The Semantic Web—the phrase brings an outburst of enthusiasm from some, while putting a frown on the faces of others who suggest that it means little, if anything. Opinions vary wildly: The Semantic Web will give you software agents that will tackle your daily needs quietly, effectively, even with common sense, finding the information they need and negotiating with other agents on your behalf. Or, the Semantic Web will be a little more of what we have now—a little faster, a little smarter, with more complex software. Or, the Semantic Web is all hype. It all depends on whom you talk to.
This book claims that there is a degree of overlap between the views of various proponents: a commonality that goes beyond hype. If anything like these visions comes to pass, certain developments, infrastructure, and technology will be needed. Some are here now, some are almost here, some have barely begun. We’ll look at the notion of the Semantic Web, what it might be, what it might do for people, and some of the base technologies that would support it. This book intends to bring you two kinds of understanding. First, you’ll get a look at a number of these key ideas and how they might work together. This involves a degree of judicious guessing, since we can’t know for sure which of them will turn out to be the most important. Second, the book will give you some background about the technical issues—enough to help you get started, should you be interested in going further. It can’t go deeply into any one subject, for the work would expand like an accordion to an unwieldy size. Also, some of the areas are too undeveloped to reduce to standard practice.
The book concentrates on concepts and design rather than straight programming and code, but it includes markup examples to illustrate ideas and some basics of the technologies. It attempts to bring simplicity and clarity to the ideas, even though there is an inherent complexity that can’t be totally disguised. To support these goals, the book uses non-mainstream techniques to enhance clarity. They include placing mind maps at the start of chapters and using a simplified format instead of the standard RDF/XML format for some RDF and OWL examples.
Think of this book as a field guide. In a field guide to birds, you would get a picture of their plumage and some facts about their environments, lives, and habits. To learn more, you would have to consult specialized material. So too with this work.
The first part of the book sets the stage with potential scenarios for the Semantic Web and highlights important technological features of the World Wide Web as we know it. It also shows the great range of ideas people have about what the Semantic Web is. The bulk of the book covers technologies and areas of interest or significance to the Semantic Web in an order that roughly represents both the current state of their development and the degree to which they will be built on by later, more complex developments.
The book starts with the representation of knowledge, represented by topic maps and RDF (chapters 1 through 3), moves to application areas like search and annotation (chapters 4 and 5), and proceeds to technologies and disciplines like logic and ontology that are being layered over the more developed technologies (chapters 6 and 7). Then the book examines web services and how they fit into the Semantic Web picture (chapter 8). Next come intelligent agents and then the difficult and complex areas of distributed trust and belief (chapters 9 and 10). The book ends with an attempt to put all the previous material into a useful perspective. How much is practical now, how much will be in the near or mid future, and what parts may be asking too much? I provide suggestions and guidelines for possible use in your upcoming projects that can assist in orienting your work more toward the Semantic Web as it starts to materialize and evolve (chapter 11).
Who will find this book useful? Essentially, anyone with a software-related interest in the future World Wide Web, from programmers to designers, requirements engineers, and managers. This book is not a programming manual, but focuses on foundations and concepts. Markup examples are included to illustrate certain key points and to give some sense of how the technologies work. Clarity, simplicity, and readability are emphasized above technical completeness.
Because Extensible Markup Language (XML) plays a prominent role in some of the important technologies such as RDF, Topic Maps, and OWL, a reading acquaintance with XML will be useful. If you’ve written a little HTML, that should be enough to read the examples. If you haven’t encountered either XML or HTML before, a basic introduction to XML would be helpful (you won’t need any of the more esoteric parts).
There is no code, in the sense of programming language fragments, in this book. However, there are a number of examples of markup language. These are typeset in a fixed width font. Lines that are too long to fit on the page have been wrapped. Whenever possible, these lines have been broken in a manner that is acceptable for the markup. Many of the markup listings have been annotated with numbered bullets that link to explanations following the listings. In a few cases, comments have been inserted into the markup at strategic locations.
Each chapter in this book starts with a mind map that captures the essential pieces of the material presented in the chapter and their interrelations. For example, here’s the mind map from chapter 3.
Mind-mapping is an informal way to capture ideas and associations between them. Tony Buzan invented the technique; his book (The Mind Map Book: How to Use Radiant Thinking to Maximize Your Brain’s Untapped Potential) is well worth reading. Among other uses, mind maps are excellent for notes and lecture plans. They can be helpful in stimulating creativity. They aren’t well known, though, so they merit a few words of explanation.
Mind maps have a central focal topic and branches on which ride keywords. The branching pattern connects associated concepts, thoughts, ideas, or facts—whatever is important or notable to the map’s creator. It’s also possible to use color, graphic images, cross links, and other features to enhance your mind maps and to make them more effective. The branches of a mind map aren’t usually ordered, although you can order them if you like.
Purchase of Explorer’s Guide to the Semantic Web includes free access to a private web forum run by Manning Publications where you can make comments about the book, ask technical questions, and receive help from the author and from other users. To access the forum and subscribe to it, point your web browser to www.manning.com/passin. This page provides information on how to get on the forum once you are registered, what kind of help is available, and the rules of conduct on the forum.
Manning’s commitment to our readers is to provide a venue where a meaningful dialog between individual readers and between readers and the authors can take place. It is not a commitment to any specific amount of participation on the part of the author, whose contribution to the AO remains voluntary (and unpaid). We suggest you try asking the author some challenging questions lest his interest stray! The Author Online forum and the archives of previous discussions will be accessible from the publisher’s web site as long as the book is in print.
Thomas Passin is Principal Systems Engineer with Mitretek Systems, a non-profit systems and information engineering company. He has been involved in data modeling and a variety of XML-related projects. He has created several complex database-backed web sites and has also been engaged in a range of conceptual modeling approaches and graphical modeling technologies. He was a key member of a team that developed several demonstration XML-based web service applications and he worked on creating XML versions of draft message standards originally written in ASN.1.
He graduated with a B.S. in physics from the Massachusetts Institute of Technology, then studied graduate-level physics at the University of Chicago. He became involved with XML-related work in 1998 and with Topic Maps in 1999 and developed the open-source TM4JScript Javascript topic map engine. Thomas Passin is also coauthor of the book Signal Processing in C. He lives in Reston, Virginia.
The figure on the cover of Explorer’s Guide to the Semantic Web is a “Muger Arabe del desierto de Zara,” an Arab woman who lives in the Sahara Desert. At 3.5 million square miles, the Sahara Desert in northern Africa is the world’s largest desert. In spite of the inhospitable climate, it is home to over two million people. The illustration is taken from a Spanish compendium of regional dress customs first published in Madrid in 1799. The book’s title page states:
Coleccion general de los Trages que usan actualmente todas las Nacionas del Mundo desubierto, dibujados y grabados con la mayor exactitud por R.M.V.A.R. Obra muy util y en special para los que tienen la del viajero universal
which we translate, as literally as possible, thus:
General collection of costumes currently used in the nations of the known world, designed and printed with great exactitude by R.M.V.A.R. This work is very useful especially for those who hold themselves to be universal travelers
Although nothing is known of the designers, engravers, and workers who colored this illustration by hand, the “exactitude” of their execution is evident in this drawing. The Muger Arabe del desierto de Zara is just one of many figures in this colorful collection. Their diversity speaks vividly of the uniqueness and individuality of the world’s towns and regions just 200 years ago. This was a time when the dress codes of two regions separated by a few dozen miles identified people uniquely as belonging to one or the other. The collection brings to life a sense of isolation and distance of that period—and of every other historic period except our own hyperkinetic present.
Dress codes have changed since then and the diversity by region, so rich at the time, has faded away. It is now often hard to tell the inhabitant of one continent from another. Perhaps, trying to view it optimistically, we have traded a cultural and visual diversity for a more varied personal life. Or a more varied and interesting intellectual and technical life.
We at Manning celebrate the inventiveness, the initiative, and, yes, the fun of the computer business with book covers based on the rich diversity of regional life of two centuries ago, brought back to life by the pictures from this collection.