The Shape of Thing to Come: Using Multi-Dimensional Separation of Concerns with Hyper/J to (Re)Shape Evolving Software

Separation of concerns is a key guiding principle of software engineering. It refers to the ability to identify, encapsulate, and manipulate only those parts of software that are relevant to a particular concept, goal, or purpose. Concerns are the primary criteria for decomposing software into smaller, more manageable and comprehensible parts that have meaning to a software engineer.

As software becomes more pervasive and its life expectancy increases, it becomes subject to greater pressures to integrate and interact with other pieces of software—often off-the-shelf software that has been written by entirely separate organizations—and to evolve and adapt to uses in new and unanticipated contexts, both technological (e.g., new hardware, operating systems, software configurations, standards) and sociological (e.g., new domains, business practices, processes and regulations, users). When the concerns a software engineer has in a particular context are ones that have been identified and encapsulated, evolution is simpler and less costly—changes are localized and easier, and the impact of change is smaller. Reuse is facilitated because developers can reuse exactly what they need and not be burdened with extraneous parts, which might be costly or incompatible with the reuse context. Integration is also simplified, because developers need only address the relevant interactions among components. Thus, while separation of concerns to aid initial development is important, as it allows developers to manage software complexity, separation of concerns to promote evolution, integration, and reuse is even more critical, as the majority of software engineering effort is expended on these activities.

Much of software evolution, reuse and integration are of an unanticipated nature. This is not necessarily because of poor design, but rather because the world is changing so fast that it is impossible to predict the paths of software evolution and use with accuracy. It is not even possible to predict exactly which concerns will arise or become important during the lifetime of a system. It is certain that different kinds of concerns will be relevant to different developers in different roles, with different goals and tasks, and/or at different stages of the software lifecycle.

This paper describes an approach called multi-dimensional separation of concerns (MDSOC), which addresses these issues. It permits effective encapsulation of arbitrary kinds of concerns simultaneously, even when the concerns were not anticipated originally, and the integration of separate concerns. We illustrate the properties and use of MDSOC with a running example.