Toward a Reasoning Framework for Dependability

Architecting dependable systems is a daunting task since it requires trade-offs among attributes such as relia bility with a precisely computed, ordinal value and attributes such as security whose value is neither precisely computed nor ordi nal. Quality attribute driven architecture design [1] techniques rely on models of the attributes that provide values that can be comp ared and ranked. Qualitative modeling techniques hold promise f or being able to model quality attributes such as confidentiali ty and integrity for which quantitative models do not exist, but which need to be compared against one another. In this position pap er we present a sketch of our work in progress on a hybrid approac h that blends quantitative and qualitative techniques to des ign a dependable system. We introduce a chain of existing archite cture modeling tools and briefly describe how they can be integrate d to support the development of a rigorous design.