8.5 Looking Ahead

Analysis is all about the development of a consistent logical model that describes all possible acceptable solutions to a problem. Design is about optimization selecting a particular solution that optimizes some set of design criteria in a way that is consistent with the analysis model.

Architectural design consists of the specification of the kind and quantity of devices, the media and rules they use to communicate, and the large-scale software components mapping to the physical architecture. The units of software architecture are subsystems and threads. Subsystems are typically layered sets of packages arranged in a hierarchical fashion. Threads cut through all layers, although they are rooted in a single active object.

The software architecture must map to the set of physical devices. The UML shows this mapping with the deployment diagram. This diagram shows not only nodes and communication paths, but also large-scale software components.

The iterative refinement implementation strategy builds these layered subsystems using vertical slices passing through all layers as well. Each vertical slice constitutes an iterative prototype. Prototypes build on the services defined in the previous prototypes. The order of prototypes is determined by the required services as well as the level of risk. Elaborating high-risk prototypes early lowers overall project risk with a minimum of rework.

The specification of the concurrency model is very important to performance in real-time systems. The concurrency model identifies a relatively small number of threads and populates these threads with the objects identified in the analysis model. Inter-task communication allows threads to share information and to synchronize control. This is often accomplished using a rendezvous pattern to ensure robust exchange of information.

The next step is to specify the middle layer of design, known as mechanistic design. This level of design focuses on the collaboration of small groups of classes and objects. In the process of mechanistic design we add classes to optimize information or control flow and to specify details that have been so far ignored.