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.
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