Advancements under the moniker of the Internet of Things (IoT) allow things to network and become the primary producers of data in the Internet.14 IoT makes the state and interactions of real-world available to Web applications and information systems with minimal latency and complexity.25 By enabling massive telemetry and individual addressing of "things," the IoT offers three prominent benefits: spatial and temporal traceability of individual real-world objects for thief prevention, counterfeit product detection and food safety via accessing their pedigree; enabling ambient data collection and analytics for optimizing crop planning, enabling telemedicine and assisted living; and supporting real-time reactive systems such as smart building, automatic logistics and self-driving, networked cars.11 Realizing these benefits requires the ability to discover and resolve queries for contents in the IoT. Offering these abilities is the responsibility of a class of software system called the Internet of Things search engine (IoTSE).
IoTSE is a complicated and relatively immature research topic. The diversity of its solution space is, arguably, a primary challenge hindering its advance. Such diversity manifests itself in terms of the type of operations within an IoTSE instance (for example, discover content, index, and resolve queries), and the types of IoT content on which those operations are applied. Each combination of operation and content type represents a research area within the Io-TSE literature with its own set of technical, social, and political issues. For instance, the IoTSE instances that discover and resolve queries on realtime sensing data from IoT-enabled sensors face the challenge of ensuring the "freshness" of data used for processing queries while minimizing the costly operation of pulling the data from sensors. IoTSE instances working with the actuating functionalities of IoT-enabled things, on the other hand, concern more with understanding the semantics of these functionalities. Due to the diversity of the IoTSE solution space and the lack of a shared vision of what IoTSE is and what it does, it is challenging to communicate the problems and the solutions related to this system. The lack of such models and constructs for the communication of IoTSE inhibits more extensive research and development efforts that span research communities over an extended time, which are necessary for the advance of the IoTSE. As the existing studies on IoTSE have primarily focused only on technical issues related to a particular "IoTSE operation - IoT content type" combination, and as the existing reviews and surveys on IoTSE have primarily focused on a particular type of IoTSE, the lack of models and constructs to communicate and classify IoTSE, which are applicable to its diverse solution space, has not been addressed in the existing literature.
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