OVERVIEW OF MAJOR COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 44

log file, while updates take place in the database. Recovery manager uses the log
file and brings the database to a consistent state.
Multilevel Data Independence Remember that one of the significant advantages
of the database approach relates to data independence. In the earlier file-oriented
systems, data definitions and application logic were intricately interwoven within
application programs. Every time data structures changed programs had to be
modified, causing serious productivity problems. This is not the case in a database
environment. DBMS provides data independence. Let us see how.
DBMS supports a three-schema architecture that separates the level of details
about the data as indicated below.
Internal level. Deals with physical storage; describes details of storage and access
mechanisms. At this level, DBMS keeps track of the files, records, blocks, and
physical locations.
Conceptual level. At this level, DBMS keeps the structure of the whole database
for the entire user community. This level concentrates on entities, relationships, con-
straints, and user data access operations. Details of physical storage are hidden from
this level.
External level. User views of individual user groups form this level. From this level,
only the data covered by specific user views are available for individual user groups.
For example, the user view of the accounting group may consist of just parts of cus-
tomer and billing data.
The DBMS separates out the three level schemas. Requests for data go through
a mapping between the external level to the conceptual level and then between the
conceptual level and the internal level. So, what do these levels have to do with data
independence? Very simply, a change of the schema at one level need not result in
a change at another level.
You may rearrange or modify the conceptual level schema. For example, you may
add some new fields to customer data. This change will affect only those user views
where the new fields are required. All other user views need not be modified. The
ability to change the conceptual schema without changing the external schema or
the application programs refers to logical data independence. Also, you may
shift the physical files and move them to new storage media without having to
change the conceptual or external schemas. This is physical data independence. With
a three-schema architecture, the DBMS provides for logical and physical data
independence.
DBMS Classifications
In any database environment, DBMS forms the most significant software compo-
nent. We have briefly looked at the major functions of DBMS. We have reviewed
the reasons why you need a specialized set of software modules in a database envi-
ronment. If every DBMS serves the same purposes, are all DBMSs the same? Will
any DBMS work in every environment?