The Web of Data

This book’s main goals are to bring together in a concise way all the methodologies, standards and recommendations related to Data, Queries, Links, Semantics, Validation and other issues concerning machine-readable data on the Web, to describe them in detail, to provide examples of their use, and to discuss how they contribute to – and how they have been used thus far on – the “Web of Data”. As the content of the Web becomes increasingly machine readable, increasingly complex tasks can be automated, yielding more and more powerful Web applications that are capable of discovering, cross-referencing, filtering, and organizing data from numerous websites in a matter of seconds. The book is divided into nine chapters, the first of which introduces the topic by discussing the shortcomings of the current Web and illustrating the need for a Web of Data. Next, “Web of Data” provides an overview of the fundamental concepts involved, and discusses some current use-cases on the Web where such concepts are already being employed. “Resource Description Framework (RDF)” describes the graph-structured data model proposed by the Semantic Web community as a common data model for the Web. The chapter on “RDF Schema (RDFS) and Semantics” presents a lightweight ontology language used to define an initial semantics for terms used in RDF graphs. In turn, the chapter “Web Ontology Language (OWL)” elaborates on a more expressive ontology language built upon RDFS that offers much more powerful ontological features. In “SPARQL Query Language” a language for querying and updating RDF graphs is described, with examples of the features it supports, supplemented by a detailed definition of its semantics. “Shape Constraints and Expressions (SHACL/ShEx)” introduces two languages for describing the expected structure of – and expressing constraints on – RDF graphs for the purposes of validation. “Linked Data” discusses the principles and best practices proposed by the Linked Data community for publishing interlinked (RDF) data on the Web, and how these techniques have been adopted. The final chapter highlights open problems and rounds out the coverage with a more general discussion on the future of the Web of Data. The book is intended for students, researchers and advanced practitioners interested in learning more about the Web of Data, and about closely related topics such as the Semantic Web, Knowledge Graphs, Linked Data, Graph Databases, Ontologies, etc. Offering a range of accessible examples and exercises, it can be used as a textbook for students and other newcomers to the field. It can also serve as a reference handbook for researchers and developers, as it offers up-to-date details on key standards (RDF, RDFS, OWL, SPARQL, SHACL, ShEx, RDB2RDF, LDP), along with formal definitions and references to further literature. The associated website webofdatabook.org offers a wealth of complementary material, including solutions to the exercises, slides for classes, raw data for examples, and a section for comments and questions. Springer Preface 6 Acknowledgements 9 Contents 10 Chapter 1 Introduction 15 1.1 The Latent Web 17 1.2 The Current Web 20 1.1.1 Hypertext Markup Language (HTML) 21 1.1.2 Interpreting HTML Content 23 Chapter 2 Web of Data 29 2.1 Overview 30 2.2 Web of Data: Concepts 30 2.2.1 Data 30 2.2.2 Queries 35 2.2.3 Semantics 39 2.2.4 Constraints 49 2.2.5 Links 51 2.3 The Current Web of Data 54 2.3.1 Wikidata 55 2.3.2 Knowledge Graphs 59 2.3.3 Schema.org and the Open Graph Protocol 62 2.3.4 Linking Open Data 67 2.4 Summary 70 2.5 Discussion 71 Chapter 3 Resource Description Framework 72 3.1 Overview 73 3.2 Terms 74 3.2.1 Internationalised Resource Identifiers (IRIs) 74 3.2.2 Literals 76 3.2.3 Blank Nodes 82 3.2.4 Defining RDF Terms 83 3.3 Triples 84 3.4 Graphs 87 3.5 Vocabulary and Modelling 90 3.5.1 Classes and Properties 90 3.5.2 n-ary Relations 92 3.5.3 Reification 94 3.5.4 Containers and Collections 96 3.6 Datasets 97 3.7 Blank Nodes 100 3.7.1 Isomorphism of RDF Graphs 100 3.7.2 Merging RDF Graphs 103 3.7.3 Lean Graphs 105 3.7.4 Blank Nodes in RDF Datasets 106 3.8 Syntaxes 107 3.8.1 RDF/XML 107 3.8.2 N-Triples 110 3.8.3 Turtle 111 3.8.4 RDFa 114 3.8.5 JSON-LD 117 3.8.6 Summary of RDF Graph Syntaxes 119 3.8.7 RDF Dataset Syntaxes 120 3.9 Summary 121 3.10 Discussion 122 Chapter 4 RDF Schema and Semantics 123 4.1 Overview 126 4.2 RDFS Vocabulary 127 4.2.1 Sub-class, Sub-property, Domain and Range 127 4.2.2 Annotations 132 4.2.3 Meta-Classes 133 4.2.4 Containers 138 4.3 RDF(S) Model-Theoretic Semantics 140 4.3.1 Simple Semantics 143 4.3.2 Datatype (D) Semantics 165 4.3.3 RDF Semantics 169 4.3.4 RDFS Semantics 175 4.4 RDF(S) Inference 183 4.4.1 RDF(S) Rules 184 4.4.2 RDF(S) Axiomatic Triples 187 4.4.3 Generalised Triples 188 4.4.4 RDF(S) Inference Procedure 189 4.4.5 Extensional Semantics 190 4.5 Summary 193 4.6 Discussion 194 Chapter 5 Web Ontology Language 196 5.1 Overview 198 5.2 Ontologies 199 5.3 OWL 2 Model Theory 205 5.4 OWL 2 Vocabulary 207 5.4.1 Equality and Inequality 207 5.4.2 Properties 211 5.4.3 Classes 226 5.4.4 Negation 254 5.4.5 Datatypes 255 5.4.6 Annotations 261 5.4.7 Features Not Supported by OWL 2 262 5.5 OWL 2 Reasoning Tasks 265 5.6 OWL 2 Languages 275 5.6.1 OWL 2 Full 276 5.6.2 OWL 2 DL 285 5.6.3 OWL 2 Profiles 308 5.6.4 Complexity 330 5.7 Summary 330 5.8 Discussion 332 Chapter 6 SPARQL Query Language 334 6.1 Overview 336 6.2 SPARQL Query Language 336 6.2.1 Basic Graph Patterns 339 6.2.2 Unions of Graph Patterns 344 6.2.3 Optional Graph Patterns 347 6.2.4 Filtering and Binding Values 349 6.2.5 Negation of Graph Patterns 357 6.2.6 Property Paths 362 6.2.7 Aggregation 366 6.2.8 Solution Modifiers 374 6.2.9 Sub-Select Queries 376 6.2.10 In-line Values 378 6.2.11 Querying Multiple Graphs 381 6.2.12 Query Types 389 6.2.13 In Practice 395 6.3 SPARQL Semantics and Complexity 397 6.3.1 SPARQL Solutions and Abstract Syntax 398 6.3.2 SPARQL Algebra and Evaluation 403 6.3.3 SPARQL Complexity 415 6.4 SPARQL Federation 420 6.5 SPARQL Update 425 6.5.1 Inserting/Deleting Triples 426 6.5.2 Inserting/Deleting with Queries 428 6.5.3 Loading Triples from the Web 432 6.5.4 Managing Graphs 433 6.6 SPARQL Entailment Regimes 435 6.6.1 Extending Simple Entailment 436 6.6.2 RDF/RDFS/D Entailment 439 6.6.3 OWL Entailment 444 6.6.4 RIF Entailment 448 6.6.5 Beyond Basic Graph Patterns 449 6.7 SPARQL Service 451 6.7.1 Output Formats 451 6.7.2 SPARQL Protocol 453 6.7.3 SPARQL Service Description 454 6.8 Summary 458 6.9 Discussion 458 Chapter 7 Shape Constraints and Expressions 460 7.1 Shape Constraints Language (SHACL) 464 7.1.1 Shapes Graph 465 7.1.2 Shapes 468 7.1.3 Connecting and Combining Shapes 478 7.1.4 Core Constraints 485 7.1.5 SHACL-SPARQL Constraints 503 7.1.6 SHACL Compact Syntax 511 7.1.7 SHACL Advanced Features 512 7.2 Shape Expressions Language (ShEx) 512 7.2.1 ShEx Compact (ShExC) Syntax 512 7.2.2 ShEx JSON (ShExJ) Syntax 518 7.3 Comparing SHACL and ShEx 519 7.4 Shapes Definitions 519 7.5 Summary 523 7.6 Discussion 524 Chapter 8 Linked Data 525 8.1 Overview 526 8.2 Linked Data Principles 528 8.2.1 The Four Principles 529 8.2.2 Recipes for Dereferencing 532 8.2.3 Content Negotiation 537 8.3 Linked Data Best Practices 539 8.3.1 Naming 540 8.3.2 Modelling 542 8.3.3 Linking 545 8.3.4 Vocabularies 549 8.4 Linking Open Data 554 8.4.1 Five-Star Open Data 555 8.4.2 Linked Open Datasets 556 8.4.3 Linked Open Vocabularies 561 8.5 Publishing Linked Data 564 8.5.1 Relational Data to RDF (RDB2RDF) 566 8.5.2 Other Legacy Formats to RDF 585 8.5.3 Link Discovery 600 8.5.4 Hosting Linked Data 605 8.6 Linked Data Platform (LDP) 616 8.6.1 Resources and Containers 618 8.6.2 HTTP RESTful Interface 621 8.7 Summary 633 8.8 Discussion 634 Chapter 9 Conclusions 636 9.1 Research Trends 637 9.1.1 Data Quality 638 9.1.2 Link Discovery 639 9.1.3 Context 639 9.1.4 Legacy Formats 640 9.1.5 Graph Analytics 641 9.1.6 Inductive Semantics 642 9.1.7 Ontology Engineering 642 9.1.8 Ontology-Based Data Access 643 9.1.9 Linked Vocabularies 644 9.1.10 Web Reasoning 645 9.1.11 Web Querying 645 9.1.12 Query Interfaces 646 9.1.13 Usage Control 647 9.1.14 Read–Write Linked Data 648 9.1.15 Knowledge Graphs 649 9.2 Final Remarks 649 Glossary 652 References 668 semantic,web,linked,data,rdf,knowledge,graphs,web,of,data This book concisely brings together the key standards and best practices relating to modelling, querying, validating and linking machine-readable data and semantics on the Web. Alongside practical examples and formal definitions, the book shows how these standards contribute to - and have been used thus far on - the "Web of Data": a machine readable evolution of the Web marked by increased automation, enabling powerful Web applications capable of discovering, cross-referencing, and organising data from numerous websites in a matter of seconds. The book is divided into nine chapters, the first of which highlights the fundamental shortcomings of the current Web that illustrate the need for increased machine readability. The next chapter outlines the core concepts of the "Web of Data", discussing use-cases on the Web where they have already been deployed. "Resource Description Framework (RDF)" describes the graph-structured data model proposed by the Semantic Web community as a common data model for the Web. The chapter on "RDF Schema (RDFS) and Semantics" presents a lightweight ontology language used to define an initial semantics for RDF graphs. In turn, the chapter "Web Ontology Language (OWL)" elaborates on a much more expressive ontology language built upon RDFS. In "SPARQL Query Language" a language for querying and updating RDF graphs is described. "Shape Constraints and Expressions (SHACL/ShEx)" introduces two languages for describing the expected structure of - and expressing constraints over - RDF graphs for the purposes of validation. "Linked Data" discusses the principles and best practices by which interlinked (RDF) data can be published on the Web, and how they have been adopted. The final chapter highlights open problems and concludes with a general discussion on the future of the Web of Data. The book is intended for students, researchers and advanced practitioners interested in learning more about the Web of Data, an d about closely related topics such as the Semantic Web, Knowledge Graphs, Linked Data, Graph Databases, Ontologies, etc. Offering a range of accessible examples and exercises, it can be used as a textbook for students and other newcomers to the field. It can also serve as a reference handbook for researchers and developers, as it offers up-to-date details on key standards (RDF, RDFS, OWL, SPARQL, SHACL, ShEx, RDB2RDF, LDP), along with formal definitions and references to further literature. The associated website webofdatabook.org offers a wealth of complementary material, including solutions to the exercises, slides for classes, interactive examples, and a section for comments and questions Front Matter ....Pages i-xvii Introduction (Aidan Hogan)....Pages 1-14 Web of Data (Aidan Hogan)....Pages 15-57 Resource Description Framework (Aidan Hogan)....Pages 59-109 RDF Schema and Semantics (Aidan Hogan)....Pages 111-183 Web Ontology Language (Aidan Hogan)....Pages 185-322 SPARQL Query Language (Aidan Hogan)....Pages 323-448 Shape Constraints and Expressions (Aidan Hogan)....Pages 449-513 Linked Data (Aidan Hogan)....Pages 515-625 Conclusions (Aidan Hogan)....Pages 627-642 Back Matter ....Pages 643-680