The Semantic Web, sometimes known as Web 3.0, is an extension of the World Wide Web through standards set by the World Wide Web Consortium (W3C). The goal of the Semantic Web is to make Internet data machine-readable.
To enable the encoding of semantics with the data, technologies such as Resource Description Framework (RDF) and Web Ontology Language (OWL) are used. These technologies are used to formally represent metadata. For example, ontology can describe concepts, relationships between entities, and categories of things. These embedded semantics offer significant advantages such as reasoning over data and operating with heterogeneous data sources. These standards promote common data formats and exchange protocols on the Web, fundamentally the RDF. According to the W3C, "The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries." The Semantic Web is therefore regarded as an integrator across different content and information applications and systems.
History The term was coined by Tim Berners-Lee for a web of data (or data web) that can be processed by machines—that is, one in which much of the meaning is machine-readable. While its critics have questioned its feasibility, proponents argue that applications in library and information science, industry, biology and human sciences research have already proven the validity of the original concept.
The idea of adding semantics to the Web predates the term itself. Berners-Lee discussed the need for semantics in the Web at the first International World Wide Web Conference in 1994. In 1998, he published a design document titled "Semantic Web Road map", outlining the architecture for a web of machine-processable data. The first patent for the creation of a semantic web was filed by Amit Sheth et al. on 30 October 2001.
Berners-Lee originally expressed his vision of the Semantic Web in 1999 as follows:
The 2001 Scientific American article by Berners-Lee, Hendler, and Lassila described an expected evolution of the existing Web to a Semantic Web. In 2006, Berners-Lee and colleagues stated that: "This simple idea…remains largely unrealized". In 2013, more than four million Web domains (out of roughly 250 million total) contained Semantic Web markup.
Example In the following example, the text "Paul Schuster was born in Dresden" on a website will be annotated, connecting a person with their place of birth. The following HTML fragment shows how a small graph is being described, in RDFa-syntax using a schema.org vocabulary and a Wikidata ID: <div vocab="https://schema.org/" typeof="Person"> <span property="name">Paul Schuster</span> was born in <span property="birthPlace" typeof="Place" href="https://www.wikidata.org/entity/Q1731"> <span property="name">Dresden</span>. </span> </div>
The example defines the following five triples (shown in Turtle syntax). Each triple represents one edge in the resulting graph: the first element of the triple (the subject) is the name of the node where the edge starts, the second element (the predicate) the type of the edge, and the last and third element (the object) either the name of the node where the edge ends or a literal value (e.g. a text, a number, etc.). _:a <https://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://schema.org/Person> . _:a <https://schema.org/name> "Paul Schuster" . _:a <https://schema.org/birthPlace> <https://www.wikidata.org/entity/Q1731> . <https://www.wikidata.org/entity/Q1731> <https://schema.org/itemtype> <https://schema.org/Place> . <https://www.wikidata.org/entity/Q1731> <https://schema.org/name> "Dresden" .
The triples result in the graph shown in the given figure.
One of the advantages of using Uniform Resource Identifiers (URIs) is that they can be dereferenced using the HTTP protocol. According to the so-called Linked Open Data principles, such a dereferenced URI should result in a document that offers further data about the given URI. In this example, all URIs, both for edges and nodes (e.g. , , ) can be dereferenced and will result in further RDF graphs, describing the URI, e.g. that Dresden is a city in Germany, or that a person, in the sense of that URI, can be fictional.
The second graph shows the previous example, but now enriched with a few of the triples from the documents that result from dereferencing (green edge) and (blue edges).
Additionally to the edges given in the involved documents explicitly, edges can be automatically inferred: the triple _:a <https://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://schema.org/Person> .
from the original RDFa fragment and the triple <https://schema.org/Person> <http://www.w3.org/2002/07/owl#equivalentClass> <http://xmlns.com/foaf/0.1/Person> .
from the document at (green edge in the figure) allow to infer the following triple, given OWL semantics (red dashed line in the second Figure): _:a <https://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Person> .
Background
The concept of the semantic network was used in the 1950s and 1960s by researchers such as the Richard H. Richens, cognitive scientist Allan M. Collins, computational linguist M. Ross QuillianQuillian, R. A notation for representing conceptual information: An application to semantics and mechanical English para- phrasing. SP-1395, System Development Corporation, Santa Monica, 29 October 1963. https://www.semanticscholar.org/paper/A-NOTATION-FOR-REPRESENTING-CONCEPTUAL-INFORMATION.-Quillian/0f9b084ecc74cac339bc46b3e0d3961048689564Quillian, M. Ross, Semantic Memory, pages 227-270 in Semantic Information Processing, editor Minsky, Marvin Lee, MIT Press, Cambridge, Mass., 1968 https://archive.org/details/semanticinformat00mins and psychologist Elizabeth F. Loftus to represent semantically structured knowledge, both conceptually and in computer systems.
The term "Semantic Web" was coined by Tim Berners-Lee, the inventor of the World Wide Web and director of the World Wide Web Consortium ("W3C"), which oversees the development of proposed Semantic Web standards. He defines the Semantic Web as "a web of data that can be processed directly and indirectly by machines". The Semantic Web extends the network of hyperlinked human-readable web pages by inserting machine-readable metadata about pages and how they are related to each other. This enables automated agents to access the Web more intelligently and perform more tasks on behalf of users.
Many of the technologies proposed by the W3C already existed before they were positioned under the W3C umbrella. These are used in various contexts, particularly those dealing with information that encompasses a limited and defined domain, and where sharing data is a common necessity, such as scientific research or data exchange among businesses. In addition, other technologies with similar goals have emerged, such as microformats.
Limitations of HTML Many files on a typical computer can be loosely divided into either human-readable documents, or machine-readable data. Examples of human-readable document files are mail messages, reports, and brochures. Examples of machine-readable data files are calendars, address books, playlists, and spreadsheets, which are presented to a user using an application program that lets the files be viewed, searched, and combined.
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 provide a method by which computers can categorize the content of web pages. In the examples below, the field names "keywords", "description" and "author" are assigned values such as "computing", and "cheap widgets for sale" and "John Doe".
<meta name="keywords" content="computing, computer studies, computer" /> <meta name="description" content="Cheap widgets for sale" /> <meta name="author" content="John Doe" />
Because of this metadata tagging and categorization, other computer systems that want to access and share this data can easily identify the relevant values.
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 denoting "emphasis" rather than , 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 extend HTML syntax to create machine-readable semantic markup about objects including people, organizations, events and products. Similar initiatives include RDFa, Microdata and Schema.org.
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.
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,Artem Chebotko and Shiyong Lu, "Querying the Semantic Web: An Efficient Approach Using Relational Databases", LAP Lambert Academic Publishing, , 2009. 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>blog</item>
Encoding similar information in a semantic web page might look like this: <item rdf:about="https://example.org/semantic-web/">Semantic Web</item>
Tim Berners-Lee calls the resulting network of Linked Data the Giant Global Graph, in contrast to the HTML-based World Wide Web. Berners-Lee posits that if the past was document sharing, the future is data sharing. His answer to the question of "how" provides three points of instruction. One, a URL should point to the data. Two, anyone accessing the URL should get data back. Three, relationships in the data should point to additional URLs with data.
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