Software Component Dependability - a Subdomain-based Theory

A proole-dependent \dependability" for software can be deened using a functional proole (a set of input-space subdomains breaking down the behavior of a component according to its speciication { these are the subdomains used in the usual \functional testing"). It is argued that measurements of dependability using the squeeze play (1) are proole dependent in general; but, (2) when the proole is deened by weighted subdomains, the squeeze-play predictions depend only on the subdomains, not on their weighting, hence this dependability is proole independent. A component developer can deene functional subdomains for the component and measure the dependability, then market the component with its dependability description. A system developer could select components with attached dependabilities, combine the components in a veriied structure, and measure { from the component dependabilities alone { a system dependability. However, the system dependability is not proole independent. Measurements for both component-and system developer can be supported by tools.