8.1 Overview of Design

By now you should have a good grasp of the process and products of analysis. Analysis identifies the criteria of acceptance of any solution. The first part of analysis studies system-environment interaction and explores and captures this interaction with context and use case diagrams. The second part of analysis drills down inside the system to identify the fundamental concepts that must be represented in the system's structure and dynamics. These concepts are captured as classes and objects.

Design is the process of specifying a specific solution that is consistent with the analysis model. Design is all about optimization and therefore is driven by the set of required quality of service properties of the system, such as reusability, timeliness, schedulability, throughput, memory usage, safety, reliability, and so on. The ROPES process divides design into three categories architectural, mechanistic, and detailed design as shown in Figure 8-1. Architectural design details the largest software structures, such as subsystems, packages, and tasks. The mechanistic design includes classes working together to achieve common goals. Detailed design specifies the internal primitive data structures and algorithms within individual classes. The three categories are described in greater detail in Table 8-1.

Figure 8-1. Three Levels of Design

Table 8-1. Phases of Design

Design Phase	Scope	What Is Specified
Architectural	System-wide Processor-wide	Subsystems and components Concurrency and resource management Distribution across multiple address spaces Safety and reliability management Deployment of software onto hardware elements
Mechanistic	Collaboration-wide	Instances of design patterns of multiple objects collaborating together Containers and design-level classes and objects Medium-level error handling policies
Detailed	Intra-object	Algorithmic detail within an object Details of data members (types, ranges, structure) Details of function members (arguments, internal structure)

For simple systems, most of the design effort may be spent in the mechanistic and detailed levels. For larger systems, including avionics and other distributed real-time systems, the architectural level is crucial to project success. This chapter focuses on architectural design.

The design process can be either translative or elaborative. Translative design takes the analysis model and, using a translator, produces an executable system more or less autonomously. Great care must be put into the design of the translator, which is often highly customized for a particular problem domain and business environment. Translation is the focus of the model-driven architecture (MDA) and query/view/translate (QVT) initiatives within the OMG. As with any approach, there are benefits and detriments to translation. On the one hand, these approaches separate the realization details from the logical aspects, facilitating the portability and reuse of the intellectual property held in the models. On the other hand, the applications may have lowered performance and increased complexity. Nonetheless, the MDA approach is popular, particularly for designs that are expected to have a long lifetime.

Elaborative design adds increasing amounts of design detail until the system is fully specified. This is done either by adding detail to the analysis model, maintaining a single model of the system of increasing detail, or by maintaining two separate models. Each approach has pros and cons. Maintaining a single model is less work and less error prone than maintaining two models, but the design information "pollutes" the analysis model. On the other hand, maintaining two models by hand is error-prone but has the advantage of separation of concerns.

In practice, I recommend a combination of elaborative and translative design. Certain aspects are easy to add via translation (such as target source language and OS targeting), while others, such as the application of architectural design patterns, as most easily applied via elaboration. The UML is process-independent and applies equally to whatever design is selected.