Smart cities generally aim at efficiently organizing and managing city resources through a digital layer on top of the legacy infrastructure. As the digitalization trend goes on with an increasing pace and with the involvement of a diverse set of actors, proper management of this digital layer as well as the services deployed over it becomes ever more crucial. This paper presents our methodology of transforming the complex smart city concept and its digital layer into a structured model for

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... ting a dynamic and adaptive service ecosystem in the digital cities of the future. An overview of the smart city concept and the three-tier architecture that emerges through the digitalization of cities is presented first, together with the main challenges in attaining coherent smart city service environments that can avoid fragmentation, ensure scalability, and allow reuse. The three major enablers that are identified in this direction are 1) a semantic functional description of city objects, representing physical devices or abstract services; 2) a distributed service directory that embodies available city services for service lookup and discovery; and 3) planning tools for selecting and chaining basic services to compose new complex services. For each of these, a highlevel overview of the available tools and results from the research literature are provided as well as the relevant standards. This overview is complemented with our own approach and design choices in project ISCO (Internet of smart city objects). Through the implementation of two distinct use cases, this paper illustrates how ISCO components can jointly enrich service creation and consumption of various stakeholders in smart cities. INDEX TERMS Smart cities, smart city objects, service discovery, service composition, semantic service description, adaptive planning.