Death, taxes, & scalability

The London Ambulance System and the online tax return system of Her Majesty's Revenue and Customs are two systems that have experienced significant problems of scalability. However, we've never seen a precise discussion of their scalability concerns. In this column we use those systems to illustrate how the scalability of software systems can be more precisely understood and characterized, and we describe recent research on goal-obstacle analysis that can help elaborate the scalability requirements of such systems, in an effort to mitigate scalability problems early in development.