a subset of the database. For instance a department head may merely be interested in the departmental finances and student enrolments but not the library information. The librarian wouldn’t be expected to have any interest in the information about academic staff. The payroll office would have no attention in student enrolments.
The conceptual vision is the information model of the enterprise and contains the view of the whole enterprise without any concern for the physical implementation. This vision is normally more stable than the other two views. In a database it perhaps desirable to change the internal view to improve performance while there has been no change in the conceptual view of the database. The conceptual vision is the overall community view of the database and it comprises all the information that is going to be represented in the database. The conceptual vision is defined by the conceptual schema which includes definitions of each of the various types of data.
The internal view is the vision about the actual physical storage of data. It notify us what data is stored in the database and how. In any case the subsequent aspects are considered at this level- Storage allocation example hashing, B-trees and so on. Access paths example- specification of primary and secondary keys indexes and pointers and sequencing.
Miscellaneous for example- data compression and encryption techniques and optimization of the internal structures.
Competence considerations are the most important at this level and the data structures are chosen to provide an efficient database. The internal view doesn’t deal with the physical devices directly. In its place it views a physical device as a collection of physical pages as well as allocates space in terms of logical pages.
The separation of the conceptual vision from the internal view enables us to provide a logical description of the database without the need to specify physical structures. This is habitually called physical data independence. Separating the external visions from the conceptual view enables us to change the conceptual view without affecting the external views. This separation is occasionally called logical data independence.
Assuming the three level vision of the database a number of mappings are needed to enable the users working with one of the external views. For instance the payroll office may have an external view of the database that consists of the following information only- Staff number and name and address.
Staff tax information example number of dependents.
Staff bank data where salary is deposited.
Staff employment salary level, status, leaves information etc.
The conceptual vision of the database may contain general staff, academic staff and casual staff and so on. A mapping will require to be created where all the staff in the different categories is combined into one category for the payroll office. The conceptual vision would include information about every staff's position, the date employment started and full-time or part time and so on. This will require to be mapped to the salary level for the salary office. As well if there is some change in the conceptual view the external view can stay the similar if the mapping is changed.
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