The adaptivity demands of ubiquitous computing motivate applications structured around implementation choices that are (a) made at runtime, reflecting available resources; (b) reevaluated and potentially changed during application operation, accommodating failures and resource discovery; and (c) locally extensible beyond choices anticipated (and fixed) by central application code. To this end, we explore a new organization for adaptive applications involving the formalization of Goals as representations of abstract services and a universe of Techniques which compete to provide them. In our model the selection of Techniques to satisfy each Goal is performed by an application-generic runtime process which continues so long as the Goal remains active, allowing applications to adapt to runtime changes in available resources. The set of available Techniques is potentially open, distributed, and incrementally extensible, providing paths for the decentralized evolution of applications.
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