Flexibility investment costs for product lines include the costs of domain engineering to characterize the commonalities and variabilities across the domain; the costs of determining how broad a scope across which to develop and maintain a product line; the costs of developing more reusable components; the costs of verifying that the reusable components will operate satisfactorily across the product line; the costs of operating a repository of reusable components; and the costs of evolving the product line architecture and the reusable components (Boehm-Scherlis, 1992; Poulin, 1998). Again, the costs may include effects of tradeoffs between product line generality and other desired –ilities. The resulting cost savings will depend on the relative costs of the commonalities and the variabilities, which will generally not be constant across the product line, and the degree to which the commonalities result in components that can be reused without modification (black-box) or which require some modification (white or glass box). The effects of evolution of reusable commercial-off-the-shelf (COTS) or purchased services involve more effort that reduces the benefits.
A final challenge is the difficulty of predicting the useful lifetime of reuse architectures and reusable components in a world of rapid change. Again, though, once rough models for determining such effects are available, they can be extended as measured experience is built up.
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