Databases Reference
In-Depth Information
Figure 12.4
Principles of plug-in style CDBMS.
A typical case of component in this kind of CDBMS is families of base
and abstract data types or implementations of some DBMS function, such as
new access paths. The DBMS architecture, among others, defines a number
of plugs components can use, for example, interfaces of functions the
DBMS will invoke and which the component must thus implement. In
other words, the architecture formulates expectations concerning interfaces
that the component must meet to be integrated successfully.
12.3.3.2 Middleware DBMSs
The typical aim of systems falling into the category of middleware DBMSs is
to integrate existing data stores, that is, to leave data items under the control
of their original (external) management systems, while integrating them into
a common DBMS-style framework. For instance, existing data stores should
be integrated into query processing or transaction management of the entire
system (the integrating DBMS ). External systems will in many cases exhibit
different capabilities, such as query languages with varying power or no que-
rying facilities at all. The different data stores might also have different data
models (i.e., different data definition and structuring means) or no explicit
data model at all. Nevertheless, users and applications should be shielded
from that kind of heterogeneity and be provided with a uniform and inte-
grated view of the entire system. That is accomplished by a CDBMS acting
as middleware [20] between the data stores and the applications of the inte-
gration. The overall aim of systems in this group is similar to that of multida-
tabase systems [7, 8], although the latter typically considers only integration
of DB systems instead of any kind of data store.
That goal is achieved by componentizing a DBMS in the following
way (see Figure 12.5). The architecture introduces a common (intermediate)
format into which the local data formats can be translated. Components
are introduced that are able to perform this kind of translation. Second,
