Architecting and developing a system of systems (SoS) is a complex and daunting task. We are all familiar with integration and operational problems between a system and its software architecture due to inconsistencies, ambiguities, and omissions in addressing quality attributes. These problems are further exacerbated in an SoS because you are typically dealing with some number of existing systems that are evolving themselves and being integrated together to provide new capabilities. In this context, you are dealing with different program offices, contractors, engineering disciplines, and program life cycles that are being mashed together to form the SoS. Functionality and capability are critically important, but the architecture must be driven by the quality attributes. Specifying and addressing quality attributes early and evaluating the architecture (system and SoS) to identify risks are key to success.
Carnegie Mellon Software Engineering Institute (SEI) has developed a number of methods that, when combined, form the basis for a software architecture-centric engineering approach. The Quality Attribute Workshop (QAW) and the Architecture Tradeoff Analysis Method (ATAM) were two of the methods developed to facilitate the discussion with stakeholders at the software architecture level. We wanted to extend the approach to effectively treat SoS considerations. We were familiar with a system engineering concept of mission threads (or workflows), a sequence of steps conducted at the various nodes in the SoS in response to a stimulus. The resulting Mission Thread Workshop (MTW) is a facilitated stakeholder-centric exercise whose purpose is to help elicit and define requirements, address engineering considerations, and uncover architectural challenges and capability gaps for an SoS.
We conceived of extending the concept of a mission thread to include quality attributes in the same manner as scenarios had extended use cases. However, we discovered that developing the mission threads at the MTW in a way similar to developing scenarios in a QAW was overly cumbersome. Developing mission threads in advance of the MTW by working with subject matter experts was a key aspect of a successful MTW. Operational, sustainment, development, and acquisition mission threads are created and refined with a few key leads of the sponsoring organization to create a starting point for the MTW. This preparation enables the stakeholders participating in the MTW to focus on validating the threads, determining gaps in them, identifying architectural and engineering issues, and considering the quality attributes at individual steps in the thread and in the whole thread during the MTW. After the MTW, the findings are summarized as a set of challenges facing the SoS architects. Like its predecessors, the MTW is a repeatable technique that works equally well in commercial and DoD contexts.
The presentation is based on an upcoming MTW technical report that describes the steps of a MTW and the engagement approach developed from lessons learned with commercial and DoD organizations. The engagement approach consists of three phases, a timeline for the different activities, inputs and outputs associated with each activity, and examples of artifacts associated with the activities.