Model standard. A structural model is new to the software process and falls directly out of a systems engineering process as it is applied to Ada software development. The structural model is the framework through which components, attributes, and inter-relationships within the system are expressed. The structural model enforces a consistency in the software structure, thus aiding understanding. The structural model does not need to be confused with structural analysis. Structural analysis is a

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... process of software component definition and refinement. In contrast, the structural model becomes an important part of the architecture in which the results of the structural analysis are implemented. An architecture, as we intend to use the term, consists of (a) a partitioning strategy and (b) a coordination strategy 1 . The partitioning strategy leads to dividing the entire system into discrete, non-overlapping parts or components. The coordination strategy leads to explicitly defined interfaces between those parts. These two strategies provide an engineering approach to bridging the gap between the system as a whole (as represented by its specification) and the design (the plan to build the product from primitive parts, such as computer instructions, metal struts, and switches). The reach of an architecture can extend from a single system (an architecture that solves a unique problem) to an entire family or product line of systems. In the latter case, once the partitioning methods and coordination rules are determined, multiple products can be generated using the same methods and rules. SOFTWARE DEVELOPMENT METHODOLOGY Architecture development and its component, structural modeling, are members of a family of methodologies used in a defined, repeatable, improvable software development process. A methodology should spell out general steps to follow. It should be specific enough to give guidance but be general enough to apply to most software situations. It should not be 1. This definition is similar to the SEI's definition of structural model [SEI93]. However, as we hope to make clear, we consider the requirements for an architecture to include the content of the partitioning, not just a strategy for partitioning. ABSTRACT A structural model is the architectural map for a large software system or family of systems (domain). The structural model used in a domain represents the point of convergence for tradeoffs between maintainability and performance, quality and efficiency. As such, different domains will likely have different structural models. The idea of a structural model evolved out of the Ada Simulator Validation Program (ASVP), which established the efficacy of Ada for real-time training simulation. In the years since this program a great deal of work has been done toward defining architectures and structural models in the air vehicle training system (flight simulation) domain. This is evidenced by recent initiatives such as the SEI's Structural Modeling, Mod Sim, STARS, DIS, ARPA's DSSA, etc. This paper discusses the concept of structural modeling within a software design methodology, the generic description of a structural model, and the structural model developed by Boeing Defense & Space Group, the Domain Architecture for Reuse in Training Systems (DARTS).