W3C Recommendation Research Articles

Towards counterfactual explanations for ontologies

Debugging and repairing Web Ontology Language (OWL) ontologies has been a key field of research since OWL became a W3C recommendation. One way to understand errors and fix them is through explanations. These explanations are usually extracted from the reasoner and displayed to the ontology authors as is. In the meantime, there has been a recent call in the eXplainable AI (XAI) field to use expert knowledge in the form of knowledge graphs and ontologies. In this paper, a parallel between explanations for machine learning and for ontologies is drawn. This link enables the adaptation of XAI methods to explain ontologies and their entailments. Counterfactual explanations have been identified as a good candidate to solve the explainability problem in machine learning. The CEO (Counterfactual Explanations for Ontologies) method is thus proposed to explain inconsistent ontologies using counterfactual explanations. A preliminary user study is conducted to ensure that using XAI methods for ontologies is relevant and worth pursuing.

The W3C Data Catalog Vocabulary, Version 2: Rationale, Design Principles, and Uptake

ABSTRACT DCAT is an RDF vocabulary designed to facilitate interoperability between data catalogs published on the Web. Since its first release in 2014 as a W3C Recommendation, DCAT has seen a wide adoption across communities and domains, particularly in conjunction with implementing the FAIR data principles (for findable, accessible, interoperable and reusable data). These implementation experiences, besides demonstrating the fitness of DCAT to meet its intended purpose, helped identify existing issues and gaps. Moreover, over the last few years, additional requirements emerged in data catalogs, given the increasing practice of documenting not only datasets but also data services and APIs. This paper illustrates the new version of DCAT, explaining the rationale behind its main revisions and extensions, based on the collected use cases and requirements, and outlines the issues yet to be addressed in future versions of DCAT.

Semantic rule checking of cross-domain building data in information containers for linked document delivery using the shapes constraint language

Information exchange is a crucial process throughout the lifecycle of built assets. Building Information Modeling (BIM) and standardized information delivery enable exchanging, storing, and linking cross-domain information bundled in containers such as the standardized Information Container for Linked Document Delivery (ICDD). While assessing the quality of the interlinking and metadata is essential to allow all parties to interpret an information container correctly, most research efforts have focused on code compliance of domain-specific models. In order to address this gap, this paper proposes an approach for validating ICDD information containers using the Shapes Constraint Language (SHACL), the W3C recommendation for Resource Description Framework (RDF) data validation. For optimized querying and validation, a case study applies SHACL inference rules to the linking structure of ICDD to infer predicates between linked entities in four ICDD containers in varying dataset sizes and different linking methods. Afterward, the validation of these containers is analyzed using three types of validation rules with increasing complexity for another SHACL implementation. Results show the feasibility of cross-domain building information validation in ICDD with SHACL and how performance enhancements can be obtained on an inferred dataset compared to the original dataset.

Interconversion of captions described by ARIB-TTML and IMSC

ARIB-TTML is a subtitle and caption standard for broadcasting systems in Japan. TTML profiles for Internet media subtitles and captions (IMSC) is a W3C recommendation. ARIB-TTML and IMSC are widely used in UHD satellite broadcasting in Japan and OTT operators; however, they have incompatible features and different description methods. These differences impede program exchange between broadband and broadcast. In this study, we propose a method for the interconversion of subtitles and captions described in ARIB-TTML and IMSC. We organized the features of each standard and proposed a conversion algorithm. The interconverted caption files were confirmed using a JavaScript-based caption renderer.

Learning SHACL shapes from knowledge graphs

Knowledge Graphs (KGs) have proliferated on the Web since the introduction of knowledge panels to Google search in 2012. KGs are large data-first graph databases with weak inference rules and weakly-constraining data schemes. SHACL, the Shapes Constraint Language, is a W3C recommendation for expressing constraints on graph data as shapes. SHACL shapes serve to validate a KG, to underpin manual KG editing tasks, and to offer insight into KG structure. Often in practice, large KGs have no available shape constraints and so cannot obtain these benefits for ongoing maintenance and extension. We introduce Inverse Open Path (IOP) rules, a predicate logic formalism which presents specific shapes in the form of paths over connected entities that are present in a KG. IOP rules express simple shape patterns that can be augmented with minimum cardinality constraints and also used as a building block for more complex shapes, such as trees and other rule patterns. We define formal quality measures for IOP rules and propose a novel method to learn high-quality rules from KGs. We show how to build high-quality tree shapes from the IOP rules. Our learning method, SHACLearner, is adapted from a state-of-the-art embedding-based open path rule learner (Oprl). We evaluate SHACLearner on some real-world massive KGs, including YAGO2s (4M facts), DBpedia 3.8 (11M facts), and Wikidata (8M facts). The experiments show that our SHACLearner can effectively learn informative and intuitive shapes from massive KGs. The shapes are diverse in structural features such as depth and width, and also in quality measures that indicate confidence and generality.

Decentralized Identifiers and Self-Sovereign Identity in 6G

A key challenge for mobile network operators in 6G is to bring together and orchestrate a variety of new emerging players of today's mobile eco-systems in order to provide economically viable and seamless mobile connectivity in the form of a multi-stakeholder service. With each new player, be it a cloud, edge, or hardware provider, the need for interfaces with secure authentication and authorization mechanisms increases, as does the complexity and operational costs of the public key infrastructures required for the identity and key management. While today's centralized public key infrastructures have proven to be technically feasible in confined and trusted spaces, they do not provide the required security for access control once centralized identity providers must be avoided because of limited cross-domain interoperability, national data protection legislation, or geopolitical-strategic reasons. Recent decentralized identity management concepts, such as the W3C recommendation of decentralized identifiers, provide a secure, tamper-proof, and cross-domain identity management alternative for future multi-stakeholder 6G networks without relying on centralized identity providers or certification authorities. This article introduces the concept of decentralized identifiers together with the principles of self-sovereign identity, and discusses opportunities and potential benefits of their application and usage for cross-domain and privacy-preserving identity and key management in 6G networks.

Satisfiability and containment of recursive SHACL

The Shapes Constraint Language (SHACL) is the recent W3C recommendation language for validating RDF data, by verifying certain shapes on graphs. Previous work has largely focused on the validation problem, while the standard decision problems of satisfiability and containment, crucial for design and optimisation purposes, have only been investigated for simplified versions of SHACL. Moreover, the SHACL specification does not define the semantics of recursively-defined constraints, which led to several alternative recursive semantics being proposed in the literature. The interaction between these different semantics and important decision problems has not been investigated yet. In this article we provide a comprehensive study of the different features of SHACL, by providing a translation to a new first-order language, called SCL, that precisely captures the semantics of SHACL. We also present MSCL, a second-order extension of SCL, which allows us to define, in a single formal logic framework, the main recursive semantics of SHACL. Within this language we also provide an effective treatment of filter constraints which are often neglected in the related literature. Using this logic we provide a detailed map of (un)decidability and complexity results for the satisfiability and containment decision problems for different SHACL fragments. Notably, we prove that both problems are undecidable for the full language, but we present decidable combinations of interesting features, even in the face of recursion.

Fixpoint Semantics for Recursive SHACL

SHACL is a W3C-proposed language for expressing structural constraints on RDF graphs. The recommendation only specifies semantics for non-recursive SHACL; recently, some efforts have been made to allow recursive SHACL schemas. In this paper, we argue that for defining and studying semantics of recursive SHACL, lessons can be learned from years of research in non-monotonic reasoning. We show that from a SHACL schema, a three-valued semantic operator can directly be obtained. Building on Approximation Fixpoint Theory (AFT), this operator immediately induces a wide variety of semantics, including a supported, stable, and well-founded semantics, related in the expected ways. By building on AFT, a rich body of theoretical results becomes directly available for SHACL. As such, the main contribution of this short paper is providing theoretical foundations for the study of recursive SHACL, which can later enable an informed decision for an extension of the W3C recommendation.

Documenting flooding areas calculation: a PROV approach

Flooding events related to waste-lake dam ruptures are one of the most threatening natural disasters in Brazil. They must be managed in advance by public institutions through the use of adequate hydrographic and environmental information. Although the Open Data paradigm offers an opportunity to share hydrographic data sets, their actual reuse is still low because of metadata quality. Our previous work highlighted a lack of detailed provenance information. The paper presents an Open Data approach to improve the release of hydrographic data sets. We discuss a methodology, based on W3C recommendations, for documenting the provenance of hydrographic data sets, considering the workflow activities related to the study of flood areas caused by the waste-lakes breakdowns. We provide an illustrative example that documents, through W3C PROV metadata model, the generation of flooding area maps by integrating land use classification, from Sentinel images, with hydrographic data sets produced by the Brazilian National Institute for Space Research.

Documenting Context-Based Quality Assessment of Controlled Vocabularies

Access to e-Government data is challenging due to the heterogeneity and complexity of the public information ecosystem. Controlled Vocabularies (CVs) provide a key to disclosing the potential of Open Government data, by supplying common terms for marking up metadata and data in a consistent and coherent way. However, quality information is needed to help public institutions decide whether to adopt an existing or newly created CV. The paper discusses how to evaluate and document CV quality thus facilitating a comparison of different controlled vocabularies based on contextual information. The Analytical Hierarchy Process (AHP) is adopted to assess the overall quality and rank of a controlled vocabulary, by integrating various quality dimensions according to the decision maker's needs. A set of e-Government controlled vocabularies that facilitate the semantic interoperability of e-Government data are selected as a testbed, and updated quality values are made available as Linked Data. Multi-step guidelines are also defined promoting and complementing the adoption of W3C recommendations to provide machine-readable quality metadata. This fosters reliability and re-usability by providing consumers with information on the assessment process carried out and the outcomes achieved. We illustrate the application of these guidelines by focusing on provenance and quality documentation.

Autonomous search in a social and ubiquitous Web

Recent W3C recommendations for the Web of Things (WoT) and the Social Web are turning hypermedia into a homogeneous information fabric that interconnects heterogeneous resources: devices, people, information resources, abstract concepts, etc. The integration of multi-agent systems with such hypermedia environments now provides a means to distribute autonomous behavior in worldwide pervasive systems. A central problem then is to enable autonomous agents to discover heterogeneous resources in worldwide and dynamic hypermedia environments. This is a problem in particular in WoT environments that rely on open standards and evolve rapidly—thus requiring agents to adapt their behavior at run time in pursuit of their design objectives. To this end, we developed a hypermedia search engine for the WoT that allows autonomous agents to perform approximate search queries in order to retrieve relevant resources in their environment in (weak) real time. The search engine crawls dynamic WoT environments to discover and index device metadata described with the W3C WoT Thing Description, and exposes a SPARQL endpoint that agents can use for approximate search. To demonstrate the feasibility of our approach, we implemented a prototype application for the maintenance of industrial robots in worldwide manufacturing systems. The prototype demonstrates that our semantic hypermedia search engine enhances the flexibility and agility of autonomous agents in a social and ubiquitous Web.

Role of SPARQL in Leveraging Sematic Web Technology

Semantic web is not just a matter of translation from HTML to RDF/OWL languages. It is a matter of understanding the content of the web through knowledge graphs. Entities need to be related with relationships. This content is composed of resources (web pages) that contain, for example, text, images and audio. Thus, there is the need of extracting entities from these resources. Currently, most of the web content is in HTML5 format which is a W3C recommendation which enables describing the structure marginally with the help of annotations. The main challenge here is to transform unstructured data from plain HTML files to structured data (e.g RDF or OWL). The current work provides the first hand information for dealing with unstructured heterogeneous data residing on web using Twinkle, a Java tool for SPARQL query execution on FOAF (Friend Of A Friend) document.

A semantic web methodological framework to evaluate the support of integrity in thesaurus tools

With the Semantic Web, thesauri regain a relevant role supporting semantic searches and other added-value services. Thesaurus standards define the constructs a thesaurus can have and the integrity rules it must comply with. Thesaurus editors can be helped in their work if thesaurus tools offer them support for integrity, warning when integrity rules are violated and/or helping them to correct these mistakes. The most recent thesaurus standard is ISO 25964, which supersedes ISO 2788, evolving towards concept-based thesauri and better aligned with the Semantic Web approach than the term-based thesauri of ISO 2788. However, the W3C recommendation for Knowledge Organisation System (KOS) representation in the semantic web context is Simple Knowledge Organisation Systems (SKOS), which is in fact prior to ISO 25964. This article focuses on thesaurus integrity and the evolution from ISO 2788 to ISO 25964. Its effect on integrity issues is analysed. A methodological proposal for evaluating integrity support in thesaurus tools, arising from the results of this work, is presented. Its target audience is professionals in charge of thesaurus editing. Besides being adapted to the most recent thesaurus standard, ISO 25964, it also includes the comparison of ISO standards with SKOS. The article concludes with the presentation of the results obtained by applying the framework to three thesaurus tools.

The support of constructs in thesaurus tools from a Semantic Web perspective: Framework to assess standard conformance

Thesauri are conceptual tools useful to achieve semantic interoperability and reusability, which are relevant goals in the Semantic Web. Thesaurus standards establish, among other issues, the constructs that can appear in a thesaurus. The ISO 25964 standard for thesauri, which supersedes ISO 2788, is the evolution of the ISO thesauri standard to a conceptual approach closer to the Semantic Web. However, it appeared when SKOS -the W3C Recommendation- was already consolidated as the standard for KOS (Knowledge Organization System) representation in the Semantic Web, including thesauri. The evolution from ISO 2788 to ISO 25964, and the relationships between constructs in ISO 2788/ISO 25964 and SKOS, are studied in this paper. From the analysis of this comparison, a methodological framework, that focuses on the construct support, is proposed to evaluate the conformance quality of thesaurus management tools. Target readers are professionals in charge of thesauri edition. A Semantic Web perspective is taken to characterize the effect that using SKOS to represent thesauri can have on the results of the assessment. A proof of concept for the model's feasibility was performed on two tools and the analysis of the results of this experimental validation is presented. The conclusions highlight the model's suitability for assessing conformance to the standards concerning support for thesaurus constructs.

The modular SSN ontology: A joint W3C and OGC standard specifying the semantics of sensors, observations, sampling, and actuation

The joint W3C (World Wide Web Consortium) and OGC (Open Geospatial Consortium) Spatial Data on the Web (SDW) Working Group developed a set of ontologies to describe sensors, actuators, samplers as well as their observations, actuation, and sampling activities. The ontologies have been published both as a W3C recommendation and as an OGC implementation standard. The set includes a lightweight core module called SOSA (Sensor, Observation, Sampler, and Actuator) available at: http://www.w3.org/ns/sosa/, and a more expressive extension module called SSN (Semantic Sensor Network) available at: http://www.w3.org/ns/ssn/. Together they describe systems of sensors and actuators, observations, the used procedures, the subjects and their properties being observed or acted upon, samples and the process of sampling, and so forth. The set of ontologies adopts a modular architecture with SOSA as a self-contained core that is extended by SSN and other modules to add expressivity and breadth. The SOSA/SSN ontologies are able to support a wide range of applications and use cases, including satellite imagery, large-scale scientific monitoring, industrial and household infrastructures, social sensing, citizen science, observation-driven ontology engineering, and the Internet of Things. In this paper we give an overview of the ontologies and discuss the rationale behind key design decisions, reporting on the differences between the new SSN ontology presented here and its predecessor [9] developed by the W3C Semantic Sensor Network Incubator group (the SSN-XG). We present usage examples and describe alignment modules that foster interoperability with other ontologies.

Knowledge Organization Systems (KOS) in the Semantic Web: a multi-dimensional review

Since the Simple Knowledge Organization System (SKOS) specification and its SKOS eXtension for Labels (SKOS-XL) became formal W3C recommendations in 2009 a significant number of conventional knowledge organization systems (KOS) (including thesauri, classification schemes, name authorities, and lists of codes and terms, produced before the arrival of the ontology-wave) have made their journeys to join the Semantic Web mainstream. This paper uses "LOD KOS" as an umbrella term to refer to all of the value vocabularies and lightweight ontologies within the Semantic Web framework. The paper provides an overview of what the LOD KOS movement has brought to various communities and users. These are not limited to the colonies of the value vocabulary constructors and providers, nor the catalogers and indexers who have a long history of applying the vocabularies to their products. The LOD dataset producers and LOD service providers, the information architects and interface designers, and researchers in sciences and humanities, are also direct beneficiaries of LOD KOS. The paper examines a set of the collected cases (experimental or in real applications) and aims to find the usages of LOD KOS in order to share the practices and ideas among communities and users. Through the viewpoints of a number of different user groups, the functions of LOD KOS are examined from multiple dimensions. This paper focuses on the LOD dataset producers, vocabulary producers, and researchers (as end-users of KOS).

The ACORN-SAT linked climate dataset

In 2012 the Australian Bureau of Meteorology published a dataset, ACORN-SAT, containing the homogenised daily temperature observations of 112 locations throughout Australia for the last 100 years. The dataset employs the latest analysis techniques and takes advantage of newly digitised observational data to monitor climate variability and change in Australia. The observations in ACORN-SAT were initially published only as comma separated values, whereas the metadata was published in a PDF report. In 2013 we converted the metadata and the observation data into RDF and published the result as Linked Open Data, accessible online via a pilot government linked data service built on the Linked Data API. In this article we describe the process of transforming the original tabular data into a Linked Sensor Data Cube (in: Proc. of the 5th International Workshop on Semantic Sensor Networks, SSN12, CEUR-WS.org, 2012, pp. 1-16) based on the W3C Semantic Sensor Network ontology (Web Semantics: Science, Services and Agents on the World Wide Web 17 (2012), 25-32) and the W3C RDF Data Cube vocabulary (The RDF Data Cube Vocabulary, W3C Recommendation, 16 January 2014). We further discuss how the dataset has since been used and interlinked with near-real time weather observations for the 112 sensing locations of the ACORN-SAT that are published by the Bureau of Meteorology. Both the original ACORN-SAT dataset and the weather observation data are accessible online at lab.environment.data.gov.au.

User-Generated Captions: From Hackers, to the Disability Digerati, to Fansubbers

User-Generated Captions: From Hackers, to the Disability Digerati, to Fansubbers

Representing Time and Space for the Semantic Web

Representation of temporal and spatial information for the Semantic Web often involves qualitative defined information (i.e., information described using natural language terms such as “before” or “overlaps”) since precise dates or coordinates are not always available. This work proposes several temporal representations for time points and intervals and spatial topological representations in ontologies by means of OWL properties and reasoning rules in SWRL. All representations are fully compliant with existing Semantic Web standards and W3C recommendations. Although qualitative representations for temporal interval and point relations and spatial topological relations exist, this is the first work proposing representations combining qualitative and quantitative information for the Semantic Web. In addition to this, several existing and proposed approaches are compared using different reasoners and experimental results are presented in detail. The proposed approach is applied to topological relations (RCC5 and RCC8) supporting both qualitative and quantitative (i.e., using coordinates) spatial relations. Experimental results illustrate that reasoning performance differs greatly between different representations and reasoners. To the best of our knowledge, this is the first such experimental evaluation of both qualitative and quantitative Semantic Web temporal and spatial representations. In addition to the above, querying performance using SPARQL is evaluated. Evaluation results demonstrate that extracting qualitative relations from quantitative representations using reasoning rules and querying qualitative relations instead of directly querying quantitative representations increases performance at query time.

On organisational involvement and collaboration in W3C standards through editorship

Over time, a number of open standards have been developed and implemented in software for addressing a number of challenges, such as lock-in, interoperability and longevity of software systems and associated digital artefacts. An understanding of organisational involvement and collaboration in standardisation is important for informing any future policy and organisational decisions concerning involvement in standardisation. The overarching goal of the study is to establish how organisations contribute to open standards development through editorship. Specifically, the focus is on open standards development in the World Wide Web Consortium (W3C). Through an analysis of editorship for all W3C recommendations we contribute novel findings concerning organisational involvement and collaboration, and highlight contributions from different types of organisations and countries. We make five principal contributions. First, we establish an overall characterisation of organisational involvement in W3C standardisation. Second, we report on organisational involvement in W3C standardisation over time. Third, we establish how different organisations, organisation types, and countries are involved in W3C technologies. Fourth, we report on organisational involvement in relation to standard development time. Fifth, we establish how organisations collaborate in W3C standardisation through social network analysis.