Context-awareness has become a distinguishing feature of Ubiquitous systems. Contrary to desktop and web applications, Ubiquitous applications gather environmental context and provide services without user intervention. In this paper, we propose a message-oriented application model that is capable of modeling both user and system initiative interaction paradigms. Systems designed based on this model are inherently loosely-coupled and scalable. Results of this research include a systematic

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... pment procedure and its supporting middleware. We show the feasibility of this model by constructing several Ubiquitous applications for two dissimilar demo sites, and discuss experiences when adopting this model.