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Chapter 5. Understanding distribution of responsibility 83
Design trades
Design trades is the name of a common systems engineering technique: Building
a set of alternate design approaches; analyzing the cost, quality, and feasibility of
the alternatives; and then choosing the best solution. The locality view supports
design trades by containing more than one locality diagram, each representing a
different conceptual approach to the physical decomposition and distribution
viewpoint of the system. It also supports reasoning about the various parameters
associated with the localities through their tagged values in UML and the
parametrics in SysML. These associated parameters can be used to drive
simulations in external programs such as Matlab.
Figure 5-1 and Figure 5-2 are locality diagrams that document different
engineering approaches to a click-and-mortar enterprise with a number of retail
stores, central warehouses, and a Web presence.
The first solution (Figure 5-1) shows processing capability in the stores. The
second solution (Figure 5-2) shows all terminals connected directly to a central
office processor. In each case, characteristics can be set for the localities that are
required to realize the design:
The first solution uses in-store caching to improve performance, because
system performance might be constrained by network bandwidth. This
architecture, however, can come at a maintenance and hardware
procurement cost due to distributed nature of hardware and software.
Upgrades to software will have to be performed across the whole network.
The second example becomes more attractive as bandwidth across the
network increases, due, let us say, to the introduction of fiber optics. In this
case, there is not so much a performance penalty, and maintenance and
upgrades become easier and less expensive due to the centralized nature of
the processing.
It is precisely for reasoning about these kinds of issues that we use localities and
connections. Today, most people would agree that Figure 5-1 represents a better
design; however, the solution in Figure 5-2 might be considered superior in a few
years, as cost of increased bandwidth decreases and network reliability
increases.