Degree of homogeneity of DBMS
If all servers (or individual local DBMSs) use identical software and
all users (clients) use identical software, the DDBMS is called homogeneous; otherwise, it is called heterogeneous. Another factor related
to the degree of homogeneity is the degree of local autonomy. If there is no
provision for the local site to function as a stand-alone DBMS, then the system
has no local autonomy. On the other hand, if direct access by local
transactions to a server is permitted, the system has some degree of local
autonomy.
At one extreme of the autonomy spectrum, we have a DDBMS that
"looks like" a centralized DBMS to the user. A single conceptual
schema exists, and all access to the system is obtained through a site that is
part of the DDBMS—which means that no local autonomy exists. At the other extreme
we encounter a type of DDBMS called a federated DDBMS (or a multidatabase
system). In such a system, each server is an independent and autonomous
centralized DBMS that has its own local users, local transactions, and DBA and
hence has a very high degree of local autonomy. The term federated database
system (FDBS) is used when there is some global view or schema of the
federation of databases that is shared by the applications. On the other hand, a
multidatabase system does not have global schema and interactively constructs
one as needed by the application. Both systems are hybrids between distributed
and centralized systems and the distinction we made between them is not
strictly followed. We will refer to them as FDBSs in a generic sense.
In a heterogeneous FDBS, one server may be a relational DBMS, another
a network DBMS, and a third an object or hierarchical DBMS; in such a case it
is necessary to have a canonical system language and to include language
translators to translate subqueries from the canonical language to the language
of each server.
