is a family of World Wide Web Consortium (W3C) Specification s originally designed as a Metadata Model but which has come to be used as a general method of modeling information, through a variety of syntax formats.
The RDF metadata model is based upon the idea of making denoting "has the color", and an object denoting "blue".
This mechanism for describing resources is a major in which automated software can store, exchange, and use machine-readable information distributed throughout the web, in turn enabling users to deal with the information with greater efficiency and Certainty . RDF's simple data model and ability to model disparate, abstract concepts has also led to its increasing use in Knowledge Management applications unrelated to Semantic Web activity.
There were several ancestors to W3C's . Technically the closest was MCF , a project initiated by Ramanathan V. Guha while at Apple Computer and continued, with contributions from Tim Bray , during his tenure at Netscape Communications Corporation . Ideas from the Dublin Core community, and from PICS , the Platform for Internet Content Selection (W3C's early Web content labelling system) were also key in shaping the direction of the RDF project.
The W3C published a specification for RDF's data model and XML syntax as a Recommendation in 1999. Work then began on a new version that was published as a set of related specifications in 2004. While there are a few implementations based on the 1999 Recommendation that have yet to be completely updated, adoption of the improved specifications has been rapid since they were developed in full public view, unlike some earlier W3C technologies. Most newcomers to RDF are unaware that the older specifications even exist.
MIME media type application/rdf+xml was registered by RFC 3870. It recommends RDF documents to follow the new specifications.
In addition to serialising RDF as XML, the W3 introduced Notation 3 (or N3) as a non-XML serialization of RDF models designed to be easier to scribble when getting started.
See Also: Ontology (computer science)
A collection of RDF statements intrinsically represents a Labeled, Directed Pseudo-graph . As such, an RDF-based Data Model is more naturally suited to certain kinds of Knowledge Representation than the Relational Model and other Ontological models traditionally used in computing today. However, in practice, RDF data is often stored in Relational Database representations sometimes also called Triple Store s. As RDFS and OWL demonstrate, additional Ontology Language s can be built upon RDF.
The subject of an RDF statement is a Resource , possibly as named by a Uniform Resource Identifier (URI). Some resources are unnamed and are called Blank Node s or anonymous resources. They are not directly identifiable. The predicate is a resource as well, representing a relationship. The object is a resource or a Unicode String Literal .
In Semantic Web applications, and in relatively popular applications of RDF like , nor does it need to represent a tangible, network-accessible resource — such a URI could denote the abstract notion of world peace, if desired.
Therefore, it is necessary for producers and consumers of RDF statements to be in agreement on the semantics of resource identifiers. Such agreement is not inherent to RDF itself, although there are some controlled vocabularies in common use, such as Dublin Core Metadata, which is partially mapped to a URI space for use in RDF.
Certain concepts in RDF are taken from Logic and Linguistics , where subject-predicate and subject-predicate-object structures have Meaning s similar to, yet distinct from, the uses of those terms in RDF. This example demonstrates:
In the English Language statement '' 'New York has the postal abbreviation NY' '','' 'New York' '' would be the subject, '' 'has the postal abbreviation' '' the predicate and '' 'NY' '' the object.
Encoded as an RDF triple, the subject and predicate would have to be resources named by URIs. The object could be a resource or literal element. For example, in the N-Triples form of RDF, the statement might look like:
: <http://purl.org/dc/terms/alternative> "NY" .
In this example, "urn:states:New%20York" is the URI for a resource that denotes the U.S. state New York , "http://purl.org/dc/terms/alternative" is the URI for a predicate (whose human-readable definition can be found at {Link without Title} ), and "NY" is a literal string. Note that the URIs chosen here are not standard, and don't need to be, as long as their meaning is known to whatever is reading them.
N-Triples is just one of several standard Serialization formats for RDF. The triple above can also be equivalently represented in the standard RDF/XML format as:
xmlns:terms="http://purl.org/dc/terms/">
NY
However, because of the restrictions on the syntax of QName s (such as terms:alternative above), there are some RDF graphs that are not representable with RDF/XML.
In a like manner, given that "http://en.wikipedia.org/wiki/Tony_Benn" identifies a particular resource (regardless of whether that URI could be traversed as a hyperlink, or whether the resource is ''actually'' the Wikipedia article about Tony Benn ), to say that the title of this resource is "Tony Benn" and its publisher is "Wikipedia" would be two assertions that could be expressed as valid RDF statements. In the N-Triples form of RDF, these statements might look like the following:
: "Tony Benn" .
: "Wikipedia" .
And these statements might be expressed in RDF/XML as:
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/">
Tony Benn
Wikipedia
To an English-speaking person, the same information could be represented simply as:
The title of this resource, which is published by Wikipedia, is 'Tony Benn'
However, RDF puts the information in the formal way that a machine can understand. The purpose of RDF is to provide an Encoding and interpretation mechanism so that Resources can be described in a way that particular Software can understand it; in other words, so that software can access and use data that it otherwise couldn't.
Both versions of the statements above are wordy because one requirement for an RDF resource (as a subject or a predicate) is that it be unique. The subject resource must be unique in an attempt to pinpoint the exact resource being described. The predicate needs to be unique in order to reduce the chance that the idea of for the Concept of a title established by the Dublin Core Metadata Initiative), it will also know that this title is different from a land title or an honorary title or just the letters t-i-t-l-e put together.
The following example shows how such simple claims can be elaborated on, by combining multiple RDF vocabularies. Here, we note that the primary topic of the Wikipedia page is a "Person" whose name is "Tony Benn":
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:foaf="http://xmlns.com/foaf/0.1/"
xmlns:dc="http://purl.org/dc/elements/1.1/">
Tony Benn
Wikipedia
Tony Benn
The body of knowledge modeled by a collection of statements may be subjected to Reification , in which each ''statement'' (that is each triple ''subject-predicate-object'' altogether) is assigned a URI and treated as a resource about which additional statements can be made, as in "''Jane says that'' John is the author of document X". Reification is sometimes important in order to deduce a level of confidence or degree of usefulness for each statement.
In a reified RDF database, each original statement, being a resource, itself, most likely has at least three additional statements made about it: one to assert that its subject is some resource, one to assert that its predicate is some resource, and one to assert that its object is some resource or literal. More statements about the original statement may also exist, depending on the application's needs.
Borrowing from concepts available in Logic (and as illustrated in graphical notations such as Conceptual Graphs and Topic Map s), some RDF model implementations acknowledge that it is sometimes useful to group statements according to different criteria, called ''situations'', ''contexts'', or ''scopes'', as discussed in articles by RDF specification co-editor Graham Klyne [http://www.ninebynine.org/RDFNotes/UsingContextsWithRDF.html . For example, a statement can be associated with a context, named by a URI, in order to assert an "is true in" relationship. As another example, it is sometimes convenient to group statements by their source, which can be identified by a URI, such as the URI of a particular RDF/XML document. Then, when updates are made to the source, corresponding statements can be changed in the model, as well.
Implementation of scopes does not necessarily require fully reified statements. Some implementations allow a single scope identifier to be associated with a statement that has not been assigned a URI, itself. [http://librdf.org/notes/contexts.html
In First-order Logic , as facilitated by RDF without scopes, the only metalevel relation is Negation , but the ability to generally state propositions about nested contexts allows RDF to comprise a metalanguage that can be used to define modal and Higher-order Logic .
See Also: RDF query language
Several Query Language s for RDF graphs have emerged. RDF query languages allow expressions to be written that can be evaluated against a collection of statements in order to produce, for example, a narrowed set of statements, resources, or object values, or to perform comparisons and operations on such items. RDF queries can be used by knowledge management applications as a basis for Inference actions.
Modeled loosely after SQL , the query language SPARQL is emerging as the de-facto RDF query language. On the track towards status of W3C Recommendation , it was released as a Candidate Recommendation in April 2006, but is back to Working Draft status since October 2006, due to open issues.
Other notable RDF query and inference languages include:
RDF has been criticised on the following grounds: RDF and RDF Schema: An Overview , p 27
- The RDF syntax is too verbose.
- The triplet (object, predicate, subject) notation is not expressive enough.
- RDF's ability to reify statements is handled ambiguously.
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