Exploring Non-functional Coupling in Systems of Systems

Many software systems, and especially systems of systems (SoS), depend on other systems to function correctly or are themselves composed of interdependent systems. In that context, behavioral changes in a dependency may have consequences for a dependent system. This includes changes to non-functional properties such as latency or availability. In this paper we use the term nonfunctional coupling to mean the extent to which a system is affected by changes to non-functional properties in other systems. We argue that non-functional coupling has implications for the reliability and performance of an overall system, as (for example) degradations in non-functional properties of one system may cascade to other systems if coupling is high. In this paper we propose a process for systematically analyzing the coupling in a model of an SoS and illustrate that process with a simple case study showing how it can provide insights into system behavior, and how those insights can help with design decisions.