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3.1.3. Loose Coupling Software coupling is the measure of module interdependency. For example, in a conventional CAD/CAM application, application attribute data from a drawing management system would be exported to an independent data file. These data would then be imported from the file by a supportive database management system. Thus, for a change in one system to be reflected in the other, the process of data exportation and importation would have to be invoked. The loose coupling architecture illustrated in Figure 2, represents an application that has been decomposed into separate ES and MM modules. Intermodular communication occurs via a linked data file. In operation, the ES would use a data-passing capability (inherent in most ES shells) to export system-acquired user data to an external database file for storage and use by the MM system; or, it would import MM acquired user data for knowledge-based manipulation. As Table 2 indicates for this Level 2 integration, advantages include improved synergy and ease of systems development because of available commercial tools, simplicity of design, and reduced data maintenance. Disadvantages include reduction in speed of operations because of communications overhead and overlap in data input and processing, poor overall system maintainability, and the ripple effect caused by an input error.
3.1.4. Tight Coupling As Figure 2 illustrates, a tight coupling software architecture consists of linked, separate modules. In this Level 3 integration, the application is decomposed into independent modules. Modular decomposition eliminates interdependency on intermediate files and provides virtual seamless integration. Intermodular communication is accomplished by direct parameter or data passing. An example of a tightly integrated system is an application that takes advantage of the capability of many ES shells to call executable programs directly and then return control to the ES when the called program is finished. In a more advanced variation, the ES directly controls the output of the MM system; for example, to display still or motion images in support of ES user needs. Tight coupling has been found to be especially practical when a small subset of ES techniques is needed, or vice versa, so the designer can focus on one environment in order to provide a better user interface and less system maintenance. As Table 2 indicates, advantages of this architecture include improved run-time operation because of reduced interprogram communications, retention of modularity and flexibility in design, and improved robustness. Disadvantages include a lack of integrated commercial tools, increased development and maintenance complexity, redundant parameter and data processing, and issues of system validation and verification.
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