Resource Description Framework
The Resource Description Framework (RDF, Eng. Mutatis mutandis " system for the description of resource " ) refers to a technical approach on the Internet to formulate logical statements about any things (resources). It was originally designed as a standard for describing metadata RDF from the World Wide Web Consortium (W3C). Meanwhile RDF is considered a fundamental building block of the Semantic Web. RDF is similar to the classical methods for the modeling of concepts such as UML class diagrams and entity-relationship model. In the RDF model is any statement from the three units subject, predicate and object, a resource as a subject with another resource or only one value ( literal) is described as an object. With another resource as a predicate these three units form a triple ( "3- tuple "). In order to have globally unique identifiers for resources, they are formed according to this convention as URLs. URLs generally frequently used features, such as metadata, RDF developers are well known and can be used throughout the world for the same purpose, which makes it possible, inter alia, programs, in turn, present the data for the human sense.
RDF Model
The RDF data model is a model with a well defined formal semantics based on the directed graph. Data in RDF are statements about resources. These statements are modeled as triples. The amount of the triple is a (mathematical) graph, and is referred to as an RDF model. A triple is an elementary statement, consisting of subject, predicate and object.
Example
The triple represents a statement in which subject and object relate to each other ( relation) are set. Relations are directed from subject to object and named by the predicate. Triples that relate to the same subjects or objects that form a semantic network, which is often presented in tabular form or graphically. Colloquially seen any statement in RDF is a simple sentence. about:
Transferred to the modeling means of RDF:
- Subject = ACME
- Predicate = produced
- Object = batteries
In the following example table (supplemented by other statements ), each row is a triple:
Resource URI and literal
A resource is something that is clearly indicated and what you want to say something. Subject and predicate are always resources. The object may be either a resource or a literal only. Literals are strings that are interpreted may still based on a specified data type. As literals can be, for example, truth-values , numbers or dates specified. RDF resources are identified by unique identifiers (URIs). The URIs make it possible to combine data coming from different sources. Most resources are identified by a URI, which is similar to a URL in shape. URLs are special URIs that serve to uniquely identify web pages. URIs are not necessarily reachable on the network.
Examples:
About the resources used as a predicate can in turn make statements in RDF and stored as a metadata format. Other RDF authors can continue to use these vocabularies by reference. A prominent example is the representation of Dublin Core in RDF. On the other hand form RDF statement itself resources that may be referenced by other statements. This technique of statements about statements is called reification.
In addition, RDF maintains predefined data types for lists and quantities ready to summarize groups of resources. Resources that have no explicit URI, but only used to group other objects are modeled usually by so-called "blank nodes". An example of this is the assignment of a name that consists of separate strings for first and last name.
Representation
RDF is independent of a specific ( textual ) representation. It is usual XML as well as a shorter syntax notation called 3 (N3 ). The W3C also has the Turtle language defined in 2011, which represents a reduced subset of N3 and should thus contribute to a wider dissemination.
For the storage of RDF in databases and data structures there are different concepts ( Triple goals), as a pure drop of the triples in a relational table is not very efficient for many queries.
Retrieval
To search in RDF data different query languages were designed. The RDF Query Language ( RDQL ), reminiscent in form strongly to SQL. In January 2008, the W3C SPARQL adopted as a W3C Recommendation, and thus quasi standard for RDF query languages made , which is why there is also many implementations of SPARQL.
History
As a precursor of the RDF Meta Content Framework can (MCF) in XML are valid, a language that was developed by Ramanathan V. Guha 1995 to 1997 and submitted after joining Netscape in June 1997 at the W3C. As part of the browser wars MCF was also a reaction to the Channel Definition Format from Microsoft. Instead of MCF to give preference, it was decided at the W3C to develop a common language for the formulation of metadata, which should bear the name of RDF. The first RDF standard was presented as a draft in August 1997 and published in February 1999 as a recommendation. From 1999 began with the development of RDF Schema.