The Web of Data

689 Pages·2020·15.16 MB·English

by  Aidan Hogan

Aidan Hogan
The Web
of Data
The Web of Data
Aidan Hogan
The Web of Data
Aidan Hogan
Department of Compu ter Science
Universidad de Chile
Santiago de Chile, Chile

ISBN 978-3-030-51579-9 ISBN 978-3-030-51580-5 (eBook)
https://doi.org/10.1007/978-3-030-51580-5

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For all those without whom this book
would not have been possible.
To all those without whom it would have
been finished sooner.
Preface
The idea of a “Web of Data”, as discussed in this book, has been around
since at least 1998, when Berners-Lee referenced his plan “for achieving a
set of connected applications for data on the Web in such a way as to form a
consistent logical web of data”. Myriad developments have taken place since
thentowardsrealisingaWebofData,includingstandards,languages,proto-
cols, tools, optimisations, theorems, and more besides. These developments
have more recently been deployed on millions of websites, where most users
interact with the Web of Data on a daily basis, perhaps without realising it.
The core objective of the Web of Data is to publish content on the Web
in formats that machines can process more easily and accurately than the
human-friendlyHTMLdocumentsformingthecurrent“WebofDocuments”.
As the Web becomes increasingly machine readable, increasingly complex
tasks can be automated on the Web, yielding more and more powerful Web
applications that are capable of discovering, cross-referencing, filtering, and
organising data from numerous websites in a matter of seconds.
Assume, for example, that we are running a recipe website. Rather than
only describing the recipe in the text of a paragraph – like “This easy dessert
recipe for Pecan Pie takes around 20–25 minutes ...” – the first step of the
WebofDataistopublishstructuredDataabouttherecipethatiseasierfor
machines to process than natural language. The data can be populated from
theunderlyingdatabaseweuseforthewebsite,andcanbeembeddedwithin
the webpages we already have, or can be published as separate documents.
Ideally we can use a data format that is also used by other recipe sites, such
that external applications – e.g., search engines – can compile together these
data from different sources, allowing users to quickly Query for recipes by
type, difficulty, duration, etc. Such applications will then point users back to
our website for more details about the recipes of interest to them.
Nextwewouldliketoaddallergyinformationtoallowuserstofilterrecipes
thatarenotsafeforthemtoeat;forexample,ifauserisallergictotreenuts,
theywouldliketoquicklyruleoutrecipesusingalmonds,cashews,chestnuts,
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pecans, pistachios, or ingredients made using such nuts, including marzipan,
certain liqueurs, etc. However, allergies are potentially complex to model
in our data, and we need to avoid offering false or misleading information.
Rather than manage this sort of information ourselves, we can Link the
ingredientsofourrecipestoanofficial,centralrepositoryofnutritionaldata,
whichdefinestheSemanticsofdifferenttypesoffoods,suchastostatethat
cashews,chestnuts,etc.,aretypesoftreenuts;thatalltreenutsareculinary
nuts; that legumes (like peanuts) are culinary nuts, but not tree nuts; etc.
What started out as some simple data about recipes slowly expands over
time and becomes more and more diverse, allowing our users to find which
of our recipes are of more relevance to them through increasingly detailed
queries.Butasnewrecipesareadded,westarttofindrecipeswithincomplete
data (e.g., durations are missing in some cases), or inconsistent information
(e.g., recipes requiring a pasta machine are marked “easy”). Hence we start
to implement mechanisms for Validation, checking that every recipe has
a title, a description, a duration, etc., that recipes marked as easy require
equipment from a common list of utensils, etc. These validation mechanisms
help us to identify, prioritise and address potential quality issues that might
affect our users as the recipe data expand and grow more diverse.
Finally,wewillconsiderauserwhohasa(hypothetical,asofyet)personal
softwareagentwithlocalaccesstosensitivedata,suchastheuser’slocation,
medical issues, culinary preferences, available utensils, etc. Based on a query
fromtheuser,suchanagentmaythenventureoutonthisexpandingWebof
Data, discover recipe sites like ours, discover product lists of local supermar-
kets,discovernutritionalrecommendationsassociatedwiththeusers’medical
issues(ifany),andpullallofthesedatatogetherinordertorecommendalist
of safe recipes they can make at home with ingredients available for delivery
from a nearby supermarket. The user can then browse this list of recipes,
ordering or filtering by price, duration, calories, etc., perhaps even visiting
our website for the details of a recipe that catches their eye.
The scenario we describe here, of course, is not nearly as straightforward
to put into practice as we seem to suggest. Considering each part in detail
raises various technical questions and challenges regarding how we structure
data, how queries should be formulated, how links to remote data can be
specified, the types of semantics we need, how constraints are formulated
for validation, and so forth. However, there have also been over two decades
of work on proposals to address these questions, particularly within the Se-
mantic Web community, which has brought forward the most concrete and
completeinstantiationofaWebofDataseentodate,withdetailedstandards
andrecommendationsrelatingtoData,Queries,Links,Semantics,Val-
idation and more besides. These proposals have been adopted on the Web,
with the standards proposed for data being promoted by popular search en-
ginesandusedonmillionsofwebsites.Standardsforqueries,links,semantics
and validation are also being increasingly adopted for specific use-cases.
Preface ix
The primary undertaking of this book is then to draw together these pro-
posals, to motive and describe them in detail, to provide examples of their
use, and to discuss how they contribute to – and how they have been used
thusfaron–theWebofData.Thebookisaimedatstudents,researchersand
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. The book can serve as a textbook
for students and other newcomers, where it motivates the topics discussed,
offers accessible examples and exercises, etc.; it can also serve as a reference
handbook for researchers and developers, where it offers up-to-date details
ofkeystandards(RDF,RDFS,OWL,SPARQL,SHACL,ShEx,RDB2RDF,
LDP), along with formal definitions and references to further literature.
The book is structured around nine chapters, as follows:
Introduction: Chapter1introducesthebook,discussingtheshortcomings
of the current Web that illustrate the need for a Web of Data.
Web of Data: Chapter2providesanoverviewofthefundamentalconcepts
underlyingtheWebofData,anddiscussessomecurrentuse-casesonthe
Web where such concepts are already deployed.
Resource Description Framework (RDF): In Chapter 3, we describe
RDF: the graph-structured data model proposed by the Semantic Web
community as a common data model for the Web.
RDF Schema (RDFS) and Semantics: InChapter4,wedescribeRDFS:
a lightweight ontology language used to define an initial semantics for
terms used in RDF graphs.
Web Ontology Language (OWL): In Chapter 5, we describe OWL: a
more expressive ontology language built upon RDFS that offers much
more powerful ontological features.
SPARQL Query Language: In Chapter 6, we describe a language for
querying and updating RDF graphs, with examples of the features it
supports, and a detailed definition of its semantics.
Shape Constraints and Expressions (SHACL/ShEx): Chapter 7 in-
troduces two languages for describing the expected structure of – and
expressing constraints on – RDF graphs for the purposes of validation.
Linked Data: In Chapter 8, we discuss the principles and best practices
proposed by the Linked Data community for publishing interlinked
(RDF) data on the Web, and how those techniques have been adopted.
Conclusions InChapter9,wewrap-upwithopenproblemsandmoregen-
eral discussion on the future of the Web of Data.
A website associated with the book – http://webofdatabook.org/ – con-
tains complementary material, including solutions for exercises, slides for
classes, raw data for examples, a comments section, and more besides.
Santiago, Chile; April 2020 Aidan Hogan
Acknowledgements
First, I would like to thank colleagues and students past and present, who
have introduced me to this topic, worked with me on this topic, or whom I
have introduced to this topic. I have learnt a lot from all of you.
IwouldliketothankThanhTranandGongChengwhowereinstrumental
in making this book happen.
I would like to thank Ralf Gerstner of Springer for his guidance, support,
and immense patience during the lengthy preparation of this book.
I would like to thank the observant and patient reviewer whose detailed
feedback helped to improve this manuscript.
Iwouldliketothankfamilyandfriendsforremindingmeonoccasionthat
there was perhaps more to life than this book and its topic.
Finally, thanks to Analí for supporting me throughout this journey, and
doing all of those important things, both big and small, that needed to be
done so that I could keep my head buried in this endeavour.
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Contents
1 Introduction.............................................. 1
1.1 The Latent Web........................................ 3
1.2 The Current Web ...................................... 6
1.2.1 Hypertext Markup Language (HTML) .............. 7
1.2.2 Interpreting HTML Content ....................... 9
2 Web of Data.............................................. 15
2.1 Overview.............................................. 16
2.2 Web of Data: Concepts.................................. 16
2.2.1 Data............................................ 16
2.2.2 Queries ......................................... 21
2.2.3 Semantics ....................................... 25
2.2.4 Constraints...................................... 35
2.2.5 Links ........................................... 37
2.3 The Current Web of Data ............................... 40
2.3.1 Wikidata........................................ 41
2.3.2 Knowledge Graphs ............................... 45
2.3.3 Schema.org and the Open Graph Protocol ........... 48
2.3.4 Linking Open Data ............................... 53
2.4 Summary.............................................. 56
2.5 Discussion ............................................. 57
3 Resource Description Framework ......................... 59
3.1 Overview.............................................. 60
3.2 Terms................................................. 61
3.2.1 Internationalised Resource Identifiers (IRIs) ......... 61
3.2.2 Literals ......................................... 63
3.2.3 Blank Nodes..................................... 69
3.2.4 Defining RDF Terms.............................. 70
3.3 Triples ................................................ 71
3.4 Graphs................................................ 74
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