The Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
In mathematics, a graph is an abstract representation of a set of objects where some pairs of the objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices, and the links that connect some pairs of vertices are called edges...
Úriis a village and commune in the comitatus of Pest in Hungary....
, allowing descriptions to be made of that set of statements such as context, provenance information or other such metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
.
Named graphs are a simple extension of the RDF data model through which graphs can be created but the model lacks an effective means of distinguishing between them once published on the Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
at large.
Named graphs and HTTP
One conceptualization of the Web is as a graph of document nodes identified with URIs and connected by hyperlink
Hyperlink
In computing, a hyperlink is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks...
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
documents. By doing a HTTP GET on a URI (usually via a Web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
), a somehow-related document may be retrieved. This "follow your nose" approach also applies to RDF documents on the Web in the form of Linked Data
Linked Data
In computing, linked data describes a method of publishing structured data so that it can be interlinked and become more useful. It builds upon standard Web technologies such as HTTP and URIs, but rather than using them to serve web pages for human readers, it extends them to share information in a...
, where typically an RDF syntax is used to express data as a series of statements, and URI
Úri
Úriis a village and commune in the comitatus of Pest in Hungary....
s within the RDF point to other resources. This Web of data has been described by Tim Berners-Lee
Tim Berners-Lee
Sir Timothy John "Tim" Berners-Lee, , also known as "TimBL", is a British computer scientist, MIT professor and the inventor of the World Wide Web...
as the "Giant Global Graph".
Named graphs are a formalization of the intuitive idea that the contents of an RDF document (a graph) on the Web can be considered to be named by the URI of the document. This considerably simplifies techniques for managing chains of provenance for pieces of data and enabling fine-grained access control to the source data. Additionally trust can be managed through the publisher applying a digital signature to the data in the named graph. (Support for these facilities was originally intended to come from RDF reification, however that approach proved problematic ).
Named graphs and RDF stores
While named graphs may appear on the Web as simple linked documents (i.e. Linked Data
Linked Data
In computing, linked data describes a method of publishing structured data so that it can be interlinked and become more useful. It builds upon standard Web technologies such as HTTP and URIs, but rather than using them to serve web pages for human readers, it extends them to share information in a...
), they are also very useful for managing sets of RDF data within an RDF store. In particular, the scope of a SPARQL
SPARQL
SPARQL is an RDF query language; its name is an acronym that stands for SPARQL Protocol and RDF Query Language. It was made a standard by the RDF Data Access Working Group of the World Wide Web Consortium, and considered as one of the key technologies of semantic web...
query may be limited to a specific set of named graphs.
Example
Assume the following (Turtle syntax RDF document has placed in a SPARQL
SPARQL
SPARQL is an RDF query language; its name is an acronym that stands for SPARQL Protocol and RDF Query Language. It was made a standard by the RDF Data Access Working Group of the World Wide Web Consortium, and considered as one of the key technologies of semantic web...
-capable store with the name http://example.org/joe.
This data has been written in a more verbose form than necessary to show the triple structures
The homepage of the person with the email address mailto:joe@example.org can be obtained using the SPARQL
SPARQL
SPARQL is an RDF query language; its name is an acronym that stands for SPARQL Protocol and RDF Query Language. It was made a standard by the RDF Data Access Working Group of the World Wide Web Consortium, and considered as one of the key technologies of semantic web...
query:
PREFIX foaf:
SELECT ?homepage
FROM NAMED
WHERE {
?person foaf:homepage ?homepage .
?person foaf:mbox .
}
The FROM NAMED here identifies the target graph for the query.
Named graphs and quads
Prior to the publication of the papers describing named graphs, there was considerable discussion about fulfilling their role within a store by using an arity above that of RDF triple statements. Where triples have the form , quads would have a form along the lines of . Named graphs can be represented this way, as , with the advantage that the part will be a URI, giving the quad Web-global scope compared to arbitrary local statement names.
Specifications
There is currently no specification for named graphs in themselves beyond that described in and (which includes syntaxes for representing named graphs), however they do form part of the SPARQL
SPARQL
SPARQL is an RDF query language; its name is an acronym that stands for SPARQL Protocol and RDF Query Language. It was made a standard by the RDF Data Access Working Group of the World Wide Web Consortium, and considered as one of the key technologies of semantic web...
Protocol and RDF Query Language specification.
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