Abstract

Keywords
Introduction
The rapid advancement and ubiquitous penetration of mobile networks and software-defined networking technology enable us to sense, predict and control the physical world using information technology – the so-called Internet of Things (IoT).1
Also referred to as Cyber-Physical Systems (CPS).
Pervasive connectivity, smart personal devices, for example in our homes, and demand for data testify to a WoT that will continue to grow. New devices are being developed and are becoming cheaper, making their integration into everyday objects ever more feasible, and as people buy into WoT technology, economies of scale lend themselves to the creation of ever more data-centric businesses.
The capabilities of these networks of devices presents us with several new and complex challenges that need to be solved before the Web of Things can deliver its promised potential. While there are, for example, some industry initiatives to achieve interoperability between smart home devices on the communication layer, including a recent collaboration between Google, Amazon, and Apple2
At the data level this problem can be solved using an ontology-based approach. Gruber [23] introduced ontologies to Computer Science as an “explicit specification of a conceptualization” consisting of a “set of objects, and the describable relationships among them” represented in a declarative formalism. Ontologies have consequently proven to be a very useful tool for semantic interoperability between parties that exchange data. With the adoption of the Linked Data movement, as proposed by Berners-Lee [5], machine-readable connections between data expressed in RDF in combination with ontologies have since seen a large uptake on the Web. One such ontology backed by a consortia of search engine providers including Google, schema.org, has seen particular success on the Web and has been used for semantic annotations on 31.3% of all websites [25] already back in 2015. It is now responsible for a large portion of the currently available machine-readable data on the Web.
There already exists a significant amount of research focusing on applying the RDF data model and OWL ontologies in different WoT scenarios, from home automation to Industry 4.0, by showing how this approach can be applied to ease integration of diverse data sources [38,55]. Ontologies and vocabularies such as the Semantic Sensor Network Ontology (SSN) [27] have been adopted in a number of research projects [7,63,69]. Although the ontology-based approach in WoT has received significant interest and adoption in research projects, it still lacks similar levels of adoption to schema.org on the Web or adoption in industry, more generally [37].
This lack of adoption can be attributed to several challenges, the following three of which we consider as the major open challenges.
Maturity and Coverage of WoT Ontologies
Semantics in the Edge
Distributed and embedded reasoning
In the following section, we will detail and discuss these three challenges in more detail.
Maturity and Coverage of WoT Ontologies
Different standardisation bodies work towards developing data models and ontologies for the Internet and Web of Things [18,26,33,34]: the World Wide Web Consortium (W3C), the Open Geospatial Consortium (OGC), the Internet Engineering Task Force (IETF), the European Telecommunication Standards Institute (ETSI), the Open Connectivity Foundation (OCF), the IPSO Alliance, and the Open Mobile Alliance, among them. Many aspects of the data being generated in the WoT need to be described semantically and the standardisation bodies sometimes adopt conceptually different modelling perspectives. The diagram in Fig. 1 shows these aspects and presents the current state-of-the-art in ontologies available to describe those.

Data aspects that require semantics in WoT and existing established ontologies.
Real world setting A central aspect in modeling WoT applications is the description of the real world setting in which the Things/Sensors/Actuators are deployed to observe or act on some features of interest. Many ontologies [4,12,27,32,61] include patterns to describe such settings. The OGC and W3C joint standard on the Semantic Sensor Network ontology (SOSA/SSN) [26], describing networks of sensors and actuators, their capabilities, their features of interest, and their individual observations or actuations, serves as a core or as a source of inspiration to many of these ontologies, ensuring some form of interoperability between them. The ETSI SmartM2M technical committee, develops the Smart Applications REFerence (SAREF) Ontology [12],3
SAREF ontology –
Extensions of SAREF and SSN have been developed for specific domains, such as CASO for the Agriculture [50], EEPSA for Buildings [22], and the SAREF4ABCD series of SAREF extensions4
SAREF extensions –
In addition, SSN has a separate module, called SSN System, to model capabilities and operating/survival ranges of systems/things. Sagar et al. [60] discussed some remaining modeling issues of SSN in this regard and proposed the S3N5
S3N ontology –
Property qualification or quantification Sensors and actuators are deployed to observe and act on specific properties of features of interest. SOSA/SSN and SAREF both have the modeling of features of interests and their properties in their core, but do not prescribe whether a property can be reused across features of interest, or should belong to a specific feature of interest,6
See Footnote 10 in [26] and
Being able to describe quantity values and their units is a requirement that is almost ubiquitous in any WoT domain. Different ontologies have been developed to describe units, their relations, and quantities with their values. A recent survey [36] compares and evaluates eight well known ontologies for units of measurements, among which QUDT [30], OM [58] and the Units Ontology [21] are the most widely used. The survey also reports on the Wikidata corpus [68] that at the time of research contained over 4.4k measurement units and 4.1k non-prefixed units. While these ontologies are comprehensive in respect to modelling units of measurements and their relations, a comprehensive model of systems of quantity kinds is still under development, with the QUDT ontology leading the way (as mentioned above). An alternative approach relying on RDF 1.1 Datatypes is proposed by Lefrançois and Zimmermann [47], and allows for more concise representation of quantity values and queries.
Contextualization of value assignment to properties The OWL time ontology [10], the WGS84,7
W3C Basic Geo ontology –
Device functionality and their APIs A recent and important core ontology to describe thing affordances in terms of properties, actions, and events, is the Thing description [34] developed by the W3C Web of Things working group. However, the Thing description does not model how the enactment of these affordances is to be modeled. This aspect is covered by SOSA/SSN that describe sensors that implement procedures and make observation. Parallel to this, they describe actuators that implement procedures and make actuations. The PEP [44] ontology defines an ontology pattern as a generalization of these two parallel conceptual models. However, the aforementioned models are mainly concerned with the flow of information between devices, but there is little integration of device types (sensors, actuators, gateways) themselves. They need to be considered as more than raw data producers, but the data input and output of these devices and the process to execute them needs to undergo harmonisation through ontological models. This has not yet happened other than in some subdomains.
Events and processes While the PROV Ontology (PROV-O) [41] is an established W3C standard that provides a set of classes, properties, and restrictions that can be used to represent events and activities that happened, i.e. to document a workflow log, an established process ontology that can be used to describe the execution behaviour of complex functions of WoT devices is missing. PSL [24] and m3po [28] were early examples of process ontologies that could theoretically be used for WoT devices, but neither are described in OWL nor have they found use outside of academia. The WiLD ontology [35] proposed as an execution model for the Linked-Data Fu system is the closest to a process model to describe the execution behaviour of WoT devices, but it has yet to be used in actual implementations and it also lacks a mapping to the ontologies mentioned above for describing device functionality and their APIs, in particular the WoT Thing description.

Semantics in the Edge: using RDF abstractions for the content of Web resource.
WoT protocols To be part of the WoT, things, sensors, and actuators, need to be exposed on the Internet and reachable using Web protocols. This category focuses on ontologies that bind the thing’s affordances and their enactments to Web protocols. The architecture paradigm for WoT applications is intended to be stateless due to Thing constraints, therefore ontologies such as Hydra [39] are a good fit. More than the classical REST level 2 that is offered by Hydra, the WoT HCTL ontology8
WoT HCTL –
CoRAL –
A missing link between the description of affordances and the actual messages that will be sent as commands and received as results, is the description of the data model for these messages. The WoT JSON Schema ontology10
JSON Schema ontology – https://www.w3.org/2019/wot/json-schema.
Human in the loop Voice and gesture-controlled interfaces are becoming increasingly popular in the WoT. In particular, in automobiles, smart homes, computer games and Augmented Reality (AR) / Virtual Reality (VR) applications, voice, gestures and sometimes even gaze has become prevalent due to its accessibility to everyone. Designers, producers, and vendors integrating gesture interfaces into their products have also increased in numbers, giving rise to a greater variation of interaction models in utilizing them. However, different modalities that are used to interact with smart environments have not yet been formalized in vocabularies and ontologies, in particular models that formally describe voice commands, gestures and gaze interactions and how they relate to affordances of WoT devices are missing.
Other considerations are of utmost importance for the WoT as it bridges the Web and the real-world where humans need to protect their privacy and integrity. Ontologies to describe access control and security of WoT application will be important for a successful deployment of the semantic WoT. The WoT SEC ontology11
WoT SEC –
While RDF has proven to be an effective data model for interoperability on the application layer, its verbose serialisation formats (e.g. RDF/XML, NTriples, or Turtle) present a challenge on the presentation layer. Other than some approaches using the HDT [19] serialisation of RDF [29] or other binary representations of RDF [8], there has been little work and even fewer uptake in industry of providing WoT devices that consume and produce RDF.
Consequently, many data formats and data models exist and they compete with each other for adoption in devices in different WoT domains. Standardisation groups rather try to solve this problem by standardising data formats and service APIs [17,33,52]. Some work aim at tackling semantic interoperability despite the heterogeneity of data formats and service API specifications, i.e., across platforms.
The use of semantic Web technologies has been investigated to facilitate semantic interoperability among these platforms [20,49,65]. One challenge is to investigate how semantic interoperability can be obtained on the edge level, i.e. between devices directly, instead of between platforms. The work in Lefrançois [45] is a starting point to investigate how constrained devices that are not natively semantic Web enabled can still be interoperable with one another.
Figure 2(a) illustrates a typical Web service that consumes and outputs resource representations, that are octet streams typed with internet media types according to the Web architecture principles.12
Figure 2(b) illustrates a combination of two WoT services that are seemingly incompatible, but that as an abstraction generate and consume RDF, respectively. The output RDF graph, generated using a certain lifting rule, could then be lowered using the second thing’s lowering rule. In this setting, the condition for the services to be composable is that the content validation rule of the first thing is more specific than the content validation rule of the second thing. However, SHACL shape containment has not been investigated yet.
As devices became powerful enough to offer storage and processing, new architectures appeared, based on edge computing. At the same time, it is now often a requirement that the final user is able to configure the intelligent environment. This poses several research questions: (i) how to embed reasoning in edge devices with various capacities; (ii) how to efficiently distribute reasoning tasks among available heterogeneous devices; and (iii) how to allow user to easily write rules for such devices.
How to embed reasoning in devices with various capacities Edge computing allows manipulating data close to sources, saving bandwidth, lowering latency and reducing communication needs. Slider [9] is an incremental reasoner optimised in memory and processing footprint. RDF4LED [40] is a lightweight RDF engine, which comprises of RDF storage and a SPARQL processor, for small query operations in lightweight edge devices. Devices on the WoT generate and consume highly dynamic data. The recent survey on stream reasoning by Dell’aglio et al. [14] discusses several open issues left to tackle in this field.
How to distribute reasoning tasks among devices Reasoning tasks can be distributed among heterogeneous devices: some are powerful computers, some are edge devices with various constraints. One challenge is to develop computationally-efficient reasoning strategies coping with such heterogeneities [62], and as close to the data sources as possible. One such approach is HyLAR that deploys incremental reasoning tasks on both, the server and the client [64]. Assuming that devices may reason with heterogeneous entailment regimes is also a starting point for a new investigation, for example, if a constrained edge node uses a poorer entailment regime than a more powerful gateway.
End-User Development In the scenario offered by such a complex network, the End-User Development (EUD) vision [48] aims at putting customization mechanisms in the hands of end users. Starting from iCAP [16], an early rule-based system for building context-aware applications, several works demonstrated the effective applicability of EUD techniques for the personalization of the functionality of smart devices and online services in different areas, including mobile environments [11] and smart home [6]. Users can take advantage of visual programming platforms such as IFTTT and Zapier to personalize the joint behaviors of their own connected entities, by adopting the trigger-action programming paradigm, i.e., they allow the definition of IF-THEN rules. Unfortunately, despite its wide adoption, the way it is implemented nowadays presents its own set of open issues: (i) low-level of abstraction (current trigger-action programming platforms adopt highly technology-dependent representation models that work with well-known connected entities, previously associated to a specific user, only. Therefore, defining IF-THEN rules becomes a complex task for non-programmers [31]), (ii) information overload (contemporary trigger-action programming platforms do not provide users with any discovery support [66], and the explosion of new smart devices and online service results in user interfaces with too much information), (iii) and run-time problems (there is the need to provide users with instruments for understanding and debugging their IF-THEN rules, i.e., to avoid possible conflicts [6] and to assess a rules’ correctness [15]).
Overview of the special issue
The focus of this special issue is to showcase novel approaches of applying semantic technologies to solve the problems of device and data integration mentioned above. We received nine submissions, covering a wide range of points of view related to these topics. The thorough peer-review process selected three of these submissions, of three different submission types (i.e. one full paper, one survey article and one linked dataset description), as mature enough to be published in the Semantic Web journal.
The three accepted papers also deal with three popular and important application domains of the WoT [59], Smart Agriculture, Smart Building and the Industry of the Future.13
Often also called Industry 4.0, a term originating in 2011 from a project in the high-tech strategy of the German government.
The paper on Weather Data Publication on the LOD using SOSA/SSN Ontology14
The survey paper Ontologies for Observations and Actuations in Buildings: A Survey15
Industry of the Future or Industry 4.0 is a third interesting application domain, where devices, machines, production modules and products are comprised as Cyber-Physical Systems (CPS) that are autonomously exchanging information, triggering actions and controlling each other. Factories are developing into intelligent environments that enable dynamic re-engineering processes and the ability to respond flexibly to failures. In particular, business-specific knowledge must therefore be modeled as self-contained bundles, and inserted into the system at run-time when needed. To address this issues, the paper EDR: A Generic Approach for the Distribution of Rule-Based Reasoning in a Cloud-Fog continuum16
by Nicolas Seydoux, Khalil Drira, Nathalie Hernandez, and Tierry Monteil, proposes an original architecture which exploits the complementarity of Cloud and Fog computing. In this model, reasoning rules are used to capture business level logic and are distributed across nodes and executed as close as possible to where the data is produced, in order to enable low-latency decision making. At the same time remote powerful Cloud computation resources are exploited in order to benefit from the Cloud stability and permanent availability. Moreover, as IoT networks are open and evolutive, the computation is dynamically distributed across Fog nodes according to the transformation of the network topology.Contributions to this special issue have shown that ontologies, linked data, and reasoning, have a wide range of research directions on the Web of Things and can be applied to a wide range of application domains. However, further advances are needed to cover gaps in existing ontologies, bring semantics in the edge, and develop distributed semantic reasoning approaches.
Footnotes
Acknowledgements
We take this opportunity to sincerely thank the authors for their invaluable and inspiring contributions to this special issue. We are also grateful to all reviewers for reviewing the submissions and helping us publish an interesting special issue (non-anonymous reviewers only, including reviews on rejected papers and in alphabetical order): Jose María Alvarez-Rodríguez, Payam Barnaghi, Eva Blomqvist, Michel Böhms, Simon Cox, Aidan Hogan, Andreas Kamilaris, Sebastian Neumaier, Alessandro Oltramari, Antonio Piccinno, Ana Roxin, Antoine Zimmermann. Finally we thank the editors-in-chief of the Semantic Web Journal, Pascal Hitzler and Krzysztof Janowicz, for their continuous support and help and for making the Semantic Web Journal Open Review and Open Access.
