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Named graphs are a key concept of Semantic Web architecture in which a set of Resource Description Framework statements (a
graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discre ...
) are identified using a URI, allowing descriptions to be made of that set of statements such as context, provenance information or other such metadata. 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 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 arcs which are expressed within the
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaSc ...
documents. By doing an
HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
GET on a URI (usually via a
Web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used o ...
), 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 In computing, linked data (often capitalized as Linked Data) is structured data which is interlinked with other data so it becomes more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but ...
, where typically an RDF syntax is used to express data as a series of statements, and URIs within the RDF point to other resources. This Web of data has been described by Tim Berners-Lee 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 In computing, linked data (often capitalized as Linked Data) is structured data which is interlinked with other data so it becomes more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but ...
), they are also very useful for managing sets of RDF data within an RDF store. In particular, the scope of a
SPARQL SPARQL (pronounced " sparkle" , a recursive acronym for SPARQL Protocol and RDF Query Language) is an RDF query language—that is, a semantic query language for databases—able to retrieve and manipulate data stored in Resource Description ...
query may be limited to a specific set of named graphs.


Example

Assume the following (
Turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
) RDF document has been placed in a
SPARQL SPARQL (pronounced " sparkle" , a recursive acronym for SPARQL Protocol and RDF Query Language) is an RDF query language—that is, a semantic query language for databases—able to retrieve and manipulate data stored in Resource Description ...
-capable store with the name . @prefix foaf: . a foaf:Person . foaf:homepage . foaf:mbox . foaf:name "Joe Lambda" . ''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 can be obtained using the
SPARQL SPARQL (pronounced " sparkle" , a recursive acronym for SPARQL Protocol and RDF Query Language) is an RDF query language—that is, a semantic query language for databases—able to retrieve and manipulate data stored in Resource Description ...
query: PREFIX foaf: SELECT ?homepage FROM NAMED WHERE 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 ''subject predicate object'', ''quads'' would have a form along the lines of ''subject predicate object context''. Named graphs can be represented this way, as ''subject predicate object graphname'', with the advantage that the ''graphname'' part will be a URI, giving the quad Web-global scope compared to arbitrary local statement names. This way of representing quads resp. quad-statements was incorporated in the specification of N-Quads.


Formal definition

A paper from the WWW 2005 conference by Carroll et al. includes a formal definition of named graphs.


Specifications

There is currently no specification for named graphs in themselves beyond that described in Carroll et al. (2005) and Carroll and Stickler (2004)TriX : RDF Triples in XML
/ref> (which includes syntaxes for representing named graphs), but they do form part of the
SPARQL SPARQL (pronounced " sparkle" , a recursive acronym for SPARQL Protocol and RDF Query Language) is an RDF query language—that is, a semantic query language for databases—able to retrieve and manipulate data stored in Resource Description ...
Protocol and RDF Query Language specification.


Proposed specifications

* TriX - Named Graphs in
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
* TriG - Named Graphs in
Turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
* N-Quads - Named Graphs in
N-Triples N-Triples is a format for storing and transmitting data. It is a line-based, plain text serialisation format for RDF (Resource Description Framework) graphs, and a subset of the Turtle (Terse RDF Triple Language) format. N-Triples should not be ...


See also

*


References

{{DEFAULTSORT:Named Graph Semantic Web Graphs