Producing a good software design involves not only writing a definition that satisfies the syntax of the chosen language or structural constraints of a design paradigm. It also involves upholding a variety of expectations about the behavior of the system—the semantic expectations. These expectations may apply not only at the code level, but also to more abstract system structures such as software architectures. Such high-level design paradigms provide a vocabulary of components or other constructs and ways to compose those constructs, but not all expressible designs are well-formed, and even well-formed designs may fail to satisfy the expectations of the paradigm.

Unfortunately, these expectations are often implicit or documented only informally, so they are challenging to discover, let alone uphold. They may for example, require correct use of complex structures, internal consistency, compliance with external standards, adherence with design principles, etc. Further, the reasons for design decisions that uphold these expectations are often not explicit in the code or other representation of the system. I introduce the idea of design obligations, which are constraints on allowable designs within a given design paradigm that help to assure appropriate use of the paradigm. To illustrate this idea, I discuss design obligations for two paradigms: data abstraction and a class of adaptive based on feedback control.

Mary Shaw is the Alan J. Perlis University Professor of Computer Science in the Software and Societal Systems Department at Carnegie Mellon University. Her research focuses on software engineering and software design, particularly software architecture and design of systems created and used by real people. She has received the United States’ National Medal of Technology and Innovation, the ACM SIGSOFT Outstanding Research Award (with David Garlan), and the IEEE Computer Society TCSE’s Lifetime Achievement, Distinguished Educator, and Distinguished Women in Software Engineering Awards,. She is an elected Fellow and Life Member of the ACM, the IEEE, and the American Association for the Advancement of Science.