Network-enabled sensing and actuation devices are key enablers to connect real-world objects to the cyberudworld. The Internet of Things (IoT) consists of the network-enabled devices and communication technologiesudthat allow connectivity and integration of physical objects (Things) into the digital world (Internet). Enormousudamounts of dynamic IoT data are collected from Internet-connected devices. IoT data is usually multi-variantudstreams that are heterogeneous, sporadic, multi-modal and spatio-temporal. IoT data can be disseminatedudwith different granularities and have diverse structures, types and qualities. Dealing with the data delugeudfrom heterogeneous IoT resources and services imposes new challenges on indexing, discovery and rankingudmechanisms that will allow building applications that require on-line access and retrieval of ad-hoc IoT data.udHowever, the existing IoT data indexing and discovery approaches are complex or centralised which hindersudtheir scalability. The primary objective of this paper is to provide a holistic overview of the state-of-the-art onudindexing, discovery and ranking of IoT data. The paper aims to pave the way for researchers to design, develop,udimplement and evaluate techniques and approaches for on-line large-scale distributed IoT applications andudservices.
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