Semantic Web

The Semantic Web is an evolution of the current web that seeks to provide granular access to the underlying data that fuels the World Wide Web. It's a manifestation of W3C director Tim Berners-Lee's vision of the Web as a universal medium for data, information, and knowledge exchange.

At its core the Semantic Web consists of a data model called 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) that facilitate formal description of concepts, terms, and relationships within a given domain.

The burgeoning Semantic Web comprises newly created and/or transformed web data sources endowed with computer-processable meaning (semantics).

Purpose

Humans are capable of using the Web to carry out tasks such as finding the Finnish word for "car", to reserve a library book, or to search for the cheapest DVD and buy it. 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 web pages that are understandable by computers, so that they can search websites and perform actions in a standardized way.

For example, a computer might be instructed to list the prices of flat screen HDTVs larger than 40 inches with 1080p resolution at shops in the nearest town that are open until 8pm on Tuesday evenings. To do this today requires search engines that are individually tailored to every website being searched. A semantic web would provide a single standard for all websites to publish the relevant information.

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

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 (Demo).

Tim Berners-Lee has further stated^[1]:

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, A 'more revolutionary' Web

Relationship to the Hypertext Web

Markup

Currently, the World Wide Web is based primarily 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.

HTML, as it is generally deployed, has limited ability to classify the blocks of text on a page, apart from the roles they play in a typical document's organization and in the desired visual layout.

For example, 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 unambiguously assert that, say, 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.

Descriptive, and extensible

The Semantic Web addresses this shortcoming, using the descriptive technologies Resource Description Framework (RDF) and Web Ontology Language (OWL), and the data-centric, customizable Extensible Markup Language (XML). These technologies are combined in order to provide descriptions that supplement or replace the content of Web documents. Thus, content may manifest as descriptive data stored in Web-accessible databases, or as markup within documents (particularly, in Extensible HTML (XHTML) interspersed with XML, or, more often, purely in XML, with layout/rendering cues stored separately). The machine-readable descriptions enable content managers to add meaning to the content, thereby facilitating automated information gathering and research by computers.

Components

XML, XML Schema, RDF, OWL, SWRL

The Semantic Web comprises the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL. The OWL Web Ontology Language Overview describes the function and relationship of each of these components of the Semantic Web:

XML provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.
XML Schema is a language for restricting the structure and content elements of XML documents.
RDF is a simple data model for referring to objects ("resources") and how they are related. An RDF-based model can be represented in XML syntax.
RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-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.
SWRL is a proposal for a Semantic Web rules-language, combining sublanguages of the OWL (OWL DL and Lite) with those of the Rule Markup Language.

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

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.)
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 metadata
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).

RDF - URI, XML, namespaces

The primary facilitators of this technology are URIs (which identify resources) along with XML and namespaces. These, together with a bit of logic, form RDF, which can be used to say anything about anything. As well as RDF, many other technologies such as Topic Maps and pre-web artificial intelligence technologies are likely to contribute to the Semantic Web.

FOAF

A popular application of the Semantic Web is Friend of a Friend (or FoaF), which describes relationships among people and other agents in terms of RDF.

Semantic Web Browsers and Interconnection

An implementation of a Semantic Web Browser is the BigBlogZoo. Over 60,000 xml feeds have been categorised using the DMOZ schema and can be spidered. It is free. The commercial version, MediaMiner, allows you to mine these feeds for information.

The SIOC Project - Semantically-Interlinked Online Communities provides methods for interconnecting discussion methods such as blogs, forums and mailing lists to each other.

The Semantic Web Ping Service is a repository indexing updates of RDF, DOAP, FOAF, SIOC documents.

Piggy Bank

Another freely downloadable tool is the new plug-in to Firefox, Piggy Bank. Piggy Bank works by extracting or translating web scripts into RDF information and storing this information on the user’s computer. This information can then be retrieved independently of the original context and used in other contexts, for example by using Google Maps to display information. Piggy Bank works with a new service, Semantic Bank, which combines the idea of tagging information with the new web languages. Piggy Bank was developed by the Simile Project, which also provides RDFizers, tools that can be used to translate specific types of information, for example weather reports for US zip codes, into RDF. Efforts like these could ease a potentially troublesome transition between the web of today and its semantic successor.

Weak AI

A Semantic Web is a type of weak AI. The concept of machine-understandable documents does not imply an artificial intelligence which allows machines to comprehend human-defined concepts. But it indicates a machine's ability to solve a well-defined problem by performing well-defined operations on existing well-defined data. In this respect, the Semantic Web is like a large-scale expert system.

Even though it is simple to define, RDF at the level with the power of a semantic web will be a complete language, capable of expressing paradoxes and tautologies, and in which it will be possible to phrase questions whose answers would to a machine require a search of the entire web and an unimaginable amount of time to resolve. Each mechanical RDF application will use a schema to restrict its use of RDF to a deliberately limited language. However, when links are made between the RDF webs, the result will be an expression of a huge amount of information. It is clear that because the Semantic Web must be able to include all kinds of data to represent the world, the language itself must be completely expressive.

The semantic web is the next generation web containing action-able information i.e. information derived from data through a semantic theory so that it can be processed directly and indirectly by machines.

To classify the data from multiple domains based on its properties and its relationship with other data, we need to use descriptive technologies e.g. RDF, RDFS, OWL, XML (these are officially recommended by W3C) to add meaning to the contents of web documents to facilitate automated information gathering and research by computers.

RDF is an XML-based standard for describing resources that exist on the web. Resources on the web are identified by URIs, which uses a global naming convention. RDF statements describe a resource, the resource’s properties, and the values of those properties. RDF statements are often referred to as “triples” that consist of a subject, predicate, and object, which correspond to a resource (subject) a property (predicate), and a property value (object).

RDFS is used to create vocabularies that describe groups of related RDF resources and the relationships between those resources.

OWL defines the types of relationships that can be expressed in RDF using an XML vocabulary to indicate the hierarchies and relationships between different resources.

Criticism

Practical feasibilty

Some critics question the basic feasibility of a complete or even partial fulfillment of the semantic web. Some approach the critique from the perspective of human behavior and personal preferences, which ostensibly diminish the likelihood of its fulfillment (see e.g., metacrap). Other commentators suggest there are limitations that stem from the current state of software engineering itself. (see e.g., Leaky abstraction).

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 a lot 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 (software) files and Geolocation 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 as there would need to be two formats for one piece of data. One format would need to be specialized for human viewing and the other would have to be specialized for machines. With this being the case, it would be much less likely for companies to adopt these practices as it would only slow down their progress. 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.

Client-server

Another criticism suggests that the Semantic Web is based on a traditional web architecture. URIs within RDF are often HTTP URIs, which are based on the social system of domain name ownership. The persitence of the identifiers rests on the custodianship of their owning organization. Peer to peer architectures such as the academic DBin project and the commercial Information Commons investigate the use of peer to peer protocols for distributing RDF data.

This criticism of the semantic web would also apply to the hypertext web, which has already scaled to vast number of documents. This scaling is accomplished partly because the actual architecture used to look up HTTP identifiers is much richer than simple client-server architecture, including cacheing, proxies, and systems like that of Akamai.

External links

References

Michael C. Daconta, Leo J. Obrst, Kevin T. Smith (30 May 2003). The Semantic Web: A Guide to the Future of XML, Web Services, and Knowledge Management. John Wiley & Sons. ISBN 0-471-43257-1. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
Dieter Fensel, Wolfgang Wahlster, Henry Lieberman, James Hendler (15 November 2002). Spinning the Semantic Web: Bringing the World Wide Web to Its Full Potential. MIT Press. ISBN 0-262-06232-1. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
Steffen Staab, Rudi Studer (2004). Handbook on Ontologies. Heidelberg: Springer Verlag. ISBN 3-540-40834-7. {{cite book}}: Unknown parameter |month= ignored (help)
Vladimir Geroimenko, Chaomei Chen (Eds.), ed. (17 January 2003). Visualizing the Semantic Web. Springer Verlag. ISBN 1-85233-576-9. {{cite book}}: Check date values in: |date= (help)
John Davies, Dieter Fensel, Frank van Harmelen (21 January 2003). Towards the Semantic Web: Ontology-Driven Knowledge Management. John Wiley & Sons. ISBN 0-470-84867-7. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
Grigoris Antoniou, Frank van Harmelen (1 April 2004). A Semantic Web Primer. The MIT Press. ISBN 0-262-01210-3. {{cite book}}: Check date values in: |date= (help)
Jeffrey T. Pollock, Ralph Hodgson (21 July 2004). Adaptive Information: Improving Business Through Semantic Interoperability, Grid Computing, and Enterprise Integration. ISBN 0-471-48854-2. {{cite book}}: Check date values in: |date= (help)

Christopher Walton (12 October 2006). Agency and the Semantic Web. Oxford University Press. ISBN 978-0-19-929248-6. {{cite book}}: Check date values in: |date= (help)

^ Victoria Shannon (2006-06-26). "A 'more revolutionary' Web". International Herald Tribune. Retrieved 2006-05-24.

[1] Victoria Shannon (2006-06-26). "A 'more revolutionary' Web". International Herald Tribune. Retrieved 2006-05-24.

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