A commonly used 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
an interim report in 1972 followed by a final report in 1977. The reports
proposed an architectural framework for databases. Under this approach, a
database is considered as containing data about an enterprise. The three
levels of the architecture are three different views of the data:
1. External - individual user view
2. Conceptual - community user view
3. Internal - physical or storage view
The three level database architecture allows 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 is able to separate the three different views of data is likely to
be flexible and adaptable. This flexibility and adaptability is data
independence that we have discussed earlier.
The external level is the view that the individual
user of the database has. This view 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 and even the applications programmers
are only interested in a subset of the database. For example, a department head
may only be interested in the departmental finances and student enrolments but
not the library information. The librarian would not be expected to have any
interest in the information about academic staff. The payroll office would have
no interest in student enrolments.
The conceptual view is the information model of the
enterprise and contains the view of the whole enterprise without any concern
for the physical implementation. This view is normally more stable than the
other two views. In a database, it may be desirable to change the internal view
to improve performance while there has been no change in the conceptual view of
the database. The conceptual view is the overall community view of the database
and it includes all the information that is going to be represented in the
database. The conceptual view is defined by the conceptual schema which
includes definitions of each of the various types of data.
The internal view is the view about the actual
physical storage of data. It tells us what data is stored in the database and
how. At least the following aspects are considered at this level:
1.
Storage allocation e.g. B-trees, hashing etc.
2.
Access paths e.g. specification of primary and
secondary keys, indexes and
pointers and sequencing.
3.
Miscellaneous e.g. data compression and encryption
techniques, optimization of the internal structures.
Efficiency considerations are the most important at
this level and the data structures are chosen to provide an efficient database.
The internal view does not deal with the physical devices directly. Instead it
views a physical device as a collection of physical pages and allocates space
in terms of logical pages.
The separation of the conceptual view from the
internal view enables us to provide a logical description of the database
without the need to specify physical structures. This is often called physical
data independence. Separating the external views from the conceptual view
enables us to change the conceptual view without affecting the external views.
This separation is sometimes called logical data independence.
Explain the Three level Architecture of a Database.
Reviewed by enakta13
on
March 18, 2013
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