User:Rcygania2/RDF Concepts Introduction

From RDF Working Group Wiki
Jump to: navigation, search

This page contains a proposed new Introduction section for RDF Concepts. The goal is to replace the following sections from RDF Concepts 2004:

The replacement should be:

  • shorter
  • more relevant
  • reflect 2011 understanding
  • informatively introduce more useful terms missing from the 2004 version (resource, vocabulary, concrete syntax, …)
  • less redundant with the later normative sections (!!!)

A few items from the old Section 3, in particular the datatype examples, are quite nice and the plan is to fold them into the later normative sections (as informative examples).

Introduction (Informative)

The Resource Description Framework (RDF) is a framework for representing information in the Web.

This document defines an abstract syntax (a data model) which serves to link all RDF-based languages and specifications, including:

  • Serialization syntaxes for storing and exchanging RDF (e.g., Turtle [TURTLE-TR] and RDF/XML [RDF-SYNTAX-GRAMMAR]),
  • the SPARQL Query Language ([SPARQL]),
  • the RDF Vocabulary Description Language ([RDF-SCHEMA]),
  • a formal model-theoretic semantics for RDF [RDF-MT].

Graph-based Data Model

The core structure of the abstract syntax is a collection of triples, each consisting of a subject, a predicate and an object. A set of such triples is called an RDF graph. This can be illustrated by a node and directed-arc diagram, in which each triple is represented as a node-arc-node link; hence the term “graph”.

  Subject --Predicate--> Object

There may be three kinds of nodes in an RDF graph: IRIs, literals, and blank nodes.

Resources and Statements

Any IRI and literal denotes some thing in the domain of discourse. These things are called resources. Anything can be a resource, including physical things, documents, and abstract concepts. Literals denote datatyped resources such as strings, numbers, and dates.

The assertion of an RDF triple says that some relationship, indicated by the predicate, holds between the resources denoted by the subject and object. This statement corresponding to an RDF triple is known as an RDF statement. The predicate itself is an IRI and denotes a relationship type, also known as a property.

The assertion of an RDF graph amounts to asserting all the triples in it, so the meaning of an RDF graph is the conjunction (logical AND) of the statements corresponding to all the triples it contains.

The statement corresponding to a triple with a blank node says that something with the given relationship exists, without explicitly naming it.

The Referent of an IRI

The resource denoted by an IRI is also called its referent. What exactly is denoted by any given IRI is out of scope of this specification and is treated in documents like [AWWW] and [Cool URIs]. A very brief and partial account follows:

  • By social convention, the IRI owner gets to say what an IRI denotes. They do this when “minting” a new IRI.
  • The IRI owner can establish the intended referent by means of a specification or other document that explains what is denoted. For example, [RDF Schema] specifies the referents of IRIs that start with http://www.w3.org/2000/01/rdf-schema#.
  • A good way of communicating the intended referent to the world is to set up the IRI so that it resolves to such a document.
  • Such a document can, in fact, be an RDF document that describes the denoted resource by means of RDF statements.

An RDF vocabulary is a collection of IRIs with clearly established referents intended for use in RDF graphs. For example, the IRIs documented in [RDF Schema] are the RDF Schema vocabulary. RDF Schema can itself be used to define and document additional RDF vocabularies, some of which are mentioned in the Primer [RDF-PRIMER].

Formal Meaning and Entailment

The idea of meaning in RDF is underpinned by the formal concept of entailment. In brief, an RDF graph A is said to entail another RDF graph B if every possible arrangement of things in the world that makes A true also makes B true. On this basis, if the truth of A is presumed or demonstrated then the truth of B can be inferred. An account of meaning and entailment in RDF, using the formalism of model theory, is given in [RDF-MT].

Merging and Managing RDF Graphs

Issue: Add language here about working with multiple graphs and about the fact that graphs merge easily once a design has been approved. Make sure this aligns with official terminology

An RDF document is a document that encodes an RDF graph in a concrete RDF syntax, such as Turtle [TURTLE-TR], RDFa [RDFA-PRIMER], RDF/XML [RDF-SYNTAX-GRAMMAR], or N-Triples [N-TRIPLES].