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W3C's Semantic Web logo

The Semantic Web is an evolving development of the World Wide Web in which the meaning (semantics) of information and services on the web is defined, making it possible for the web to "understand" and satisfy the requests of people and machines to use the web content.[1][2] It derives from World Wide Web Consortium director Sir Tim Berners-Lee's vision of the Web as a universal medium for data, information, and knowledge exchange.[3]

At its core, the semantic web comprises a set of design principles,[4] collaborative working groups, and a variety of enabling technologies. Some elements of the semantic web are expressed as prospective future possibilities that are yet to be implemented or realized.[2] Other elements of the semantic web are expressed in formal specifications.[5] Some of these include Resource Description Framework (RDF), a variety of data interchange formats (e.g. RDF/XML, N3, Turtle, N-Triples), and notations such as RDF Schema (RDFS) and the Web Ontology Language (OWL), all of which are intended to provide a formal description of concepts, terms, and relationships within a given knowledge domain.

Contents

Purpose

Humans are capable of using the Web to carry out tasks such as finding the Finnish word for "monkey", reserving a library book, and searching for a low price for a DVD. However, a computer cannot accomplish the same tasks without human direction because web pages are designed to be read by people, not machines. The semantic web is a vision of information that is understandable by computers, so that they can perform more of the tedious work involved in finding, combining, and acting upon information on the web.

Tim Berners-Lee originally expressed the vision of the semantic web as follows:[6]

I have a dream for the Web [in which computers] become capable of analyzing all the data on the Web – the content, links, and transactions between people and computers. A ‘Semantic Web’, which should make this possible, has yet to emerge, but when it does, the day-to-day mechanisms of trade, bureaucracy and our daily lives will be handled by machines talking to machines. The ‘intelligent agents’ people have touted for ages will finally materialize.

Tim Berners-Lee, 1999

Semantic publishing will benefit greatly from the semantic web. In particular, the semantic web is expected to revolutionize scientific publishing, such as real-time publishing and sharing of experimental data on the Internet. This simple but radical idea is now being explored by W3C HCLS group's Scientific Publishing Task Force.

Semantic Web application areas are experiencing intensified interest due to the rapid growth in the use of the Web, together with the innovation and renovation of information content technologies. The Semantic Web is regarded as an integrator across different content and information applications and systems, and provide mechanisms for the realisation of Enterprise Information Systems. The rapidity of the growth experienced provides the impetus for researchers to focus on the creation and dissemination of innovative Semantic Web technologies, where the envisaged ’Semantic Web’ is long overdue. Often the terms ’Semantics’, ’metadata’, ’ontologies’ and ’Semantic Web’ are used inconsistently. In particular, these terms are used as everyday terminology by researchers and practitioners, spanning a vast landscape of different fields, technologies, concepts and application areas. Furthermore, there is confusion with regards to the current status of the enabling technologies envisioned to realise the Semantic Web. In a paper presented by Gerber, Barnard and Van der Merwe [7] the Semantic Web landscape are charted and a brief summary of related terms and enabling technologies are presented. The architectural model proposed by Tim Berners-Lee is used as basis to present a status model that reflects current and emerging technologies [8].

Web 3.0

Tim Berners-Lee has described the semantic web as a component of 'Web 3.0'.[9]

People keep asking what Web 3.0 is. I think maybe when you've got an overlay of scalable vector graphics - everything rippling and folding and looking misty — on Web 2.0 and access to a semantic Web integrated across a huge space of data, you'll have access to an unbelievable data resource."

Tim Berners-Lee, 2006

Relationship to the hypertext web

Limitations of HTML

Many files on a typical computer can be loosely divided into documents and data. Documents like mail messages, reports, and brochures are read by humans. Data, like calendars, addressbooks, playlists, and spreadsheets are presented using an application program which lets them be viewed, searched and combined in many ways.

Currently, the World Wide Web is based mainly on documents written in Hypertext Markup Language (HTML), a markup convention that is used for coding a body of text interspersed with multimedia objects such as images and interactive forms. Metadata tags, for example

<meta name="keywords" content="computing, computer studies, computer">
<meta name="description" content="Cheap widgets for sale">
<meta name="author" content="John Doe">

provide a method by which computers can categorise the content of web pages.

With HTML and a tool to render it (perhaps web browser software, perhaps another user agent), one can create and present a page that lists items for sale. The HTML of this catalog page can make simple, document-level assertions such as "this document's title is 'Widget Superstore'", but there is no capability within the HTML itself to assert unambiguously that, for example, item number X586172 is an Acme Gizmo with a retail price of €199, or that it is a consumer product. Rather, HTML can only say that the span of text "X586172" is something that should be positioned near "Acme Gizmo" and "€199", etc. There is no way to say "this is a catalog" or even to establish that "Acme Gizmo" is a kind of title or that "€199" is a price. There is also no way to express that these pieces of information are bound together in describing a discrete item, distinct from other items perhaps listed on the page.

Semantic HTML refers to the traditional HTML practice of markup following intention, rather than specifying layout details directly. For example, the use of <em> denoting "emphasis" rather than <i>, which specifies italics. Layout details are left up to the browser, in combination with Cascading Style Sheets. But this practice falls short of specifying the semantics of objects such as items for sale or prices.

Microformats represent unofficial attempts to extend HTML syntax to create machine-readable semantic markup about objects such as retail stores and items for sale.

Semantic Web solutions

The Semantic Web takes the solution further. It involves publishing in languages specifically designed for data: Resource Description Framework (RDF), Web Ontology Language (OWL), and Extensible Markup Language (XML). HTML describes documents and the links between them. RDF, OWL, and XML, by contrast, can describe arbitrary things such as people, meetings, or airplane parts. Tim Berners-Lee calls the resulting network of Linked Data the Giant Global Graph, in contrast to the HTML-based World Wide Web.

These technologies are combined in order to provide descriptions that supplement or replace the content of Web documents. Thus, content may manifest itself as descriptive data stored in Web-accessible databases [10], or as markup within documents (particularly, in Extensible HTML (XHTML) interspersed with XML, or, more often, purely in XML, with layout or rendering cues stored separately). The machine-readable descriptions enable content managers to add meaning to the content, i.e., to describe the structure of the knowledge we have about that content. In this way, a machine can process knowledge itself, instead of text, using processes similar to human deductive reasoning and inference, thereby obtaining more meaningful results and helping computers to perform automated information gathering and research.

An example of a tag that would be used in a non-semantic web page:

<item>cat</item>

Encoding similar information in a semantic web page might look like this:

<item rdf:about="http://dbpedia.org/resource/Cat">Cat</item>

Relationship to object oriented programming

A number of authors highlight the similarities which the Semantic Web shares with object-oriented programming (OOP).[11][12] Both the semantic web and object-oriented programming have classes with attributes and the concept of instances or objects. Linked Data uses Dereferenceable Uniform Resource Identifiers in a manner similar to the common programming concept of pointers or "object identifiers" in OOP. Dereferenceable URIs can thus be used to access "data by reference". The Unified Modeling Language is designed to communicate about object-oriented systems, and can thus be used for both object-oriented programming and semantic web development.

When the web was first being created in the late 1980s and early 1990s, it was done using object-oriented programming languages[citation needed] such as Objective-C, Smalltalk and CORBA. In the mid-1990s this development practice was furthered with the announcement of the Enterprise Objects Framework, Portable Distributed Objects and WebObjects all by NeXT, in addition to the Component Object Model released by Microsoft. XML was then released in 1998, and RDF a year after in 1999.

Similarity to object oriented programming also came from two other routes: the first was the development of the very knowledge-centric "Hyperdocument" systems by Douglas Engelbart[13], and the second comes from the usage and development of the Hypertext Transfer Protocol.[14]

Skeptical reactions

Practical feasibility

Critics (e.g. Which Semantic Web?) question the basic feasibility of a complete or even partial fulfillment of the semantic web. Cory Doctorow's critique ("metacrap") is from the perspective of human behavior and personal preferences. For example, people lie: they may include spurious metadata into Web pages in an attempt to mislead Semantic Web engines that naively assume the metadata's veracity. This phenomenon was well-known with metatags that fooled the AltaVista ranking algorithm into elevating the ranking of certain Web pages: the Google indexing engine specifically looks for such attempts at manipulation. Peter Gärdenfors and Timo Honkela point out that logic-based semantic web technologies cover only a fraction of the relevant phenomena related to semantics [15] [16].

Where semantic web technologies have found a greater degree of practical adoption, it has tended to be among core specialized communities and organizations for intra-company projects.[17] The practical constraints toward adoption have appeared less challenging where domain and scope is more limited than that of the general public and the World-Wide Web.[17]

The potential of an idea in fast progress

The original 2001 Scientific American article by Berners-Lee described an expected evolution of the existing Web to a Semantic Web.[18] A complete evolution as described by Berners-Lee has yet to occur. In 2006, Berners-Lee and colleagues stated that: "This simple idea, however, remains largely unrealized."[19] While the idea is still in the making, it seems to evolve quickly and inspire many. Between 2007-2010 several scholars have already explored first applications and the social potential of the semantic web in the business and health sectors, and for social networking [20] and even for the broader evolution of democracy, specifically, how a society forms its common will in a democratic manner through a semantic web [21]

Censorship and privacy

Enthusiasm about the semantic web could be tempered by concerns regarding censorship and privacy. For instance, text-analyzing techniques can now be easily bypassed by using other words, metaphors for instance, or by using images in place of words. An advanced implementation of the semantic web would make it much easier for governments to control the viewing and creation of online information, as this information would be much easier for an automated content-blocking machine to understand. In addition, the issue has also been raised that, with the use of FOAF files and geo location meta-data, there would be very little anonymity associated with the authorship of articles on things such as a personal blog.

Doubling output formats

Another criticism of the semantic web is that it would be much more time-consuming to create and publish content because there would need to be two formats for one piece of data: one for human viewing and one for machines. However, many web applications in development are addressing this issue by creating a machine-readable format upon the publishing of data or the request of a machine for such data. The development of microformats has been one reaction to this kind of criticism.

Specifications such as eRDF and RDFa allow arbitrary RDF data to be embedded in HTML pages. The GRDDL (Gleaning Resource Descriptions from Dialects of Language) mechanism allows existing material (including microformats) to be automatically interpreted as RDF, so publishers only need to use a single format, such as HTML.

Need

The idea of a 'semantic web' necessarily coming from some marking code other than simple HTML is built on the assumption that it is not possible for a machine to appropriately interpret code based on nothing but the order relationships of letters and words. If this is not true, then it may be possible to build a 'semantic web' on HTML alone, making a specially built 'semantic web' coding system unnecessary.

There are latent dynamic network models that can, under certain conditions, be 'trained' to appropriately 'learn' meaning based on order data, in the process 'learning' relationships with order (a kind of rudimentary working grammar). See for example latent semantic analysis

Components

The semantic web comprises the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL that are organized in the Semantic Web Stack. The OWL Web Ontology Language Overview describes the function and relationship of each of these components of the semantic web:

  • XML provides an elemental syntax for content structure within documents, yet associates no semantics with the meaning of the content contained within.
  • XML Schema is a language for providing and restricting the structure and content of elements contained within XML documents.
  • RDF is a simple language for expressing data models, which refer to objects ("resources") and their relationships. An RDF-based model can be represented in XML syntax.
  • RDF Schema is a vocabulary for describing properties and classes of RDF-based resources, with semantics for generalized-hierarchies of such properties and classes.
  • OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.
  • SPARQL is a protocol and query language for semantic web data sources.

Current ongoing standardizations include:

Not yet fully realized layers include:

  • Unifying Logic and Proof layers are undergoing active research.

The intent is to enhance the usability and usefulness of the Web and its interconnected resources through:

  • Servers which expose existing data systems using the RDF and SPARQL standards. Many converters to RDF exist from different applications. Relational databases are an important source. The semantic web server attaches to the existing system without affecting its operation.
  • Documents "marked up" with semantic information (an extension of the HTML <meta> tags used in today's Web pages to supply information for Web search engines using web crawlers). This could be machine-understandable information about the human-understandable content of the document (such as the creator, title, description, etc., of the document) or it could be purely metadata representing a set of facts (such as resources and services elsewhere in the site). (Note that anything that can be identified with a Uniform Resource Identifier (URI) can be described, so the semantic web can reason about animals, people, places, ideas, etc.) Semantic markup is often generated automatically, rather than manually.
  • Common metadata vocabularies (ontologies) and maps between vocabularies that allow document creators to know how to mark up their documents so that agents can use the information in the supplied metadata (so that Author in the sense of 'the Author of the page' won't be confused with Author in the sense of a book that is the subject of a book review).
  • Automated agents to perform tasks for users of the semantic web using this data
  • Web-based services (often with agents of their own) to supply information specifically to agents (for example, a Trust service that an agent could ask if some online store has a history of poor service or spamming)

Challenges

Some of the challenges for the Semantic Web include vastness, vagueness, uncertainty, inconsistency and deceit. Automated reasoning systems will have to deal with all of these issues in order to deliver on the promise of the Semantic Web.

  • Vastness: The World Wide Web contains at least 48 billion pages as of this writing (August 2, 2009). The SNOMED CT medical terminology ontology contains 370,000 class names, and existing technology has not yet been able to eliminate all semantically duplicated terms. Any automated reasoning system will have to deal with truly huge inputs.
  • Vagueness: These are imprecise concepts like "young" or "tall". This arises from the vagueness of user queries, of concepts represented by content providers, of matching query terms to provider terms and of trying to combine different knowledge bases with overlapping but subtly different concepts. Fuzzy logic is the most common technique for dealing with vagueness.
  • Uncertainty: These are precise concepts with uncertain values. For example, a patient might present a set of symptoms which correspond to a number of different distinct diagnoses each with a different probability. Probabilistic reasoning techniques are generally employed to address uncertainty.
  • Deceit: This is when the producer of the information is intentionally misleading the consumer of the information. Cryptography techniques are currently utilized to alleviate this threat.

This list of challenges is illustrative rather than exhaustive, and it focuses on the challenges to the "unifying logic" and "proof" layers of the Semantic Web. The World Wide Web Consortium (W3C) Incubator Group for Uncertainty Reasoning for the World Wide Web (URW3-XG) final report lumps these problems together under the single heading of "uncertainty". Many of the techniques mentioned here will require extensions to the Web Ontology Language (OWL) for example to annotate conditional probabilities. This is an area of active research.[22]

Projects

This section provides some example projects and tools, but is very incomplete. The choice of projects is somewhat arbitrary but may serve illustrative purposes. It is also remarkable that in this early stage of the development of semantic web technology, it is already possible to compile a list of hundreds of components that in one way or another can be used in building or extending semantic webs.[23]

DBpedia

DBpedia is an effort to publish structured data extracted from Wikipedia: the data is published in RDF and made available on the Web for use under the GNU Free Documentation License, thus allowing Semantic Web agents to provide inferencing and advanced querying over the Wikipedia-derived dataset and facilitating interlinking, re-use and extension in other data-sources.

FOAF

A popular application of the semantic web is Friend of a Friend (or FoaF), which uses RDF to describe the relationships people have to other people and the "things" around them. FOAF permits intelligent agents to make sense of the thousands of connections people have with each other, their jobs and the items important to their lives; connections that may or may not be enumerated in searches using traditional web search engines. Because the connections are so vast in number, human interpretation of the information may not be the best way of analyzing them.

FOAF is an example of how the Semantic Web attempts to make use of the relationships within a social context.

GoodRelations for e-commerce

A huge potential for Semantic Web technologies lies in adding data structure and typed links to the vast amount of offer data, product model features, and tendering / request for quotation data.

The GoodRelations ontology is a popular vocabulary for expressing product information, prices, payment options, etc. It also allows expressing demand in a straightforward fashion.

GoodRelations has been adopted by BestBuy, Yahoo, OpenLink Software, O'Reilly Media, the Book Mashup, and many others.

SIOC

The SIOC Project - Semantically-Interlinked Online Communities provides a vocabulary of terms and relationships that model web data spaces. Examples of such data spaces include, among others: discussion forums, weblogs, blogrolls / feed subscriptions, mailing lists, shared bookmarks, image galleries.

SIMILE

Semantic Interoperability of Metadata and Information in unLike Environments

SIMILE is a joint project, conducted by the MIT Libraries and MIT CSAIL, which seeks to enhance interoperability among digital assets, schemata/vocabularies/ontologies, meta data, and services.

NextBio

A database consolidating high-throughput life sciences experimental data tagged and connected via biomedical ontologies. Nextbio is accessible via a search engine interface. Researchers can contribute their findings for incorporation to the database. The database currently supports gene or protein expression data and is steadily expanding to support other biological data types.

Linking Open Data

Datasets in the Linking Open Data project, as of Sept 2008

Class linkages within the Linking Open Data datasets

The Linking Open Data project is a W3C-led effort to create openly accessible, and interlinked, RDF Data on the Web. The data in question takes the form of RDF Data Sets drawn from a broad collection of data sources. There is a focus on the Linked Data style of publishing RDF on the Web.

OpenPSI

OpenPSI the (OpenPSI project) is a community effort to create UK government linked data service that supports research. It is a collaboration between the University of Southampton and the UK government, lead by OPSI at the National Archive and is supported by JISC funding.

Erfgoedplus.be

Erfgoedplus.be ('heritage-plus') is a project aimed at disclosing all types of heritage from the provinces of Limburg and Vlaams-Brabant and the city of Leuven to the public by applying semantic web technology. Erfgoedplus.be uses RDF/XML, OWL and SKOS to describe relationships to heritage types, concepts, objects, people, place and time. Data are normalized and enriched by means of thesauri (AAT) and an ontology (CIDOC CRM), available for input, conversion and navigation.

Erfgoedplus.be is a regional aggregator for EuropeanaLocal (Europeana) and an example of how semantic web technology is applied within the heterogeneous context of heritage.

See also

References

  1. ^ Berners-Lee, Tim; James Hendler and Ora Lassila (May 17, 2001). "The Semantic Web". Scientific American Magazine. http://www.sciam.com/article.cfm?id=the-semantic-web&print=true. Retrieved March 26, 2008. 
  2. ^ a b "W3C Semantic Web Frequently Asked Questions". W3C. http://www.w3.org/2001/sw/SW-FAQ. Retrieved March 13, 2008. 
  3. ^ Herman, Ivan (March 7, 2008). "Semantic Web Activity Statement". W3C. http://www.w3.org/2001/sw/Activity.html. Retrieved March 13, 2008. 
  4. ^ "Design Issues". W3C. http://www.w3.org/DesignIssues/. Retrieved March 13, 2008. 
  5. ^ Herman, Ivan (March 12, 2008). "W3C Semantic Web Activity". W3C. http://www.w3.org/2001/sw/. Retrieved March 13, 2008. 
  6. ^ Berners-Lee, Tim; Fischetti, Mark (1999). Weaving the Web. HarperSanFrancisco. chapter 12. ISBN 9780062515872. 
  7. ^ Gerber, AJ, Barnard, A & Van der Merwe, Alta (2006), A Semantic Web Status Model, Integrated Design & Process Technology, Special Issue: IDPT 2006
  8. ^ Gerber, Aurona; Van der Merwe, Alta; Barnard, Andries; (2008), A Functional Semantic Web architecture, European Semantic Web Conference 2008, ESWC’08, Tenerife, June 2008.
  9. ^ Victoria Shannon (June 26, 2006). "A 'more revolutionary' Web". International Herald Tribune. http://www.iht.com/articles/2006/05/23/business/web.php. Retrieved May 24, 2006. 
  10. ^ Artem Chebotko and Shiyong Lu, "Querying the Semantic Web: An Efficient Approach Using Relational Databases", LAP Lambert Academic Publishing, ISBN 978-3-8383-0264-5, 2009.
  11. ^ Knublauch, Holger; Oberle, Daniel; Tetlow, Phil; Evan (March 9, 2006). "A Semantic Web Primer for Object-Oriented Software Developers". W3C. http://www.w3.org/2001/sw/BestPractices/SE/ODSD/. Retrieved July 30, 2008. 
  12. ^ Connolly, Daniel (August 13, 2002). "An Evaluation of the World Wide Web with respect to Engelbart's Requirements". W3C. http://www.w3.org/Architecture/NOTE-ioh-arch. Retrieved July 30, 2008. 
  13. ^ Engelbart, Douglas (1990). "Knowledge-Domain Interoperability and an Open Hyperdocument System". Bootstrap Institute. http://www.bootstrap.org/augdocs/augment-132082.htm. Retrieved July 30, 2008. 
  14. ^ Connolly, Dan. "From the editor... WebApps". W3C. http://www.w3.org/People/Connolly/9703-web-apps-essay.html. Retrieved July 30, 2008. 
  15. ^ Gärdenfors, Peter (2004), "How to make the Semantic Web more semantic", Formal Ontology in Information Systems: proceedings of the third international conference (FOIS-2004) (IOS Press): p. 17–34 
  16. ^ Timo Honkela, Ville Könönen, Tiina Lindh-Knuutila and Mari-Sanna Paukkeri (2008), "Simulating processes of concept formation and communication", Journal of Economic Methodology, http://www.informaworld.com/smpp/content~content=a903999101 
  17. ^ a b Ivan Herman (2007). "State of the Semantic Web". Semantic Days 2007. http://www.w3.org/2007/Talks/0424-Stavanger-IH/Slides.pdf. Retrieved July 26, 2007. 
  18. ^ Berners-Lee, Tim (May 1, 2001). "The Semantic Web". Scientific American. http://www.sciam.com/article.cfm?articleID=00048144-10D2-1C70-84A9809EC588EF21. Retrieved March 13, 2008. 
  19. ^ Nigel Shadbolt, Wendy Hall, Tim Berners-Lee (2006). "The Semantic Web Revisited". IEEE Intelligent Systems. http://eprints.ecs.soton.ac.uk/12614/1/Semantic_Web_Revisted.pdf. Retrieved April 13, 2007. 
  20. ^ Lee Feigenbaum (May 1, 2007). "The Semantic Web in Action". Scientific American. http://www.thefigtrees.net/lee/sw/sciam/semantic-web-in-action. Retrieved February 24, 2010. 
  21. ^ Martin Hilbert (April, 2009). "The Maturing Concept of E-Democracy: From E-Voting and Online Consultations to Democratic Value Out of Jumbled Online Chatter". Journal of Information Technology and Politics. http://www.informaworld.com/smpp/content~db=all~content=a911066517. Retrieved February 24, 2010. 
  22. ^ Lukasiewicz, Thomas; Umberto Straccia. "Managing uncertainty and vagueness in description logics for the Semantic Web". http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B758F-4SPSPKW-1&_user=147018&_coverDate=11%2F30%2F2008&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000012179&_version=1&_urlVersion=0&_userid=147018&md5=8123c273189b1148cadb12f95b87a5ef. 
  23. ^ See, for instance: Bergman, Michael K.. "Sweet Tools". AI3; Adaptive Information, Adaptive Innovation, Adaptive Infrastructure. http://www.mkbergman.com/?page_id=325. Retrieved January 5, 2009. 

Further reading

External links


Wikibooks

Up to date as of January 23, 2010
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From Wikibooks, the open-content textbooks collection

Contents

Introduction

The semantic web is an exciting new evolution of the World Wide Web (WWW) providing machine-readable and machine-comprehensible information far beyond current capabilities. In an age of information deluge, governments, individuals and businesses will come to rely more and more on automated services, which will improve in their capacity to assist humans by “understanding” more of the content on the web. This has potentially far-reaching consequences for all businesses today.

More information on the web needs to be structured in a form that machines can ‘understand’ and process rather than merely display. It relies solely on a machine’s ability to solve complex problems by performing well-defined operations on well-defined data. Sir Tim Berners-Lee, inventor of the World Wide Web, has coined the term “Semantic Web” to describe this approach. Berners-Lee, Hendler and Lassila provide the following definition:

“The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation.”

- Tim Berners-Lee, Ora Lassila, James Hendler (Scientific American May 2001)

What Is The Semantic Web?

The Semantic Web is a mesh of information linked up in such a way as to be easily processable by machines, on a global scale. You can think of it as being an efficient way of representing data on the World Wide Web, or as a globally linked database.

The Semantic Web was thought up by Tim Berners-Lee, inventor of the WWW, URIs, HTTP, and HTML. There is a dedicated team of people at the World Wide Web consortium (W3C) working to improve, extend and standardize the system, and many languages, publications, tools and so on have already been developed. However, Semantic Web technologies are still very much in their infancies, and although the future of the project in general appears to be bright, there seems to be little consensus about the likely direction and characteristics of the early Semantic Web.

What's the rationale for such a system? Data that is generally hidden away in HTML files is often useful in some contexts, but not in others. The problem with the majority of data on the Web that is in this form at the moment is that it is difficult to use on a large scale, because there is no global system for publishing data in such a way as it can be easily processed by anyone. For example, just think of information about local sports events, weather information, plane times, Major League Baseball statistics, and television guides... all of this information is presented by numerous sites, but all in HTML. The problem with that is that, is some contexts, it is difficult to use this data in the ways that one might want to do so.

So the Semantic Web can be seen as a huge engineering solution... but it is more than that. We will find that as it becomes easier to publish data in a repurposable form, so more people will want to publish data, and there will be a knock-on or domino effect. We may find that a large number of Semantic Web applications can be used for a variety of different tasks, increasing the modularity of applications on the Web. But enough subjective reasoning... onto how this will be accomplished.

The Semantic Web is generally built on syntaxes which use URIs to represent data, usually in triples-based structures, i.e., many triples of URI data that can be held in databases, or interchanged on the world Wide Web using a set of particular syntaxes developed especially for the task. These syntaxes are called "Resource Description Framework" syntaxes. URI - Uniform Resource Identifier

A URI is simply a Web identifier: like the strings starting with "http:" or "ftp:" that you often find on the World Wide Web. Anyone can create a URI, and the ownership of them is clearly delegated, so they form an ideal base technology with which to build a global Web on top of. In fact, the World Wide Web is such a thing: anything that has a URI is considered to be "on the Web".

The syntax of URIs is carefully governed by the IETF, who published RFC 2396 as the general URI specification. The W3C maintains a list of URI schemes.

RDF - Resource Description Framework

A triple can simply be described as three URIs. A language which utilises three URIs in such a way is called RDF: the W3C have developed an XML serialization of RDF, the "Syntax" in the RDF Model and Syntax recommendation. RDF XML is considered to be the standard interchange format for RDF on the Semantic Web, although it is not the only format. For example, Notation3 (which we shall be going through later on in this article) is an excellent plain text alternative serialization.

Once information is in RDF form, it becomes easy to process it, since RDF is a generic format, which already has many parsers. XML RDF is quite a verbose specification, and it can take some getting used to (for example, to learn XML RDF properly, you need to understand a little about XML and namespaces beforehand...), but let's take a quick look at an example of XML RDF right now:-

Why RDF?

When people are confronted with XML RDF for the first time, they usually have two questions: "why use RDF rather than XML?", and "do we use XML Schema in conjunction with RDF?".

The answer to "why use RDF rather than XML?" is quite simple, and is twofold. Firstly, the benefit that one gets from drafting a language in RDF is that the information maps directly and unambiguously to a model, a model which is decentralized, and for which there are many generic parsers already available. This means that when you have an RDF application, you know which bits of data are the semantics of the application, and which bits are just syntactic fluff. And not only do you know that, everyone knows that, often implicitly without even reading a specification because RDF is so well known. The second part of the twofold answer is that we hope that RDF data will become a part of the Semantic Web, so the benefits of drafting your data in RDF now draws parallels with drafting your information in HTML in the early days of the Web.

The answer to "do we use XML Schema in conjunction with RDF?" is almost as brief. XML Schema is a language for restricting the syntax of XML applications. RDF already has a built in BNF that sets out how the language is to be used, so on the face of it the answer is a solid "no". However, using XML Schema in conjunction with RDF may be useful for creating datatypes and so on. Therefore the answer is "possibly", with a caveat that it is not really used to control the syntax of RDF. This is a common misunderstanding, perpetuated for too long now.

Screen Scraping, and Forms

For the Semantic Web to reach its full potential, many people need to start publishing data as RDF. Where is this information going to come from? A lot of it can be derived from many data publications that exist today, using a process called "screen scraping". Screen scraping is the act of literally getting the data from a source into a more manageable form (i.e., RDF) using whatever means come to hand. Two useful tools for screen scraping are XSLT (an XML transformations language), and regular expression (in Perl, Python, and so on).

However, screen scraping is often a tedious solution, so another way to approach it is to build proper RDF systems that take input from the user and then store it straight away in RDF. Data such as you may enter when signing up for a new mail account, buying some CDs online, or searching for a used car can all be stored as RDF and then used on the Semantic Web.

The Power Of Semantic Web Languages

The main power of Semantic Web languages is that any one can create one, simply by publishing some RDF that describes a set of URIs, what they do, and how they should be used. We have already seen that RDF Schema and DAML are very powerful languages for creating languages.

Because we use URIs for each of the terms in our languages, we can publish the languages easily without fear that they might get misinterpreted or stolen, and with the knowledge that anyone in the world that has a generic RDF processor can use them. The Principle Of Least Power

The Semantic Web works on a principle of least power: the fewer rules, the better. This means that the Semantic Web is essentially very unconstraining in what it lets one say, and hence it follows that anyone can say anything about anything. When you look at what the Semantic Web is trying to do, it becomes very obvious why this level of power is necessary... if we started constraining people, they wouldn't be able to build a full range of applications, and the Semantic Web would therefore become useless to some people. How Much Is Too Much?

However, it has been pointed out that this power will surely be too much... won't people be trying to process their shopping lists on an inference engine, and suddenly come up with a plan for world peace, or some strange and exciting new symphony?

The answer is (perhaps unfortunately!) no. Although the basic parts of the Semantic Web, RDF and the concepts behind it are very minimally constraining, applications that are built on top of the Semantic Web will be designed to perform specific tasks, and as such will be very well defined.

For example, take a simple server log program. One might want to record some server logs in RDF, and then build a program that can gather statistics from the logs that pertain to the site; how many visitors it had in a week, and so forth. That doesn't mean that it'll turn your floppy disc drive into a toaster or anything; it'll just process server logs. The power that you get from publishing your information in RDF is that once published in the public domain, it can be repurposed (used for other things) so much easier. Because RDF uses URIs, it is fully decentralized: you don't have to beg for some central authority to publish a language and all your data for you... you can do it yourself. It's Do It Yourself data management.

The Pedantic Web

Unfortunately, there is an air of academia and corporate thinking lingering in the Semantic Web community, which has lead to the term "Pedantic Web" being coined, and a lot of mis/disinformation and unnecessary hype being disseminated. Note that this very document was devised to help clear up some common misconceptions that people may have about the Semantic Web.

For example, almost all beginners to RDF go through a sort of "identity crisis" phase, where they confuse people with their names, and documents with their titles. For example, it is common to see statements such as:-

<http://example.org/> dc:creator "Bob" .

However, Bob is just a literal string, so how can a literal string write a document? What the author really means is:-

<http://example.org/> dc:creator _:b . _:b foaf:name "Bob" .

i.e., that example.org was created by someone whose name is "Bob". Tips like these are being slowly collected, and some of them are being displayed in the SWTips guide, a collection of Semantic Web hints and tips maintained as a collaborative development project. Education And Outreach

The move away from the "Pedantic Web", to some extent, is all part of a movement to bring the power of the Semantic Web to the people. This is a well documented need:-

[...] the idea that the above URIs reveal a schema that somehow fully describes this language and that it is so simple (only two {count 'em 2} possible "statements"), yet looks like the recipe for flying to Mars is a bit daunting. Its very simplicity enables it to evaluate and report on just about anything - from document through language via guidelines! It is a fundamental tool for the Semantic Web in that it gives "power to the people" who can say anything about anything.

- EARL for dummies, William Loughborough, May 2001

RDF Schema and DAML+OIL are generally languages that need to be learned, however, so what is being done to accommodate people who have neither the time nor patience to read up on these things, and yet want to create Semantic Web applications? Thankfully, many Semantic Web applications will be lower end applications, so you'll no more need to have a knowledge of RDF than Amaya requires one to have a knowledge of (X)HTML. Trust and Proof

The next step in the architecture of the Semantic Web is trust and proof. Very little is written about this layer, which is a shame since it will become very important in the future.

In stark reality, the simplest way to put it is: if one person says that x is blue, and another says that x is not blue, doesn't the whole Semantic Web fall apart?

The answer is of course not, because a) applications on the Semantic Web at the moment generally depend upon context, and b) because applications in the future will generally contain proof checking mechanisms, and digital signatures. Context

Applications on the Semantic Web will depend on context generally to let people know whether or not they trust the data. If I get an RDF feed from a friend about some movies that he's seen, and how highly he rates them, I know that I trust that information. Moreover, I can then use that information and safely trust that it came from him, and then leave it down to my own judgement just to how much I trust his critiques of the films that he has reviewed.

Groups of people also operate on shared context. If one group is developing a Semantic Web depiction service, cataloguing who people are, what their names are, and where pictures of those people are, then my trust of that group is dependent upon how much I trust the people running it not to make spurious claims.

So context is a good thing because it lets us operate on local and medium scales intuitively, without having to rely on complex authentication and checking systems. However, what happens when there is a party that we know, but we don't know how to verify that a certain heap of RDF data came from them? That's where digital signatures come in.


In general, there are small- and large-scale systems, and interactions between the two will most likely form a huge part of the transactions that occur on the Semantic Web. Let's define what we mean by large-, medium-, and small-scale systems.

Large Scale

An example of a large-scale system is two companies that are undergoing a merger needing to combine their databases. Another example would be search engines compiling results based upon a huge range of data. Large-scale Semantic Web systems generally involve large databases, and heavy duty inference rules and processors are required to handle the databases. Medium Scale

Medium-scale Semantic Web systems attempt to make sense out of the larger-scale Semantic Web systems, or are examples of small-scale Semantic Web systems joined together. An example of the former is a company trying to partially understand two large-scale invoice formats enough to use them together. An example of the latter is of two address book language groups trying to create a super-address book language.

Small Scale

Small-scale Semantic Web systems are less widely discussed. By small-scale Semantic Web systems, we mean languages that will be used primarily offline, or piles of data that will only be transferred with a limited scope, perhaps between friends, departments, or even two companies.

Sharing data on a local level is a very powerful example of how the Semantic Web can be useful in a myriad of situations. In the next section on evolution we shall be finding out how interactions between the different sized systems will form a key part of the Semantic Web. SEM - SEmantic Memory

The concept of a SEmantic Memory was first proposed by Seth Russell, who suggested that personal database dumps of RDF that one has collected from the "rest" of the Semantic Web (a kind of Semantic Cloud) would be imperative for maintaining a coherent view of data. For example, a SEM would most likely be partitioned into data which is inherent to the whole Semantic Web (i.e., the schemata for the major languages such as XML RDF, RDF Schema, DAML+OIL, and so on), local data which is important for any Semantic Web applications that may be running (e.g. information about the logic namespace for CWM, which is currently built in), and data that the person has personally been using, is publishing, or that has been otherwise entered into the root context of the SEM.

The internal structure of a SEM will most likely go well beyond the usual triples structure of RDF, perhaps as far as quads or even pents. The extra fields are for contexts (an StID), and perhaps sequences. In other words, they are ways of grouping information within the SEM, for easy maintenance and update. For example, it should become simple to just delete any triple that was added into a certain context by removing all triples with that particular StID.

A lot of work on the Semantic Web has concentrated on making data stores (i.e., SEMs) interoperable, which is good, but that has lead to less work being conducted on what actually happens within the SEM itself, which is not good, because the representation of quads and pents in RDF is therefore up in the air. b developers to be investigating at this stage.

Evolution

A very important concept on the Semantic Web is that of evolution: going from one system into another. Two key parts of evolvability are partial understanding and transformability. We will find out next how these manifest themselves naturally when changing the scale of a system. Partial Understanding: Large Scale to Medium Scale

The concept of partial understanding is a very important one on the Semantic Web, and can often be found in older documents that came out about the same time as the Semantic Web was first being theorized.

An example of partial understanding when moving a large-scale system to a medium-scale system is of a company trying to make sense out of two invoices, one from Company A and one from Company B. The knowledge that both of the companies use similar fields in their invoices is well known, so a company trying to make sense out of the invoices can easily compile a master list of expenditures by simply scraping the data from the two invoice languages. Neither Company A nor Company B need to know that this is going on.

Indeed, TimBL included this example in his XML 2000 keynote:-

[...] what we'll end up doing in the future is converting things, so for example [...] in the Semantic Web we will have a relationship between two languages so that if you get an invoice in a language you don't understand, and you have... some business software which can pay invoices... by following links across the Semantic Web, your machine will be able to automatically convert it from one language to another, and so process it.

- Tim Berners-Lee

Transformability: Small Scale to Medium Scale

An example of a small-scale Semantic Web system joined together to make a medium-sized Semantic Web system could be two groups that have published address book formats wanting to make a larger and better address book format by merging the two current formats together. Anyone using one of the old address book formats could probably convert them into the new format, and hence there would be a greater sense of interoperability. That's generally what happens when one goes from a small-scale Semantic Web system into a medium-scale Semantic Web system, although this is often not without some disadvantages and incompatabilities. The Semantic Web takes the sting out of it by automating 99% of the process (it can convert field A into field B, but it can't fill in any new data for you... of course, new fields can always be left empty for a while).

Facilitating Evolvability

How do we document the evolution of languages? This is a very important and indeed urgent question, and one which TimBL summarized quite neatly:-

Where for example a library of congress schema talks of an "author", and a British Library talks of a "creator", a small bit of RDF would be able to say that for any person x and any resource y, if x is the (LoC) author of y, then x is the (BL) creator of y. This is the sort of rule which solves the evolvability problems. Where would a processor find it? [...]

- Semantic Web roadmap, Tim Berners-Lee

One possible answer is: third-party databases. Very often, it is not practical to have (in TimBL's example) either the LoC or or BL record the fact that two of their fields are the same, so this information will have to be recorded by a reputable third party.

One such "third party" that was set up to investigate this is SWAG, the Semantic Web Agreement Group. Co-founded by Seth Russell, Sean B. Palmer, Aaron Swartz, and William Loughborough, the group aims to ensure interoperability on the Semantic Web. They set up what is possibly the first ever third-party Semantic Web dictionary, the WebNS SWAG Dictionary.

Intertwining: Difficult, But Important

Although the Semantic Web as a whole is still very much at a grassroots kind of level, people are starting to take notice; they're starting to publish information using RDF, and thereby making it fit for the Semantic Web.

However, not enough is being done to link information together... in other words, the "Semantic" part of the "Semantic Web" is coming along nicely, but where's the "Web"? People are not using other people's terms effectively; when they use other terms, they often do so because they're aimlessly trying to help, but just generating noise in the process. If you're going to use other people's data, try to find out what the advantage is in doing that beforehand. For example, just because you use the term "dc:title" in your RDF rather than a home brewed ":title", does that mean that suddenly a Dublin Core application is going to be able to "understand" your code? Of course not. What it does mean however is that if the "dc:title" property in your instance is being put to use in such a way that information may be need to repurposed from it in the near future, then you may gain some advantage because "dc:title" is such a commonly used term, you may be able to modify a current rules file, or whatever.

Another part of the problem may be due to a problem similar to the one that the early World Wide Web experienced: why bother publishing a Web site when there is no one else's site to link to or be linked to? Why bother publishing a Web site when so few people have browsers? Why bother writing a browser when there are so few Web sites? Some people have to make the leaps for it all to happen, and that's a slow process.

What can be done about the situation? Well, it may hopefully sort itself out. Another well-known principle that applies very well to Semantic Web applications is that there is no point in reinventing the wheel; viz., if someone has already invented a schema which contains a comprehensive and well understood and used set of terms that you also need to use in your application, then there is no point in trying to redo the work that they have done. At some points this may lead to a form of "schema war", but survival of the fittest should see to it that a core of the best schemata are put to the most use. This is probably what TimBL means when he says that terms will just "emerge" out of the Semantic Cloud, that when people keep using the term "zip", rather than just recording that my term "zip" is equivalent to your term "zip" which is equivalent to someone else's term "zip", we'll all just use the same URI, and hence interoperability will be vastly improved. Does It Work? What Semantic Web Applications Are There?

I addressed this in a previous article: The Semantic Web: Taking Form, but it does bear repeating: the Semantic Web already works, and people are using it.

Semantic Web Applications

Unfortunately, the Semantic Web is dissimilar in many ways from the World Wide Web, including that you can't just point people to a Web site for them to realise how it's working, and what it is. However, there have been a number of small-scale Semantic Web applications written up. One of the best ones is Dan Connolly's Arcs and Nodes diagrams experiment:-

One of the objectives of the advanced development component of the Semantic Web activity is to demonstrate how RDF and Semantic Web technologies can be applied to the W3C Process to increase efficiency, reliability, etc. In the early stages of developing an RDF model of the W3C process, the tools I was using to visualize the model while working on it started working well enough that I started applying them to all sorts of stuff.

- Circles and arrows diagrams using stylesheet rules, Dan Connolly.

Of course, this is a rather demonstration-oriented Semantic Web project, but it does illustrate the feasibility of applications being easily built using Semantic Web toolkits.

Another good example of the Semantic Web at work is Dan Brickley et al.'s RDFWeb. RDFWeb is a RDF database-driven hypermedia blogspace, a site where all information is stored as RDF, and then that RDF used to render XHTML. Plans are underway to incorporate more advanced Semantic Web principles into the site. What Can I Do To Help?

There are many ways in which one can contribute to creating the Semantic Web. Here's a few of them:-

   * Publish some globally useful data in RDF.
   * Write an inference engine in the language of your choice.
   * Spread the word: do some education and outreach.
   * Help in the development of RDF Schema and/or DAML.
   * Contribute in representing state in RDF, a rather neglected field of research.
   * Apply your own development backgrounds to the Semantic Web, give us all a new angle to consider it from.
   * Instead of using some proprietary system for your next application, consider making it a Semantic Web project instead.

There are many other ways in which one can help as well: ask in the community for more details. What Now? Further Reading

As of 2001-09, the amount of Semantic Web Education and Outreach materials can only really be described as "pitiful" (hence this introduction, for a start). Here's a short list of some of the good primers and materials currently available, in no particular order:-

   * http://www.w3.org/2000/10/swap/Primer (Getting Into RDF & Semantic Web Using N3)
   * http://www.w3.org/DesignIssues/Semantic (Semantic Web Roadmap)
   * http://purl.org/swag/whatIsSW (What Is The Semantic Web?)
   * http://uwimp.com/eo.htm (Semantic Web Primer)
   * http://logicerror.com/semanticWeb-long (Semantic Web Introduction - Long)
   * http://www.scientificamerican.com/2001/0501issue/0501berners-lee.html (SciAm: The Semantic Web)
   * http://www.xml.com/pub/a/2001/03/07/buildingsw.html (Building The Semantic Web)
   * http://infomesh.net/2001/06/swform/ (The Semantic Web, Taking Form)
   * http://www.w3.org/2001/sw/Activity (SW Activity Statement)
   * http://www.w3.org/2000/01/sw/ (SWAD)

For more information, all the latest news etc., Dave Beckett's Resource Description Framework (RDF) Resource Guide is absolutely brilliant.

Many Semantic Web and RDF developers hang out on the RDF IG IRC chatroom, on irc.openprojects.net, #rdfig.

Ontology and Ontology Language

More on the Semantic Web


Simple English

The Semantic Web is a project of the W3C that uses metadata to let computers understand the information on the internet. This would make computers able to do more of the work involved in finding, sharing and combining information on the internet.

The Semantic Web is an idea of the inventor of the World Wide Web, Tim Berners Lee. He wants to make the web more intuitive about how to meet a user's needs. The semantics of information and services is defined in Web Ontology Language and RDF Schemas. These are used to give a formal description of concepts, terms, and relationships within a given area of knowledge.

Tim Berners-Lee's Idea was as follows[1]:

I have a dream for the Web [in which computers] become capable of analyzing all the data on the Web – the content, links, and transactions between people and computers. A ‘Semantic Web’, which should make this possible, has yet to emerge, but when it does, the day-to-day mechanisms of trade, bureaucracy and our daily lives will be handled by machines talking to machines. The ‘intelligent agents’ people have touted for ages will finally materialize.

Markup

The World Wide Web is based on HTML documents. The semantic web involves using Resource description Framework (RDF) that has data that computers use. The layout used in HTML will be stored separately such as in a CSS file. RDF and OWL can either supplement or replace the content of web documents (XHTML).

In this way a machine can use the knowledge itself. By using the information in ways similar to human reasoning it can create more meaningful results.

References

  1. Berners-Lee, Tim; Fischetti, Mark (1999). Weaving the Web. HarperSanFrancisco. pp. chapter 12. ISBN 9780062515872. 







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