Databases Reference
In-Depth Information
Some of the design issues in adopting either approach are described in the
following sections.
Interconnection of Existing Systems
Not all organizations have the luxury of developing the distributed data-
base environment from scratch. Already existing database management
applications are costly investments that are not likely to be replaced all at
once by new distributed systems. The existing environment, including
hardware, software, and databases, can be preserved by providing a mech-
anism for producing federated systems (i.e., systems composed of autono-
mous software components).
The federated approach is a practical, first-step solution toward a dis-
tributed database environment. It accommodates a legacy of existing sys-
tems while extending to incorporate new nodes. Therefore, it is important
to select distributed DBMS software that supports existing computing
hardware and allows for expansion. Within a federated system, pairs of
nodes can be coupled in ways that range from very loose (i.e., each node
is autonomous) to very tight (i.e., each node interacts directly with the
other). The various forms of coupling affect the design, execution, and
capability of the distributed applications.
The mode of coupling affects the number of translations required to
exchange information between each site. Zero translations are needed
when both components use the same representations. Some systems may
choose to translate the data produced by one site directly to the format
required by the other site. A more common method is to translate the data
into a neutral format first, and then translate into the target format.
Loosely coupled systems are the most modular and in
some ways are easier to maintain. This is because changes to the implemen-
tation of a site's system characteristics and its DBMS are not as likely to
affect other sites. The disadvantage of loosely coupled systems is that users
must have some knowledge of each site's characteristics to execute
requests. Because very little central authority to control consistency exists,
correctness cannot be guaranteed. In addition, loosely coupled systems
typically involve more translations that may cause performance problems.
Loose Coupling.
Tightly coupled systems behave more like a single, inte-
grated system. Users need not be aware of the characteristics of the sites
fulfilling a request. With centralized control, the tightly coupled systems
are more consistent in their use of resources and in their management of
shared data. The disadvantage of tight coupling is that because sites are
interdependent, changes to one site are likely to affect other sites. Also,
users at some sites may object to the loss of freedom to the central control
mechanisms necessary to maintain the tight coupling of all the systems.
Tight Coupling.
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