  Complex software systems must be maintained for years or decades, and the
effort and cost to maintain them are often high, involving continuous
refactoring to ensure their longevity in the face of changing requirements. In
this article, we introduce the notion of architectural knowledge (AK)
sustainability as a new concept to support architects dealing with the
evolution of long-lived systems. Architecture sustainability refers to the
ability of the architecture to endure over time with the minimum number of
refactoring cycles possible. We suggest that sustainability of the AK is a
function of how stable the decisions are, and we discuss a set of
sustainability criteria and metrics useful to estimate the sustainability of
this AK.
