27/11/2002Olga & Anna1 סמינר במערכות מידע והנדסת ידע סמסטר חורף תשס " ג אנה לויט אולגה טברובסקי

27/11/2002Olga & Anna1

סמסטר חורף תשס"גסמינר במערכות מידע והנדסת ידע

אנה לויט

אולגה טברובסקי

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XMLXML

XML is a markup language for documents containing structured information.

A markup language is a mechanism to identify structures in a document.

XML specifies neither semantics nor a tag set.

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XMLXML

XML provides a facility to define tags and the structural relationships between them.

Since there's no predefined tag set, there can't be any preconceived semantics.

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XMLXML

XML was created so that richly structured documents could be used over the web.

The only viable alternatives, HTML and SGML, are not practical for this purpose.

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XMLXML

XML is defined by a number of related specifications: Extensible Markup Language (XML) 1.0 - Defines the syntax of XML.XML Pointer Language (XPointer) and XML Linking Language (XLink) - Defines a standard way to represent links between resources. In addition to simple links, like HTML's <A> tag, XML has mechanisms for links between multiple resources and links between read-only resources. XPointer describes how to address a resource, XLink describes how to associate two or more resources.

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XMLXML

Extensible Style Language (XSL) - Defines the standard stylesheet language for XML.

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XMLXML

XML is nothing more than a way to standardize data formats.

In a way, it is just the next level of data above the character level, which has been standardized on such similarly unglamorous technologies as ASCII and Unicode.

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XML XML NamespacesNamespaces

Documents, containing multiple markup vocabularies, pose problems of recognition and collision.

Software modules need to be able to recognize the tags and attributes which they are designed to process.

These considerations require that document constructs should have universal names, whose scope extends beyond their containing document.

This specification describes a mechanism, XML namespaces, which accomplishes this.

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Resource Description Framework (RDF)Resource Description Framework (RDF)

The World Wide Web was originally built for human consumption, and although everything on it is machine-readable, this data is not machine-understandable. It is very hard to automate anything on the Web, and because of the volume of information the Web contains, it is not possible to manage it manually.

The solution proposed here is to use metadata to describe the data contained on the Web.

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RDF was developed by the W3C for Web-based metadata, using XML as an interchange syntax.

RDF's essential aim is to make work easier for autonomous agents, which would refine the Web by improving search engines and service directories.

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In 1989 Tim Berners-Lee invented the Web, and casinos, pornographers, and, incidentally, businesspeople found a medium of unprecedented power.

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Many limitations of the Web are widely accepted:

1. The predominance of HTML documents, which mix content with presentation

2. The difficulty of maintaining Web sites to reflect inevitable real-world changes

3. The difficulty of seamlessly presenting dynamic content

4. The seeming futility of finding precisely what one wants using a Web-crawler search engine

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The first two are supposed to give way to a future of an XML-driven Web, which would improve the maintainability and flexibility of Web data.

The W3C takes aim at the latter two with the Resource Description Framework (RDF), claiming that RDF will make the management and navigation of Web data easier to automate by providing structured Web metadata as counterpart to Web data.

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The broad goal of RDF is to define a mechanism for describing resources that makes no assumptions about a particular application domain, nor defines (a priori) the semantics of any application domain. The definition of the mechanism should be domain neutral, yet the mechanism should be suitable for describing information about any domain.

RDF is very abstract, very dry, and very academic.

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RDF can be used in a variety of application areas

For Example:

in resource discovery to provide better search engine capabilities

in cataloging for describing the content and content relationships available at a particular Web site, page, or digital library

by intelligent software agents to facilitate knowledge sharing and exchange

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in describing collections of pages that represent a single logical "document“

for describing intellectual property rights of Web pages, and for expressing the privacy preferences of a user as well as the privacy policies of a Web site

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In RDF, resources are represented by Uniform Resource Identifiers (URIs), of which URLs are a subset.

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RDF is very simple. It is no more than a way to express and process a series of simple assertions.

For example: This article is authored by Uche Ogbuji

This is called a statement in RDF and has three structural parts: a subject ("this article"), a predicate ("is authored by"), and an object ("Uche Ogbuji").

The object is a string: "Uche Ogbuji". This is called a literal in RDF, but an object could also be a resource.

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The figure shows several RDF statements combined into a single diagram.

All of RDF is pretty much an expansion of this basis. RDF defines a directed graph of statements that describe Web-based resources.

This might seem rather simple to be such an important technology, but it is RDF 's very simplicity that makes it so powerful.

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RDF allows many simple statements to be aggregated so that machine agents can apply the well-tested graph traversal techniques to glean data.

These statements are called triples because there are three predominant parts (subject, object, and predicate).

Databases of such triples have been shown to be scalable to many millions of triples, mostly because of the simplicity of this information.

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The abstract representation we have discussed above is the basis of RDF, but it is quite impractical for exchanging RDF descriptions and placing such descriptions in HTML and XML content.

To this end RDF Model & Syntax (RDF M&S) also provides a serialization format in XML for RDF.

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<rdf:RDF

xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

xmlns="http://schemas.uche.ogbuji.net/rdfexample/">

<rdf:Description about="http://uche.ogbuji.net/thisarticle">

<authored-by>

<rdf:Description ID="uche.ogbuji.net">

<name>Uche Ogbuji</name>

<nationality>Nigerian</nationality>

</rdf:Description>

</authored-by>

</rdf:Description>

</rdf:RDF>

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One constant in all RDF serializations is the use of the element rdf:RDF to wrap the RDF statements.

RDF relies heavily on XML namespaces for disambiguating names. There are several element and attribute names that must be in the namespace defined by RDF M&S.

All RDF predicates must use a namespace to clarify their meaning.

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The Advantage of RDFThe Advantage of RDF

The W3C suggests that webmasters begin the process of annotating existing Web data with RDF by embedding simple descriptions into the headers of their documents.

The advantage of RDF is that it is readily extensible with schemas that are also machine readable, bringing about an unprecedented level of automation.

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RDF SchemaRDF Schema

The RDF data model, as specified in RDF M&S, defines a simple model for describing interrelationships among resources in terms of named properties and values.

RDF properties may be thought of as attributes of resources and in this sense correspond to traditional attribute-value pairs. RDF properties also represent relationships between resources.

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As such, the RDF data model can therefore resemble an entity-relationship diagram. The RDF data model, however, provides no mechanisms for declaring these properties, nor does it provide any mechanisms for defining the relationships between these properties and other resources.

That is the role of RDF Schema.

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For describing bibliographic resources, for example, descriptive attributes including "author", "title", and "subject" are common.

For digital certification, attributes such as "checksum" and "authorization" are often required.

The declaration of these properties (attributes) and their corresponding semantics are defined in the context of RDF as an RDF schema.

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A schema defines not only the properties of the resource (Title, Author, Subject, Size, Color, etc.) but may also define the kinds of resources being described (books, Web pages, people, companies, etc.).

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The RDF Schema allows designers to specify classes of Resource types and properties to convey descriptions of those classes, and constraints on the allowed combinations of classes, properties, and values.

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XML, DTD and XML SchemasXML, DTD and XML Schemas

RDF Schemas might be contrasted with XML Document Type Definitions (DTDs) and XML Schemas. Unlike an XML, DTD and XML Schemas, which give specific constraints on the structure of a document, an RDF Schema provides information about the interpretation of the statements given in an RDF data model. The RDF/XML syntax itself provides considerable flexibility in the syntactic expression of the data model. A syntactic schema alone is not sufficient for RDF purposes. RDF Schemas may also specify consistency constraints that should be followed by these data models.

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DAMLDAML

DAML (DARPA Agent Markup Language) was created as part of a research program started in August 2000 by DARPA, a US governmental research organization. It is being developed by a large team of researchers, coordinated by DARPA. The DAML language is being developed as an extension to XML and the RDF.

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DAMLDAML

Unlike RDF DAML is not a data model; instead, it is a schema language that can be used to constrain and describe data following the RDF data model. To put it another way: DAML is an RDF schema language. RDF already has a schema language, called RDF Schema, and DAML is an extension of this language. Note that DAML also extends the RDF syntax.

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DAMLDAML

The value of DAML is thus that it allows one to describe RDF data, and so makes it possible to add more semantics to the data.

In general, what DAML adds to RDF Schema is additional ways to constrain the allowed values of properties, and what properties a class may have. In addition, it provides some properties that can be truly useful to generic software.

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OILOIL

OIL (Ontology Inference Layer) is an initiative funded by the European Union program for Information Society Technologies as part of some of its research projects.

The work has been done by participants in these projects, and the resulting specification is a specification published by the research project. OIL is intended to solve the findability problem, support e-commerce, and enable knowledge management.

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OILOIL

The original purpose of OIL was to enable the sharing – the exchange – of ontologies.

This sharing and exchange can be seen to have (at least) two dimensions:

the unequivocal sharing of semantics so that when the ontology is deployed it can be interpreted in a consistent manner

ensuring that when the ontology is viewed by an agent (in particular here a person) other than the author, the intention of the author is clear.

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OILOIL

OIL is very similar to DAML in that it, too, is an extension of RDF Schema, and the capabilities of the two languages are very similar.

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DAML + OILDAML + OIL

DAML+OIL is a semantic markup language for Web resources. It builds on earlier W3C standards such as RDF and RDF Schema, and extends these languages with richer modelling primitives.

DAML+OIL provides modelling primitives commonly found in frame-based languages.

The latest release of the language DAML+OIL provides a rich set of constructs with which to create ontologies and to markup information so that it is machine readable and understandable.

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DAML+OIL is now being proposed as a W3C standard for ontological and metadata representation.

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DAML+OIL provides a larger set of modeling concepts for describing the world, including

class hierarchies and relationships property hierarchies and relationships the cardinality of properties restrictions on which property can be used where datatype objects instance data

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The important difference between XML Schema (and DTD) and DAML+OIL (which may appear to have overlapping functionality) is that an XML Schema (and DTD) defines a class of XML documents by describing syntactic constraints on those documents, while DAML+OIL (like RDF Schema) is data-orientated, describing constraints on objects, not on documents.

DAML+OIL and RDF Schema are therefore particularly useful for describing relationships between objects which are described over several different documents, including relationships between schemas or ontologies.