The space station operations control software: a case study in architecture maintenance

Software maintenance teams are often faced with the challenge of adapting a system's architecture in response to problem reports as well as new functional requirements. More often than not, these maintenance objectives can be accomplished either through the addition of alternative, "patching" components, or by refactoring the original architecture. The latter approach usually results in a simpler, more cohesive design that is more robust, easier to maintain, and therefore should be preferred. This paper presents a case study describing how the Space Station Operations Control Software (OCS) team has handled architectural change after the initial delivery of the system. In particular, the paper analyses two specific examples: the reaction of the maintenance team to a design problem discovered during testing, and the incorporation of a major new feature into the software design.