RDF Primer

W3C Editor's Draft 24 November 2002

This version:
Latest version:
Previous version:
Frank Manola, The MITRE Corporation, fmanola@mitre.org
Eric Miller, W3C, em@w3.org
Series Editor:
Brian McBride, Hewlett-Packard Laboratories, bwm@hplb.hpl.hp.com


The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web. It is particularly intended for representing metadata about Web resources, such as the title, author, and modification date of a Web page, copyright and licensing information about a Web document, or the availability schedule for some shared resource. However, by generalizing the concept of a "Web resource", RDF can also be used to represent information about things that can be identified on the Web, even when they can't be directly retrieved on the Web. RDF provides a common framework for expressing this information so it can be exchanged between applications without loss of meaning.

This Primer is designed to provide the reader with the basic knowledge required to effectively use RDF. It introduces the basic concepts of RDF and describes its XML syntax. It describes how to define RDF vocabularies using the RDF Vocabulary Description Language, and gives an overview of some deployed RDF applications. It also describes the content and purpose of other RDF specification documents.

Status of this Document

This is a W3C RDF Core Working Group Working Draft produced as part of the W3C Semantic Web Activity. This document incorporates material developed by the Working Group designed to provide the reader with the basic knowledge required to effectively use RDF in their particular applications.

This document is being released for review by W3C members and other interested parties to encourage feedback and comments. This is the current state of an ongoing work on the Primer.

This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use it as reference material or to cite as other than "work in progress". A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.

In conformance with W3C policy requirements, known patent and IPR constraints associated with this Working Draft are detailed on the RDF Core Working Group Patent Disclosure page.

Comments on this document are invited and should be sent to the public mailing list www-rdf-comments@w3.org. An archive of comments is available at http://lists.w3.org/Archives/Public/www-rdf-comments/.

Table of Contents

  1. Introduction
  2. Making Statements About Resources
      2.3 The RDF Model
      2.4 Structured Property Values and Blank Nodes
      2.5 Typed Literals
      2.6 Concepts Summary
  3. An XML Syntax for RDF: RDF/XML
      3.1 Basic Principles
      3.2 Defining New RDF Resources
      3.3 RDF/XML Summary
  4. Other RDF Capabilities
      4.1 RDF Containers
      4.2 RDF Collections
      4.3 RDF Reification
      4.4 Miscellaneous RDF Facilities
            4.4.1 More on Structured Values: rdf:value
  5. Defining RDF Vocabularies: RDF Schema
      5.1 Defining Classes
      5.2 Defining Properties
      5.3 Interpreting RDF Schema Declarations
      5.4 Other Schema Information
      5.5 Richer Schema Languages
  6. Some RDF Applications: RDF in the Field
      6.1 Dublin Core Metadata Initiative
      6.2 PRISM
      6.3 XPackage
      6.4 RSS 1.0: RDF Site Summary
      6.5 CIM/XML
      6.6 Gene Ontology Consortium
  7. Other Parts of the RDF Specification
      7.1 RDF Semantics
      7.2 Test Cases
  8. References
      8.1 Normative References
      8.2 Informational References
  9. Acknowledgments


  A. Uniform Resource Identifiers (URIs) Survival Guide
  B. Extensible Markup Language (XML) Survival Guide
  C. Changes

1. Introduction

The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web. It is particularly intended for representing metadata about Web resources, such as the title, author, and modification date of a Web page, copyright and licensing information about a Web document, or the availability schedule for some shared resource. However, by generalizing the concept of a "Web resource", RDF can also be used to represent information about things that can be identified on the Web, even when they can't be directly retrieved on the Web. Examples include information about items available from online shopping facilities (e.g., information about specifications, prices, and availability), or the description of a Web user's preferences for information delivery.

RDF provides a common framework for expressing this information so it can be exchanged between applications without loss of meaning. Since it is a common framework, application designers can leverage the availability of common RDF parsers and processing tools. The ability to exchange information between different applications means that the information may be made available to applications other than those for which it was originally created.

RDF is based on the idea of identifying things using Web identifiers (URIs), and describing resources in terms of simple properties and property values. This enables RDF to represent simple statements about resources as a graph of nodes and arcs representing the resources, and their properties and values. To make this discussion somewhat more concrete as soon as possible, the group of statements "there is someone whose name is Eric Miller, whose email address is em@w3.org, and whose title is Dr." could be represented as the RDF graph in Figure 0:

An RDF Graph
     Describing Eric Miller
Figure 0: An RDF Graph Describing Eric Miller

Figure 0 illustrates that RDF uses URIs to identify:

RDF also provides an XML-based syntax (called RDF/XML) for recording and exchanging these graphs. The following is a small chunk of RDF in RDF/XML corresponding to the graph in Figure 0:

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

  <Person rdf:about="http://www.w3.org/People/EM/contact#me">
    <fullName>Eric Miller</fullName>
    <mailbox rdf:resource="mailto:em@w3.org"/>


Note that this RDF/XML also contains URIs, as well as properties like mailbox and fullName (in an abbreviated form), and their respective values em@w3.org, and Eric Miller.

Like HTML, this RDF/XML is machine processable, and, using URIs, can link pieces of data across the Web. However, unlike conventional hypertext, RDF URIs can refer to any identifiable thing, including things that may not be directly retrievable on the Web (such as the person Eric Miller). The result is that in addition to describing such things as Web pages, we can also describe cars, businesses, people, news events, etc. In addition, RDF properties themselves have URIs, to precisely identify the kind of relationship that exists between the linked items.

The following documents contribute to the specification of RDF:

This Primer is intended to augment these other documents, to help information system designers and application developers understand the features of RDF and how to use them. In particular, the Primer is intended to answer such questions as:

  • What does RDF look like?
  • What information can RDF represent?
  • How is RDF information created, accessed, and processed?
  • How can existing information be combined with RDF?

The Primer is a non-normative document, which means that it does not provide a definitive specification of RDF. The examples and other explanatory material in the Primer are provided to help you understand RDF, but they may not always provide definitive or fully-complete answers. In such cases, you should refer to the relevant normative parts of the RDF specification. To help you do this, we provide links pointing to the relevant parts of the normative specifications.

2. Making Statements About Resources

RDF is intended to provide a simple way to state properties of (make assertions about) Web resources, e.g., Web pages. For example, imagine that we want to state the fact that someone named John Smith created a particular Web page. A straightforward way to state this in English would be in the form of a simple statement such as:

http://www.example.org/index.html has a creator whose value is John Smith

We've underlined parts of this statement to illustrate that, in order to describe the properties of something, we need ways to name, or identify, a number of things:

  • We need a way to identify the thing we want to describe (the Web page, in this case)
  • We need a way to identify a specific property (creator, in this case) of the thing that we want to describe
  • We need a way to identify the thing we want to assign as the value of this property (who the creator is), for the thing we want to describe

In this statement, we've used the Web page's URL (Uniform Resource Locator) to identify it. In addition, we've used the word "creator" to identify the property we want to talk about, and the two words "John Smith" to identify the thing (a person) we want to say is the value of this property.

We could state other properties of this Web page by writing additional English statements of the same general form, using the URL to identify the page, and words (or other expressions) to identify the properties and their values. For example, to specify the date the page was created, and the language in which the page is written, we could write the additional statements:

http://www.example.org/index.html has a creation-date whose value is August 16, 1999
http://www.example.org/index.html has a language whose value is English

RDF is based on the idea that the things we want to describe have properties which have values, and that resources can be described by making statements, similar to those above, that specify those properties and values. RDF uses a particular terminology for talking about the various parts of statements. Specifically, the part that identifies the thing the statement is about (the Web page in this example) is called the subject. The part that identifies the property or characteristic of the subject that the statement specifies (creator, creation-date, or language in these examples) is called the predicate, and the part that identifies the value of that property is called the object. So, taking the English statement

http://www.example.org/index.html has a creator whose value is John Smith

the RDF terms for the various parts of the statement are:

  • the subject is the URL http://www.example.org/index.html
  • the predicate is the word "creator"
  • the object is the words "John Smith"

However, while English is good for communicating between (English-speaking) humans, RDF is about making machine-processable statements. To make these kinds of statements suitable for processing by machines, we need two things:

  • a system of machine-processable identifiers that allows us to identify a subject, predicate, or object in a statement without any possibility of confusion with a similar-looking identifier that might be used by someone else on the Web.
  • a machine-processable language for representing these statements and exchanging them between machines.

Fortunately, the existing Web architecture provides us with both of the necessary mechanisms. The Web's Uniform Resource Identifier (URI) provides us with a way to uniquely identify anything we want to talk about in an RDF statement, and the Extensible Markup Language (XML) provides us with a format for representing and exchanging RDF statements. The next two sections briefly describe these mechanisms.

@@Now that Secs 2.1 and 2.2 are appendices, need a smooth segue here. In particular, need to briefly introduce URIrefs and QNames.@@

@@Mention that full discussion of RDF URIrefs (and the graph model as a whole) is in the Concepts document.@@

2.3 The RDF Model

Now that we've introduced URI references for identifying things we want to talk about on the Web, and XML as a machine-processable way of representing RDF statements, we can describe how RDF lets us use URIs to make statements about resources. In the introduction, we said that RDF was based on the idea of expressing simple statements about resources, where those statements are built using subjects, predicates, and objects. In RDF, we could represent our original English statement:

http://www.example.org/index.html has a creator whose value is John Smith

by an RDF statement having:

  • a subject http://www.example.org/index.html
  • a predicate http://purl.org/dc/elements/1.1/creator
  • and an object http://www.example.org/staffid/85740

Note how we have introduced URIrefs to identify not only the subject of the original statement, but also the predicate and object, instead of using the words "creator" and "John Smith", respectively. We'll discuss this further a bit later on.

RDF models statements as nodes and arcs in a graph. In this notation, a statement is represented by:

  • a node for the subject, labeled with its URIref
  • a node for the object, labeled with its URIref
  • an arc for the predicate, labeled with its URIref, directed from the subject node to the object node.

So the RDF statement above would be represented by the graph shown in Figure 1:

A Simple RDF Statement
Figure 1: A Simple RDF Statement

Groups of statements are represented by corresponding groups of nodes and arcs. So if we wanted to also represent the additional statements

http://www.example.org/index.html has a creation-date whose value is August 16, 1999
http://www.example.org/index.html has a language whose value is English

we could, by introducing suitable URIrefs to name the properties "creation-date" and "language", use the graph shown in Figure 2:

Several Statements About the Same Resource
Figure 2: Several Statements About the Same Resource

Figure 2 illustrates that the objects of RDF statements may be either resources identified by URIrefs, or constant values (called literals) represented by character strings, in order to represent certain kinds of property values (literals may not be the subjects or predicates of RDF statements). In drawing RDF graphs, nodes that represent resources identified by URIrefs are shown as ellipses, while nodes that represent literals are shown as boxes (labeled by the literal itself). RDF graphs can be described as "labeled directed graphs", since the arcs have labels, and are "directed" (point in a specific direction, from subject to object).

@@mention that a full discussion of RDF literals is in the Concepts document.@@

Sometimes it is not convenient to draw graphs when discussing them, so an alternative way of writing down the statements, called triples, is also used. In the triples notation, each statement in the graph is written as a simple triple of subject, predicate, and object node labels (either URIref or literal), in that order. The triples representing the three statements shown in Figure 2 would be written in full as:

<http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/creator> <http://www.example.org/staffid/85740> .

<http://www.example.org/index.html> <http://www.example.org/terms/creation-date> "August 16, 1999" .

<http://www.example.org/index.html> <http://www.example.org/terms/language> "English" .

Each triple corresponds to a single arc in the graph, complete with the arc's beginning and ending nodes (the subject and object of the statement). Unlike the drawn graph (but like the original statements), the triples notation requires that a node be separately identified for each statement it appears in. So, for example, http://www.example.org/index.html appears three times (once in each triple) in the triples representation of the graph, but only once in the drawn graph. However, the triples represent exactly the same information as the drawn graph, and this is a key point: what is fundamental to RDF is the graph model of the statements. The notation used to represent or depict the graph is secondary.

The full triples notation requires that URI references be written out completely, in angle brackets, which, as the example above illustrates, can result in very long lines. For convenience, we will use a shorthand way of writing triples in the rest of this Primer, and also in other RDF specifications. In this shorthand, we can substitute a QName without angle brackets as an abbreviation of a full URI reference. So, for example, if the QName prefix foo is mapped to the namespace URI http://example.org/somewhere/, then the QName foo:bar is shorthand for the URIref http://example.org/somewhere/bar. We will also make extensive use in these examples of several "well-known" QName prefixes (which we will use without explicitly specifying them each time), defined as follows:

prefix rdf:, namespace URI: http://www.w3.org/1999/02/22-rdf-syntax-ns#
prefix rdfs:, namespace URI: http://www.w3.org/2000/01/rdf-schema#
prefix dc:, namespace URI: http://purl.org/dc/elements/1.1/
prefix daml:, namespace URI: http://www.daml.org/2001/03/daml+oil#
prefix ex:, namespace URI: http://www.example.org/ (or http://www.example.com/)
prefix xsd:, namespace URI: http://www.w3.org/2001/XMLSchema#

We will also use variations on the "example" prefix ex: as needed in the examples, where this will not cause confusion, for example,

prefix exterms:, namespace URI: http://www.example.org/terms/ (for terms used by our example organization),
prefix exstaff:, namespace URI: http://www.example.org/staffid/ (for our example organization's staff identifiers),
prefix ex2:, namespace URI: http://www.domain2.example.org/ (for a second example organization), and so on.

Using our new shorthand, we can write the previous set of triples as:

ex:index.html dc:creator exstaff:85740 .

ex:index.html exterms:creation-date "August 16, 1999" .

ex:index.html exterms:language "English" .

The examples we've just given of RDF statements begin to illustrate some of the advantages of using URIrefs as RDF's basic way of identifying things. For instance, instead of identifying the creator of the Web page in our first example by the character string "John Smith", we've assigned him a URIref, in this case (using a URIref based on his employee number) http://www.example.org/staffid/85740 . An advantage of using a URIref in this case is that we can be more precise in our identification. That is, the creator of the page isn't the character string "John Smith", or any one of the thousands of people named John Smith, but the particular John Smith associated with that URIref (whoever created the URIref defines the association). Moreover, since we have a URIref for the creator of the page, it is a full-fledged resource, and we can record additional information about him, such as his name, and age, as in the graph shown in Figure 3:

More Information About John Smith
Figure 3: More Information about John Smith

These examples also illustrate that RDF uses URIrefs as predicates in RDF statements. That is, rather than using character strings (or words) such as "creator" or "name" to identify properties, RDF uses URIrefs. Using URIrefs to identify properties is important for a number of reasons. First, it allows us to distinguish the properties we use from properties someone else may use that would otherwise be identified by the same character string. For instance, in our example, example.org uses "name" to mean someone's full name written out as a character string literal (e.g., "John Smith"), but someone else may intend "name" to mean something different (e.g., the name of a variable in a piece of program text). A program encountering "name" as a property identifier on the Web wouldn't necessarily be able to distinguish these uses. However, if example.org writes http://www.example.org/terms/name for its "name" property, and the other person writes http://www.domain2.example.org/genealogy/terms/name for hers, we can keep straight the fact that there are distinct properties involved (even if a program cannot automatically determine the distinct meanings). Another reason why it is important to use URIrefs to identify properties is that it allows us to treat RDF properties as resources themselves. Since properties are resources, we can record descriptive information about them (e.g., the English description of what example.org means by "name"), simply by adding additional RDF statements with the property's URIref as the subject.

Using URIrefs as subjects, predicates, and objects in RDF statements allows us to begin to develop and use a shared vocabulary on the Web, reflecting (and creating) a shared understanding of the concepts we talk about. For example, in the triple

ex:index.html  dc:creator  exstaff:85740 .

the predicate dc:creator, when fully expanded as a URIref, is an unambiguous reference to the "creator" attribute in the Dublin Core metadata attribute set, a widely-used set of attributes (properties) for describing information of all kinds. The writer of this triple is effectively saying that the relationship between the Web page (identified by http://www.example.org/index.html ) and the creator of the page (a distinct person, identified by http://www.example.org/staffid/85740 ) is exactly the concept identified by http://purl.org/dc/elements/1.1/creator . Moreover, anyone else, or any program, that understands http://purl.org/dc/elements/1.1/creator will know exactly what is meant by this relationship.

Of course, RDF's use of URIrefs doesn't solve all our problems because, for example, people can still use different URIrefs to refer to the same thing. However, the fact that these different URIrefs are used in the commonly-accessible "Web space" creates the opportunity both to identify equivalences among these different references, and to migrate toward the use of common references.

The result of all this is that RDF provides a way to make statements that applications can more easily process. Now an application can't actually "understand" such statements, of course, but it can deal with them in a way that makes it seem like it does. For example, a user could search the Web for all book reviews and create an average rating for each book. Then, the user could put that information back on the Web. Another web site could take that list of book rating averages and create a "Top Ten Highest Rated Books" page. Here, the availability and use of a shared vocabulary about ratings, and a shared group of URIrefs identifying the books they apply to, allows individuals to build a mutually-understood and increasingly-powerful (as additional contributions are made) "information base" about books on the Web. The same principle applies to the vast amounts of information that people create about thousands of subjects every day on the Web.

RDF statements are similar to a number of other formats for recording information, such as:

  • entries in a simple record or catalog listing describing the resource in a data processing system.
  • rows in a simple relational database.
  • simple assertions in formal logic

and information in these formats can be treated as RDF statements, allowing RDF to be used as a unifying model for integrating data from many sources.

2.4 Structured Property Values and Blank Nodes

Things would be very simple if the only types of information we had to record about things were obviously in the form of the simple RDF statements we've illustrated so far. However, most real-world data involves structures that are more complicated than that, at least on the surface. For instance, in our original example, we recorded the date the Web page was created as a single exterms:creation-date property, with a simple character string literal as its value. However, suppose we wanted to show, as the value of the exterms:creation-date property, the month, day, and year as separate pieces of information? Or, in the case of John Smith's personal information, suppose we wanted to record his address. We might write the whole address out as a character string literal, as in the triple

exstaff:85740  exterms:address  "1501 Grant Avenue, Bedford, Massachusetts 01730" .

However, suppose we wanted to record John's address as a structure consisting of separate street, city, state, and Zip code values? How do we do this in RDF?

We can represent such structured information in RDF by considering the aggregate thing we want to talk about (like John Smith's address) as a resource, and then making statements about that new resource. So, in the RDF graph, in order to break up John Smith's address into its component parts, we create a new node to represent the concept of John Smith's address, and assign that concept a new URIref to identify it, say http://www.example.org/addressid/85740 (which we will abbreviate as exaddressid:85740). We then write RDF statements (create additional arcs and nodes) with that node as the subject, to represent the additional information, producing the graph shown in Figure 4:

Breaking Up John's Address
Figure 4: Breaking Up John's Address

or the triples:

exstaff:85740      exterms:address  exaddressid:85740 .
exaddressid:85740  exterms:street   "1501 Grant Avenue" .
exaddressid:85740  exterms:city     "Bedford" .
exaddressid:85740  exterms:state    "Massachusetts" .
exaddressid:85740  exterms:Zip      "01730" .

Using this approach allows us to represent structured information in RDF, but it can involve generating numerous "intermediate" URIrefs to represent aggregate concepts such as John's address, concepts that may never need to be referred to directly from outside a particular graph, and thus don't, strictly speaking, require "universal" identifiers. In addition, in the drawing of the graph representing the group of statements shown in Figure 4, we don't really need the URIref we assigned to identify "John Smith's address", since we could just as easily have drawn the graph as in Figure 5:

Using a Blank Node
Figure 5: Using a Blank Node

In Figure 5, which is a perfectly good RDF graph, we've used a node without a label to stand for the concept of "John Smith's address". This unlabeled node, or blank node, functions perfectly well in the drawing without needing a URIref, since the node itself provides the necessary connectivity between the various other parts of the graph. (Blank nodes were previously called anonymous resources in [RDF-MS].) However, we would need some form of explicit identifier for that node if we wanted to represent this graph as triples. To see this, we can try to write the triples corresponding to what is shown in the drawn graph. What we would get would be something like:

exstaff:85740  exterms:address  ??? .
???            exterms:street   "1501 Grant Avenue" .
???            exterms:city     "Bedford" .
???            exterms:state    "Massachusetts" .
???            exterms:Zip      "01730" 

where ??? stands for something that indicates the presence of the blank node. Since a complex graph might contain more than one blank node, we would also need a way to differentiate between the various blank nodes in the triples representation of the graph. To do this, we use node identifiers, having the form _:name, to indicate the presence of blank nodes in triples. For instance, in this example we might generate the node identifier _:johnaddress to refer to the blank node, in which case the resulting triples might be:

exstaff:85740  exterms:address  _:johnaddress .
_:johnaddress  exterms:street   "1501 Grant Avenue" .
_:johnaddress  exterms:city     "Bedford" .
_:johnaddress  exterms:state    "Massachusetts" .
_:johnaddress  exterms:Zip      "01730" .

In a triples representation of a graph, each distinct blank node in the graph is given a different node identifier. Unlike URIrefs and literals, node identifiers are not considered to be actual parts of the RDF graph (this can be seen by looking at the drawn graph in Figure 5 and noting that there is no node identifier used to label the blank node). Node identifiers only have significance within the triple representation of the graph, and only for the purpose of distinguishing one blank node from another (so that two groups of triples that differ only by re-naming their node identifiers are considered to represent identical RDF graphs). Node identifiers also have significance only within the triples representing a single graph (so that two different graphs with the same number of blank nodes might use the same node identifiers to distinguish them, and it would be unwise to assume that blank nodes from different graphs having the same node identifiers referred to the same resource). If it is expected that a node in a graph will need to be referenced from outside the graph, a URIref should be assigned to identify it.

At the beginning of this section, we noted that we can represent aggregate structures, like John Smith's address, by considering the aggregate thing we want to talk about as a resource, and then making statements about that new resource. This example illustrates an important aspect of RDF: RDF directly represents only binary relationships, e.g. the relationship between John Smith and the literal representing his address. When we try to represent the relationship between John and the group of separate components of this address, we are dealing with an n-ary (n-way) relationship (in this case, n=5) between John and the street, city, state, and zip components. In order to represent such structures directly in RDF (e.g., considering the address as a group of street, city, state, and zip sub-components), we need to break this n-way relationship up into a group of separate binary relationships. Blank nodes give us one way to do this. Each time we have an n-ary relationship, we can choose one of the participants as the subject of the relationship (John in this case), and create a blank node to represent the rest of the relationship (John's address in this case). We can then represent the remaining participants in the relationship (such as the city in our example) as separate properties of the new resource represented by the blank node.

Blank nodes also give us a way to more accurately make statements about resources that may not have URIs, but that are described in terms of relationships with other resources that do have URIs. For example, when making statements about a person, say Jane Smith, it may seem natural to use that person's email address as her URI, e.g., mailto:jane@example.org. However, this approach can cause a number of problems. One obvious problem is that Jane Smith's email address may change when she changes jobs, and so it may be hard to combine information about Jane recorded at different times. Another problem is that we may want to record information about Jane's mailbox (e.g., the server it is on) as well as about Jane herself (e.g., her current address), and using a URIref for Jane based on her email address makes it difficult to know which thing we're talking about. The same problem exists when a company's Web page URL, say http://www.example.com/, is used as the URI of the company itself. Once again, we may need to record information about the Web page (e.g., who created it and when) as well as about the company, and using http://www.example.com/ as an identifier for both makes it difficult to know which thing we're talking about.

The fundamental problem is that using Jane's email address as a stand-in for Jane isn't really accurate: Jane's email address identifies a mailbox, and Jane and her mailbox are not the same thing. When Jane herself doesn't have a URI, a blank node gives us a more accurate way of modeling this situation. We can represent Jane by a blank node, and give the blank node an exterms:emailaddress property having the URIref mailto:jane@example.org as its value. We can also assign the blank node an rdf:type property with a value of exterms:Person (we will discuss types in more detail in the following sections), a exterms:name property with a value of "Jane Smith", and any other descriptive information we might want to provide, as shown in the following triples:

_:jane  exterms:emailaddress   mailto:jane@example.org .
_:jane  rdf:type       exterms:Person .
_:jane  exterms:name   "Jane Smith" .
_:jane  exterms:empID  "23748"
_:jane  exterms:age    "26" .

This says, accurately, that "there is a resource of type Person, whose electronic mailbox is identified by mailto:jane@example.org, whose name is 'Jane Smith', etc." That is, the blank node can be read as "there is a resource". Statements with that blank node as subject then provide information about the characteristics of that resource.

In practice, using blank nodes instead of URIrefs in these cases doesn't change the way we actually handle this kind of information very much. For example, if we know independently that an email address uniquely identifies someone at example.org (particularly if the address is unlikely to be reused), we can still use that fact to associate information about that person from multiple sources, even though the email address is not the person's URI. For example, if we were to find another piece of RDF on the web that described a book, and gives the author's contact information as the email address mailto:jane@example.org, we might reasonably conclude that the author's name is Jane Smith. The point is that saying something like "the author of the book is mailto:jane@example.org" is actually a shorthand for "the author of the book is someone whose email address is mailto:jane@example.org". Using a blank node to represent this "someone" is just a more accurate way to represent the real world situation. (Incidentally, some RDF-based schema languages allow specifying that certain properties are unique identifiers. This is discussed further in Section 5.5.)

2.5 Typed Literals

In the last section, we described how to handle situations in which we needed to take property values represented by character string literals, and break them up into structured values that identify the individual parts of those property values. Using this approach, instead of, say, recording the date a Web page was created as a single exterms:creation-date property, with a single character string literal as its value, we could represent the value as a structure consisting of the month, day, and year as separate pieces of information. However, so far, we've followed the practice of representing any constant values that serve as objects in RDF statements by these simple untyped literals, even when we probably intend for the value of the property to be a number (e.g., the value of a year or age property) or some other kind of more specialized value.

For example, earlier in Figure 3, we illustrated an RDF graph recording information about John Smith. In that graph, we recorded the value of John Smith's exterms:age property as the literal "27", as shown in Figure 6:

Representing John Smith's Age
Figure 6: Representing John Smith's Age

In this case, our hypothetical organization example.org probably intends for "27" to be interpreted as a number, rather than as the string consisting of the character "2" followed by the character "7". However, an application reading that literal "27" would only know how to do that if the application was explicitly given the information that the literal "27" was intended to represent a number, and knew which number the literal "27" was supposed to represent. The common practice in programming languages or database systems is to provide this kind of information by associating a datatype with the literal, in this case, a datatype like decimal or integer. An application that understands the datatype then knows, for example, whether the literal "10" is intended to represent the number ten, the number two, or the string consisting of the character "1" followed by the character "0", depending on whether the specified datatype is integer, binary, or string. In RDF, typed literals are used to provide this kind of information.

Using a typed literal, we could describe John Smith's age as being the integer number 27 using the triple:

<http://www.example.org/staffid/85740>  <http://www.example.org/terms/age> "27"^^<http://www.w3.org/2001/XMLSchema#integer> .

or, using our QName simplification for writing long URIs:

exstaff:85740  exterms:age  "27"^^xsd:integer .

or as shown in Figure 7:

A Typed Literal for John Smith's Age
Figure 7: A Typed Literal for John Smith's Age

Similarly, in the graph shown in Figure 2 describing information about a Web page, we recorded the value of the page's exterms:creation-date property as the character string literal "August 16, 1999". However, using a typed literal, we could describe the creation date of the Web page as being the date August 16, 1999, using the triple:

ex:index.html  exterms:creation-date  "1999-08-16"^^xsd:date .

or as shown in Figure 8:

A Typed Literal for a Web Page's Creation Date
Figure 8: A Typed Literal for a Web Page's Creation Date

As these examples illustrate, an RDF typed literal is formed by explicitly pairing a URIref identifying a particular datatype (in these examples, the datatypes integer and date from XML Schema Part 2: Datatypes [XML-SCHEMA2]) with a literal that the datatype uses to represent the intended value. In each case, this results in a single node in the RDF graph with the pair as its label.

Unlike typical programming languages and database systems, RDF has no built-in set of datatypes of its own, such as datatypes for integers, reals, strings, or dates. Instead, it relies on datatypes defined elsewhere that can be identified by a URIref. RDF typed literals simply provide a way to explicitly indicate, for a given literal, what datatype should be used to interpret it. As far as RDF is concerned, you can write any pair of URIref and literal you want as a typed literal. This gives RDF the flexibility to directly represent information coming from different sources without the need to perform type conversions between these sources and a native set of RDF datatypes. (Type conversions would still be required when moving information between systems with different datatype systems, but RDF would impose no extra conversions into and out of a native set of RDF types.)

The actual interpretation of a typed literal (determining the value it denotes) must be performed by an RDF processor that is programmed to "understand" that datatype. In particular, we've used XML Schema datatypes in the two examples we've just presented, and will be using XML Schema datatypes in most of our other examples as well (for one thing, XML Schema data types have URIrefs we can use to refer to them, specified in [XML-SCHEMA2]). XML Schema datatypes have a "first among equals" status in RDF. They are treated no differently than any other datatype, but they are expected to be the most widely used, and therefore the most likely to be interoperable among different software. As a result, it is expected that many RDF processors will be programmed to recognize these datatypes. However, RDF software could be programmed to process other sets of datatypes as well.

RDF datatype concepts borrow a conceptual framework from XML Schema datatypes [XML-SCHEMA2] to more precisely describe these datatype requirements. RDF's use of this framework is defined in RDF Concepts and Abstract Syntax [RDF-CONCEPTS].

@@Framework discussion removed@@

The flexibility provided by RDF typed literals comes at a price. For one thing, RDF has no way of knowing whether or not a URIref in a typed literal actually identifies a datatype. Moreover, even when a URIref does identify a datatype, RDF itself does not define the validity of pairing that datatype with a particular literal. This validity can only be determined by software built to understand that datatype. For example, you could write the triple:

exstaff:85740  exterms:age  "pumpkin"^^xsd:integer .

or the graph shown in Figure 9:

An Invalid Typed Literal for John Smith's Age
Figure 9: An Invalid Typed Literal for John Smith's Age

The typed literal in Figure 9 is valid RDF, but obviously an error as far as the xsd:integer datatype is concerned, since "pumpkin" is not defined as being in the lexical space of xsd:integer.

In general, RDF software may be called on to process RDF data that contains datatypes that it has not been programmed to understand, in which case there are some things the software will not be able to do. This includes recognizing whether or not a particular string represents a legal value for a particular datatype. In this case, RDF software not built to understand the xsd:integer datatype would not be able to recognize that "pumpkin" is not a valid xsd:integer.

2.6. Concepts Summary

Taken as a whole, RDF is simple: nodes-and-arcs diagrams interpreted as statements about things identified by URIrefs. This section has presented an introduction to these concepts. The normative (i.e., definitive) RDF specification defining these concepts is the RDF Concepts and Abstract Syntax [RDF-CONCEPTS], which should be consulted for further information. Together with the RDF Semantics [RDF-SEMANTICS], [RDF-CONCEPTS] provides the definition of the abstract syntax for RDF, together with its formal semantics (meaning). Additional background on the basic ideas underlying RDF, and its role in providing a general language for describing Web information, can be found in [WEBDATA].

@@Following para moved here from Concepts document sect 1.2 "Background reading", and needs to be fit in.@@

RDF draws upon ideas from knowledge representation, artificial intelligence and data management, including from Conceptual Graphs, logic-based knowledge representation, frames, and relational databases. Some possible sources of background information are [Sowa], [CG], [KIF], [Hayes], [Luger], [Gray].

@@Original text continues.@@

However, in addition to the basic techniques for representing RDF statements in diagrams (or triples) we've seen so far, it should be clear that we also need a way for people to define the vocabularies they intend to use in those statements, including:

  • defining types of things (like ex:Person)
  • defining properties (like ex:age and creation-date), and
  • defining the types of things that can serve as the subjects or objects of statements involving those properties (such as specifying that the value of an ex:age property should always be an xsd:integer).

The basis for describing such vocabularies in RDF is the RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY], which will be described in Section 5.

3. An XML Syntax for RDF: RDF/XML

As we described in Section 2, RDF's conceptual model is a graph. RDF provides an XML syntax for writing down and exchanging RDF graphs, called RDF/XML. Unlike triples, which are intended as a shorthand notation, RDF/XML is the normative syntax for writing RDF. RDF/XML is defined in the RDF/XML Syntax Specification [RDF-SYNTAX]. This section describes this RDF/XML syntax.

3.1. Basic Principles

We can illustrate the basic ideas behind the RDF/XML syntax using some of the examples we've presented already. Suppose we want to represent one of our initial statements:

http://www.example.org/index.html has a creation-date whose value is August 16, 1999

The RDF graph for this single statement, after assigning a URIref to the creation-date property, is shown in Figure 10:

with a triple representation of:

ex:index.html  exterms:creation-date  "August 16, 1999" .

Corresponding RDF/XML syntax for the graph in Figure 6 would be:

1. <?xml version="1.0"?>
2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.             xmlns:exterms="http://www.example.org/terms/">

4.   <rdf:Description rdf:about="http://www.example.org/index.html">
5.       <exterms:creation-date>August 16, 1999</exterms:creation-date>
6.   </rdf:Description>

7. </rdf:RDF>

(we have added line numbers to use in explaining the example).

This seems like a lot of overhead. We can understand better what is going on by considering each part of this XML in turn.

@@may be too much XML tutorial material in discussing lines 1, 2, and 3@@

Line 1, <?xml version="1.0"?>, is the XML declaration, which indicates that the following content is XML, and what version of XML it is.

Line 2 begins an rdf:RDF element. This indicates that the following XML content (starting here and ending with the </rdf:RDF> in Line 7) is intended to represent RDF. Following the rdf:RDF on this same line is an XML namespace declaration, represented as an xmlns attribute of the rdf:RDF start-tag. This declaration specifies that all tags in this content prefixed with rdf: are part of the namespace identified by the URIref http://www.w3.org/1999/02/22-rdf-syntax-ns#. This namespace is the source for the RDF-specific terms used in RDF/XML.

Line 3 specifies another XML namespace declaration, this time for the prefix exterms:. This is expressed as another xmlns attribute of the rdf:RDF element, and specifies that the namespace URIref http://www.example.org/terms/ is to be associated with the exterms: prefix. This namespace is the source for the specific terms defined by our example organization, example.org. The ">" at the end of line 3 indicates the end of the rdf:RDF start-tag. Lines 1-3 are general "housekeeping" necessary to indicate that we are defining RDF/XML content, and to identify the sources of the terms we are using.

Lines 4-6 provide the RDF/XML for the specific statement we're representing. An obvious way to talk about any RDF statement is to say it's a description, and that it's about the subject of the statement (in this case, about http://www.example.org/index.html), and this is the way RDF/XML represents the statement. The rdf:Description start tag in Line 4 indicates that we're starting a description of a resource, and goes on to identify the resource the statement is about (the subject of the statement) using the rdf:about attribute to specify the URIref of the subject resource. Line 5 provides a property element, with the QName <exterms:creation-date> as its tag, to hold the string literal August 19, 1999 of the creation-date property of the statement. It is nested within the containing rdf:Description element, indicating that this property applies to the resource specified in the rdf:about attribute of the rdf:Description element. The URIref of the creation-date property corresponding to the QName <exterms:creation-date> is obtained by appending the name creation-date to the URI of the exterms: prefix (http://www.example.org/terms/), giving http://www.example.org/terms/creation-date. Line 6 indicates the end of this particular rdf:Description element.

Finally, Line 7 indicates the end of the rdf:RDF element started on Line 2.

This example illustrates the basic ideas used by RDF/XML to encode an RDF graph as XML elements, attributes, element content, and attribute values. The URIref labels for properties and object nodes are written as XML QNames, consisting of a short prefix denoting a namespace URI, together with a local name denoting a namespace-qualified element or attribute, as described in Section 2.2. The (namespace URIref, local name) pair are chosen so that concatenating them forms the original node URIref. The URIrefs of subject nodes are stored in XML attribute values. The nodes labeled by character string literals (which are always object nodes) become element text content or attribute values.

@@In above, Section 2.2 now an appendix.@@

We could represent an RDF graph consisting of multiple statements in RDF/XML by using RDF/XML similar to Lines 4-6 in the previous example to separately represent each statement. For example, if we wanted to write the two statements:

ex:index.html  exterms:creation-date  "August 16, 1999" .
ex:index.html  exterms:language "English" .

we could write the RDF/XML as:

1.  <?xml version="1.0"?>
2.  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:exterms="http://www.example.org/terms/">

4.    <rdf:Description rdf:about="http://www.example.org/index.html">
5.        <exterms:creation-date>August 16, 1999</exterms:creation-date>
6.    </rdf:Description>

7.    <rdf:Description rdf:about="http://www.example.org/index.html">
8.        <exterms:language>English</exterms:language>
9.    </rdf:Description>

10. </rdf:RDF>

This is the same as our initial example, with the addition of lines 7-9, a second rdf:Description element to represent the second statement. We could represent an arbitrary number of additional statements in the same way, using a separate rdf:Description element for each additional statement. As this example illustrates, once the overhead of writing the XML and namespace declarations is dealt with, writing each additional RDF statement in RDF/XML is both straightforward and not too complicated.

The RDF/XML syntax provides several abbreviations to make common uses easier to write. For example, it is typical for the same resource to be described with several properties and values at the same time, as in the example above. To handle this case, RDF/XML allows multiple property elements representing those properties to be nested within the rdf:Description element that identifies the subject resource. For example, if we wanted to represent our previous group of statements about http://www.example.org/index.html:

ex:index.html  dc:creator  exstaff:85740 .
ex:index.html  exterms:creation-date  "August 16, 1999" .
ex:index.html  exterms:language "English" .

whose graph (the same as Figure 2) is shown in Figure 11:

@@Syntax doc. Sec 2.3@@

Several Statements About the Same Resource
Figure 11: Several Statements About the Same Resource

the RDF/XML syntax for the graph shown in Figure 11 could be written as:

@@Syntax doc. Sec 2.4@@

1.  <?xml version="1.0"?>
2.  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:dc="http://purl.org/dc/elements/1.1/"
4.              xmlns:exterms="http://www.example.org/terms/">

5.    <rdf:Description rdf:about="http://www.example.org/index.html">
6.         <exterms:creation-date>August 16, 1999</exterms:creation-date>
7.         <exterms:language>English</exterms:language>
8.         <dc:creator rdf:resource="http://www.example.org/staffid/85740"/>
9.    </rdf:Description>

10. </rdf:RDF>

(we have added line numbers again to use in explaining the example).

Compared with the previous two examples, we've added an additional namespace declaration (in Line 3), and an additional creator property element (in Line 8). In addition, we've nested the three property elements whose subject is http://www.example.org/index.html within a single rdf:Description element identifying that subject, rather than writing a separate rdf:Description element for each statement.

Line 8 also introduces a new form of property element. (The element tag also uses a different namespace prefix, the new namespace prefix dc: we defined in Line 3.) The exterms:language element in Line 7 is similar to the exterms:creation-date element we defined in the first example. Both these elements represent properties with character strings as property values, and such elements are specified by enclosing the character string within start- and end-tags corresponding to the property name. However, the dc:creator element on Line 8 represents a property whose value is another resource, rather than a character string. If we had written the URIref of this resource as a literal string within start- and end-tags in the same way as we wrote the literal values of the other elements, we would be saying that the value of the dc:creator element was the character string http://www.example.org/staffid/85740, rather than the resource identified by that string interpreted as a URIref. In order to indicate the difference, we've written the dc:creator element using what XML calls an empty element (it has no separate end tag), and defined the property value using an rdf:resource attribute within that empty element. The rdf:resource attribute indicates that its value is another resource, identified by its URIref. Because the URIref is being used as an attribute value, RDF/XML requires that we write out the full URIref, rather than abbreviating it as a QName, as we've done in writing element and attribute names.

It is important to understand that the RDF/XML in the example above is an abbreviation. The RDF/XML below, in which each statement is written separately, describes exactly the same RDF graph:

 <?xml version="1.0"?>
 <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

   <rdf:Description rdf:about="http://www.example.org/index.html">
       <exterms:creation-date>August 16, 1999</exterms:creation-date>

   <rdf:Description rdf:about="http://www.example.org/index.html">

   <rdf:Description rdf:about="http://www.example.org/index.html">
       <dc:creator rdf:resource="http://www.example.org/staffid/85740"/>


We will describe a few additional RDF/XML abbreviations in the following sections. However, you should consult [RDF-SYNTAX] for a more thorough description of the abbreviations that are available.

RDF/XML also allows us to represent graphs that include nodes that have no URIrefs, i.e., blank nodes. For example, Figure 12 (taken from [RDF-SYNTAX]) shows a graph saying "the document 'http://www.w3.org/TR/rdf-syntax-grammar' has a title 'RDF/XML Syntax Specification (Revised)' and has an editor, the editor has a name 'Dave Beckett' and a home page 'http://purl.org/net/dajobe/' ".

A Graph Containing a Blank Node
Figure 12: A Graph Containing a Blank Node

This illustrates an idea we discussed near the end of Section 2: the use of a blank node to represent something that does not have a URIref, but can be described in terms of other information. In this case, the blank node represents a person, the editor of the document, and the person is described by his name and home page.

RDF/XML provides several ways to represent blank nodes. These are described in [RDF-SYNTAX]. The approach we will illustrate here, and the most general approach, is to assign a blank node identifier (or bnodeID) to the blank node. A bnodeID serves to identify a blank node within a particular RDF/XML document but, unlike a URIref, is unknown outside the document in which it is assigned. A bnodeID is assigned to a blank node using an rdf:nodeID attribute in the rdf:Description element that describes the blank node. Using this approach, RDF/XML corresponding to Figure 12 could be written as follows:

@@Syntax doc. Sec 2.10@@

1.  <?xml version="1.0"?>
2.  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:dc="http://purl.org/dc/elements/1.1/"
4.              xmlns:exterms="http://example.org/stuff/1.0/">

5.     <rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
6.       <dc:title>RDF/XML Syntax Specification (Revised)</dc:title>
7.       <exterms:editor rdf:nodeID="abc"/>
8.     </rdf:Description>

9.     <rdf:Description rdf:nodeID="abc">
10.        <exterms:fullName>Dave Beckett</exterms:fullName>
11.        <exterms:homePage rdf:resource="http://purl.org/net/dajobe/"/>
12.    </rdf:Description>

13. </rdf:RDF>

In this example, the bnodeID is assigned to the blank node in Line 9, and used to reference it in Line 7 The advantage of using a bnodeID over some of the other approaches described in [RDF-SYNTAX] is that using a bnodeID allows the same blank node to be referred to in more than one place in the same RDF/XML document.

Finally, the typed literals we described in Section 2.5 may be used as property values instead of the character string literals we have used in the examples so far. A typed literal is represented in RDF/XML by adding an rdf:datatype attribute specifying a datatype URIref to the property element containing the literal.

@@Syntax doc. Sec 2.9@@

For example, to change the statement shown in Figure 10 to use a typed literal instead of a character literal for the creation-date property, the triple representation might be:

ex:index.html  exterms:creation-date  "1999-08-16"^^xsd:date .

and the corresponding RDF/XML syntax would be:

1. <?xml version="1.0"?>
2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.             xmlns:exterms="http://www.example.org/terms/">

4.   <rdf:Description rdf:about="http://www.example.org/index.html">
5.     <exterms:creation-date rdf:datatype=
6.   </rdf:Description>

7. </rdf:RDF>

In Line 5, a typed literal is given as the value of the ex:creation-date property element by adding an rdf:datatype attribute to the element's start-tag to specify the datatype. The value of this attribute is the URIref of the datatype, in this case, the URIref of the XML Schema date datatype. Since this is an attribute value, the full URIref must be written out, rather than using the QName abbreviation xsd:date that we used in the triple. A literal appropriate to this datatype is then written as the element content, in this case, the literal 1999-08-16, which is the literal representation for August 16, 1999 in the XML Schema date datatype.

For the most part, we will continue to use XML-style (untyped) character literals in our examples. However, you should be aware that typed literals from appropriate datatypes, such as XML Schema datatypes, can always be used instead.

The facilities we have illustrated so far provide a simple but general way to serialize graphs in RDF/XML. Using these facilities, an RDF graph is written in RDF/XML as follows:

  • All blank nodes are assigned blank node identifiers.
  • Each node is listed in turn as the subject of an un-nested rdf:Description element, using an rdf:about attribute if the node has a URIref, or an rdf:nodeID attribute if the node is blank.
    For each triple with this node as subject, an appropriate property element is created, with either literal content (possibly empty), an rdf:resource attribute specifying the object of the triple (if the object node has a URIref), or an rdf:nodeID attribute specifying the object of the triple (if the object node is blank).

Compared to some of the serialization approaches described in [RDF-SYNTAX], this simple serialization approach provides the most direct representation of the actual graph structure, and is particularly recommended for applications in which the output RDF/XML is to be used in further RDF processing.

3.2. Defining New RDF Resources

@@These aren't really *new* resources; look at Dave and Brian comments@@

So far, we've been describing resources that we imagine have been defined (and given URIrefs) already. For instance, in our initial examples, we've been providing descriptive information about example.org's web page, whose URIref was http://www.example.org/index.html. We referred to this resource (defined elsewhere) using an rdf:about attribute. However, obviously we also want to be able to introduce new resources. For example, suppose a company, example.com, wanted to provide an RDF-based catalog of its products as an RDF/XML document, identified by (and located at) http://www.example.com/2002/04/products. Within that resource, each product might be given a separate RDF description. This catalog, along with one of these descriptions (the catalog entry for a model of tent called the "Overnighter") might be written:

@@Syntax doc. Sec 2.14@@

1.   <?xml version="1.0"?>
2.   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.               xmlns:exterms="http://www.example.com/terms/">

4.     <rdf:Description rdf:ID="item10245">
5.          <exterms:model>Overnighter</exterms:model>
6.          <exterms:sleeps>2</exterms:sleeps>
7.          <exterms:weight>2.4</exterms:weight>
8.          <exterms:packedSize>14x56</exterms:packedSize>
9.     </rdf:Description>

  ...other product descriptions...

10.  </rdf:RDF>

(We've included the surrounding xml, RDF, and namespace information in lines 1 through 3, and line 10, but this information would only need to be defined once for the whole catalog, not repeated for each entry in the catalog).

This is similar to our previous examples in the way it represents the properties (model, sleeping capacity, weight) of the resource (the tent) being described. However, in line 4, the rdf:Description element has an rdf:ID attribute instead of an rdf:about attribute. Using rdf:ID indicates that we are using a fragment identifier, given by the value of the rdf:ID attribute ("item10245" in this case, which might be the catalog number used by example.com), as a shorthand for the complete URIref of the resource we want to describe. This fragment identifier item10245 will be interpreted relative to a base URI, in this case, the URI of the containing catalog. The full URIref for the tent is formed by taking the base URI (of the catalog), and appending #item10245 to it, giving the URIref http://www.example.com/2002/04/products#item10245.

The rdf:ID attribute is somewhat similar to the ID attribute in XML and HTML, in that it defines a label which can be used to refer to this resource. This label must be unique within the resource (in this case, the catalog) in which it is defined. Any other RDF within this catalog could refer to this resource (this particular catalog entry) by using the relative URIref #item10245 in a rdf:about attribute. This would be understood to refer to another resource defined within the catalog. We could also have introduced the URIref of the catalog entry itself by specifying rdf:about="#item10245" instead of rdf:ID="item10245" (i.e., by specifying the relative URIref directly). The two forms are essentially synonyms: the full URIref formed by RDF is the same in either case: http://www.example.com/2002/04/products#item10245.

RDF located outside the catalog could refer to this catalog entry by using the full URIref, i.e., by concatenating the relative URIref #item10245 of the catalog entry to the base URI of the catalog, forming the absolute URIref http://www.example.com/2002/04/products#item10245. For example, an outdoor sports web site exampleRatings.com might use RDF to provide ratings of various tents. The (5-star) rating given to the tent we described earlier might then be represented on exampleRatings.com's web site as:

1.  <?xml version="1.0"?>
2.  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:sportex="http://www.exampleRatings.com/terms/">

4.    <rdf:Description rdf:about="http://www.example.com/2002/04/products#item10245">
5.         <sportex:ratingBy>Richard Roe</sportex:ratingBy>
6.         <sportex:numberStars>5</sportex:numberStars>
7.    </rdf:Description>
8.  </rdf:RDF>

In this example, line 4 uses an rdf:Description element with an rdf:about attribute whose value is the full URIref of the tent's catalog entry, defined by the earlier RDF description. The use of this URIref allows the tent being referred to in the rating to be precisely identified.

This example not only shows how new resources can be defined in RDF/XML; it also illustrates one of the basic architectural principles of the Web, which is that anyone should be able say anything they want about existing resources [BERNERS-LEE98]. The example also illustrates that the RDF describing a particular resource does not need to be located all in one place; instead, it may be distributed throughout the web. This is true not only for examples like this one, in which one organization is rating or commenting on resources defined by another, but also for situations in which the original creator of a resource (or anyone else) wishes to amplify the description of that resource by providing additional information about it. This may be done either by modifying the original document in which the resource was defined, to add the properties and values needed to describe the additional information, or, as this example illustrates, by creating a separate document, and providing the additional properties and values in rdf:Description elements that refer to the original resource using rdf:about.

The previous example indicated that fragment identifiers such as #item10245 will be interpreted relative to a base URI. By default, this base URI would be the URI of the resource in which the fragment is used. However, in some cases it is desirable to be able to explicitly specify this base URI. For instance, suppose that in addition to the catalog located at http://www.example.com/2002/04/products, example.org wanted to provide a duplicate catalog on a mirror site, say at http://mirror.example.com/2002/04/products. This could create a problem, since if the catalog was retrieved from the mirror site, the URIref generated for our example tent would be http://mirror.example.com/2002/04/products#item10245, rather than http://www.example.com/2002/04/products#item10245, and hence apparently a different tent. To deal with this problem, RDF/XML supports XML Base [XML-BASE], which allows an XML document to specify a base URI other than the URI of the document itself. In this case, we would define the catalog as:

1.   <?xml version="1.0"?>
2.   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.               xmlns:exterms="http://www.example.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products">

5.     <rdf:Description rdf:ID="item10245">
6.          <exterms:model>Overnighter</exterms:model>
7.          <exterms:sleeps>2</exterms:sleeps>
8.          <exterms:weight>2.4</exterms:weight>
9.          <exterms:packedSize>14x56</exterms:packedSize>
10.    </rdf:Description>

  ...other product descriptions...

11.  </rdf:RDF>

The xml:base declaration in line 4 specifies that the base URI for the content within the rdf:RDF element (until another xml:base attribute is specified) is http://www.example.com/2002/04/products, and all relative URIrefs cited within that content will be interpreted relative to that base, no matter where the actual content is located. As a result, the relative URIref of our tent, #item10245, will generate the same absolute URIref, http://www.example.com/2002/04/products#item10245, no matter where the catalog is located.

So far, we've been talking about a single product description, a particular model of tent, from example.com's catalog. However, example.com will probably offer several different models of tents, as well as multiple instances of other categories of products, such as backpacks, hiking boots, and so on. This idea of instances of things that can be classified into different kinds or categories is similar to the programming language concept of objects having different types or classes. RDF supports this concept by providing a predefined property, rdf:type. When an RDF resource is defined as having an rdf:type property, the value of that property is considered to be a resource that defines a category or class of things, and the original resource is considered to be an instance of that category or class. Using rdf:type, example.com might indicate that our product description is that of a tent as follows:

1.   <?xml version="1.0"?>
2.   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.               xmlns:exterms="http://www.example.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products">

5.     <rdf:Description rdf:ID="item10245">
6.          <rdf:type rdf:resource="http://www.example.com/terms/Tent" />
7.          <exterms:model>Overnighter</exterms:model>
8.          <exterms:sleeps>2</exterms:sleeps>
9.          <exterms:weight>2.4</exterms:weight>
10.         <exterms:packedSize>14x56</exterms:packedSize>
11.    </rdf:Description>

  ...other product descriptions...

12.  </rdf:RDF>

Note the use of the rdf:type property to indicate that the instance belongs to class Tent. In this case, we imagine that example.com has defined its classes as part of the same vocabulary that it uses to describe its other terms (such as the property exterms:weight), so we use the absolute URIref of the class to refer to it. If example.com had defined these classes in the product catalog itself, we could have used the relative URIref #Tent to refer to it.

RDF itself does not define a vocabulary for defining application-specific classes of things, like Tent in this example. Instead, such classes would be defined in an RDF Schema. The RDF Schema vocabulary is described in Section 5. Other vocabularies for defining classes can also be defined, such as the DAML+OIL and OWL languages described in Section 5.5. In addition, RDF defines several pre-defined types of its own for various purposes. These will be described in Section 4.

Since defining resources as instances of specific types is fairly common, the RDF/XML syntax provides a special abbreviation for instances defined as members of classes using the rdf:type property. In this abbrevation, the rdf:type property and value are removed, and the rdf:Description element name is replaced by the class name. Using this abbreviation, example.com's tent from the example above could also be defined as:

@@Syntax doc. Sec 2.13; introduce term "typed node"@@

1.   <?xml version="1.0"?>
2.   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.               xmlns:exterms="http://www.example.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products">

5.     <exterms:Tent rdf:ID="item10245">
6.          <exterms:model>Overnighter</exterms:model>
7.          <exterms:sleeps>2</exterms:sleeps>
8.          <exterms:weight>2.4</exterms:weight>
9.          <exterms:packedSize>14x56</exterms:packedSize>
10.    </exterms:Tent>

  ...other product descriptions...

11.  </rdf:RDF>

Both this abbreviation and the previous description of the tent (using the full <rdf:Description rdf:ID="10245"> element) illustrate that RDF statements can be written in RDF/XML in a way that closely resembles the descriptions that might have been written directly in XML. This is an important consideration, given the increasing use of XML in all kinds of applications, since it suggests that RDF could be used in these applications without major changes in information structure being required, and that much deployed XML can be interpreted as RDF statements.

@@Note often need to add an identifier to XML as subject; clarify how XML can be interpreted as RDF.@@

3.3. RDF/XML Summary

The examples above have illustrated some of the basic ideas behind the RDF/XML syntax. For a discussion of the basic principles behind the modeling of RDF statements in XML (known as striping), and other details and examples about writing RDF in XML, refer to the RDF/XML Syntax Specification [RDF-SYNTAX].

@@mention other abbrevations covered in syntax; more nested forms@@

4. Other RDF Capabilities

RDF provides a number of additional capabilities, including some built-in types and properties for representing groups of resources and RDF statements, and capabilities for deploying RDF information in the World Wide Web. These additional capabilities are described in the following sections.

4.1. RDF Containers

There is often a need to describe groups of things. For example, we might want to say that a book was created by several authors, or to list the students in a course, or the software modules in a package. RDF provides several pre-defined types and properties that can be used to describe such groups.

First, RDF provides three predefined types (together with some associated predefined properties) for describing containers. A container is a resource that contains things. The contained things are called members. The members of a container may be resources or literals. RDF defines three types of containers:

  • rdf:Bag
  • rdf:Seq
  • rdf:Alt

A Bag (a resource having type rdf:Bag) is a group of resources or literals, possibly including duplicate members, where there is no significance in the order of the members. For example, a Bag might be used to describe a group of part numbers in which the order of entry or processing of the part numbers does not matter.

A Sequence or Seq (a resource having type rdf:Seq) is a group of resources or literals, possibly including duplicate members, where the order of the members is significant. For example, a Sequence might be used to describe a group that must be maintained in alphabetical order.

An Alternative or Alt (a resource having type rdf:Alt) is a group of resources or literals that are alternatives (typically for a single value of a property). For example, an Alt might be used to describe alternative language translations for the title of a book, or to describe a list of alternative Internet sites at which a resource might be found. An application using a property whose value is an Alt container should be aware that it can choose any one of the members of the group as appropriate.

To describe a resource as being one of these types of containers, you give the resource an rdf:type property whose value is one of the pre-defined resources rdf:Bag, rdf:Seq, or rdf:Alt (whichever is appropriate). The container resource (which may either be a blank node or a resource with a URIref) denotes the group as a whole. The members of the container can be described by defining a container membership property for each member with the container resource as its subject and the member as its object. These membership properties have names of the form rdf:_n, where n is an integer, e.g., rdf:_1, rdf_2, rdf_3, and so on, and are used specifically for describing the members of containers. Container resources may also have other properties that describe the container, in addition to the container membership properties and the rdf:type property.

It is important to understand that while these types of containers are described using pre-defined RDF types and properties, any special meanings associated with these containers, e.g., that the members of an Alt container are alternative values, are only intended meanings. These specific container types, and their definitions, are provided with the aim of establishing a shared convention among those who need to describe groups of things. All RDF does is provide the types and properties that can be used to construct the RDF graphs to describe each type of container. RDF has no more built-in understanding of what a resource of type rdf:Bag is than it has of what a resource of type ex:Tent, that we discussed in Section 3.2, is. In each case, applications must be written to behave according to the particular meaning involved for each type. This point will be expanded on in the following examples.

A typical use of a container is to indicate that the value of a property is a group of things. For example, to represent the sentence "Course 6.001 has the students Amy, Tim, John, Mary, and Sue", you could describe the course by giving it a s:students property whose value is a container of type rdf:Bag (the group of students) and then, using the container membership properties, describe the individual students as being members of that container, as in the RDF graph shown in Figure 14:

A Simple Bag Container
Figure 14: A Simple Bag Container

Since the value of the s:students property in this example is described as a Bag, there is no intended significance in the order given for the URIrefs of each student, even though the properties in the graph have integers in their names. It is up to applications creating and processing graphs that include rdf:Bag containers to ignore any (apparent) order in the names of the membership properties.

RDF/XML provides some special syntax and abbreviations to make it simpler to describe such containers. For example, the following RDF/XML describes the graph shown in Figure 14:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

   <rdf:Description rdf:about="http://example.edu/courses/6.001">
            <rdf:li rdf:resource="http://example.edu/students/Amy"/>
            <rdf:li rdf:resource="http://example.edu/students/Tim"/>
            <rdf:li rdf:resource="http://example.edu/students/John"/>
            <rdf:li rdf:resource="http://example.edu/students/Mary"/>
            <rdf:li rdf:resource="http://example.edu/students/Sue"/>

Note that RDF/XML provides li as a convenience element to avoid having to explicitly number each membership property. The numbered properties rdf:_1, rdf:_2, and so on are generated from the li elements in forming the corresponding graph. The element name li was chosen to be mnemonic with the term "list item" from HTML. Note also the use of a <rdf:Bag> element within the <s:students> property element. The <rdf:Bag> element is an example of the typed node we saw earlier in Section 3.2, and is an abbreviation of an rdf:Description element, together with an rdf:type property, describing the Bag. Since no URIref is specified, the Bag is a blank node. Its nesting within the <s:students> property element is an abbreviated way of indicating that the blank node is the value of this property. These abbreviations are described further in [RDF-SYNTAX].

The graph structure for an rdf:Seq container, and the corresponding RDF/XML, are similar to those for an rdf:Bag (the only difference is in the type, rdf:Seq). Once again, although an rdf:Seq container is intended to describe a sequence, it is up to applications creating and processing the graph to appropriately interpret the sequence of integer-valued property names.

As an illustration of an Alt container, the sentence "The source code for X11 may be found at ftp.example.org, ftp.example1.org, or ftp.example2.org" could be expressed in the RDF graph shown in Figure 15:

A Simple Alt Container
Figure 15: A Simple Alt Container

The graph in Figure 15 could be written in RDF/XML as:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

   <rdf:Description rdf:about="http://example.org/packages/X11">
            <rdf:li rdf:resource="ftp://ftp.example.org"/>
            <rdf:li rdf:resource="ftp://ftp.example1.org"/>
            <rdf:li rdf:resource="ftp://ftp.example2.org"/>

An Alt container is intended to have at least one member, identified by the property rdf:_1. This member is intended to be considered as the default or preferred value. Other than the member identified as rdf:_1, the order of the remaining elements is not significant.

The RDF in Figure 15 as written states simply that the value of the s:DistributionSite site property is the Alt container resource itself. Any additional meaning that is to be read into this graph, e.g., that one of the members of the Alt container is to be considered as the value of the s:DistributionSite site property, or that ftp://ftp.example.org is the default or preferred value, must be built into an application's understanding of how an Alt is intended to behave, and/or into the meaning defined for the particular property (s:DistributionSite in this case), which also must be understood by the application.

Alt containers are frequently used in conjunction with language tagging. For example, a work whose title has been translated into several languages might have its Title property pointing to an Alt container holding each of the language variants.

The distinction between the intended meanings of a Bag and an Alt can be further illustrated by considering the authorship of the book "Huckleberry Finn". The book has exactly one author, but the author has two names (Mark Twain and Samuel Clemens). Either name is sufficient to specify the author. Thus using an Alt container of the author's names more accurately represents the relationship than using a Bag (which might suggest there are two different authors).

Users are free to choose their ways to describe groups of resources, rather than using the ones described here. These RDF containers are merely provided as common definitions that, if generally used, could help make data involving groups of resources more interoperable.

Sometimes there are clear alternatives to using these RDF container types. For example, a relationship between a particular resource and a group of other resources could be indicated by making the first resource the subject of multiple statements using the same property. This is structurally not the same as the resource being the subject of a single statement whose object is a container containing multiple members. In some cases, these two structures may have equivalent meaning, but in other cases they may not. The choice of which to use in a given situation should be made with this in mind.

Consider as an example the relationship between a writer and her publications. We might have the sentence:

Sue has written "Anthology of Time", "Zoological Reasoning", and "Gravitational Reflections".

In this case, there are three resources each of which was written independently by the same writer. This could be expressed using repeated properties as:

exstaff:Sue exterms:publication ex:AnthologyOfTime .
exstaff:Sue exterms:publication ex:ZoologicalReasoning .
exstaff:Sue exterms:publication ex:GravitationalReflections .

In this example there is no stated relationship between the publications other than that they were written by the same person. Each of the statements is an independent fact, and so using repeated properties would be a reasonable choice. However, this could just as reasonably be represented as a statement about the group of resources written by Sue:

exstaff:Sue exterms:publication _:z
_:z rdf:type rdf:Bag .
_:z rdf:_1 ex:AnthologyOfTime .
_:z rdf:_2 ex:ZoologicalReasoning .
_:z rdf:_3 ex:GravitationalReflections .

On the other hand, the sentence:

The resolution was approved by the Rules Committee, having members Fred, Wilma, and Dino.

says that the committee as a whole approved the resolution; it does not necessarily state that each committee member individually voted in favor of the resolution. In this case, it would be potentially misleading to model this sentence as three separate exterms:approvedBy statements, one for each committee member, as shown below:

ex:resolution exterms:approvedBy ex:Fred .
ex:resolution exterms:approvedBy ex:Wilma .
ex:resolution exterms:approvedBy ex:Dino .

since these statements say that each member individually approved the resolution.

In this case, it would be better to model the sentence as a single exterms:approvedBy statement whose subject is the resolution and whose object is the committee itself. The committee resource could then be described as a Bag whose members are the members of the committee, as in the following triples:

ex:resolution exterms:approvedBy ex:rulesCommittee
ex:rulesCommittee rdf:type rdf:Bag .
ex:rulesCommittee rdf:_1 ex:Fred .
ex:rulesCommittee rdf:_2 ex:Wilma .
ex:rulesCommittee rdf:_3 ex:Dino .

Finally, when using these RDF containers, it is important to understand that you are not constructing containers, as you would a programming language data structure; instead, you are describing containers (groups of things) that actually exist. For instance, in the Rules Committee example just given, the Rules Committee is an unordered group of people, whether you describe it in RDF that way or not. When you give the Rules Committee resource an rdf:type property whose value is rdf:Bag, you are simply describing the Rules Committee as having whatever characteristics you associate with things of type rdf:Bag, not constructing a particular data structure to hold the members of the group (you could indicate that the Rules Committe was a Bag without describing any members at all). Similarly, when you use the container membership properties, you are simply describing a container resource as having certain things as members. You are not necessarily saying that the things that you describe as members are the only members that exist. For example, the triples given above to describe the Rules Committee say only that Fred, Wilma, and Dino are members of the Bag, not that they are the only members of the Bag.

4.2. RDF Collections

A limitation of the containers described in Section 4.1 is that there is no way to close them, i.e., to say "these are all the members of the container". This is because, while one graph may describe some of the members, there is no way to exclude the possibility that there is another graph somewhere that describes additional members. RDF provides support for describing groups containing only the specified members, in the form of RDF collections. An RDF collection is a group of things represented as a list structure in the RDF graph. This list structure is constructed using the predefined type rdf:List, the predefined properties rdf:first and rdf:rest, and the predefined resource rdf:nil.

To illustrate this, you could represent the sentence "The students in course 6.001 are Amy, Tim, and John" using the graph shown in Figure 16:

A Simple Alternative Container
Figure 16: An RDF Collection (list structure)

For each member of the collection, such as s:Amy, there is a corresponding resource of type rdf:List. This list resource is linked to the collection member by an rdf:first property, and to the rest of the list by an rdf:rest property. The end of the list is indicated by an rdf:rest property being the resource rdf:nil. This structure will be familiar to those who know the Lisp programming language. As in Lisp, the rdf:first and rdf:rest properties allow applications to traverse the structure.

RDF/XML provides a special notation to make it easier to describe collections. In RDF/XML, a collection is described by a property element that has the attribute rdf:parseType="Collection", and that contains a group of nested elements representing the members of the collection. The rdf:parseType="Collection" attribute indicates that the enclosed elements are to be used to create the corresponding list structure in the RDF graph.

To illustrate how this works, the following RDF/XML would result in the RDF graph shown in Figure 16:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

   <rdf:Description rdf:about="http://example.edu/courses/6.001">
      <s:students rdf:parseType="Collection" >
            <rdf:Description rdf:about="http://example.edu/students/Amy"/>
            <rdf:Description rdf:about="http://example.edu/students/Tim"/>
            <rdf:Description rdf:about="http://example.edu/students/John"/>

RDF defines no particular meaning for this structure; it is up to applications to interpret it.

@@Also need to explain that you can't really close the members, but the intent is to close them. It's more complicated to add additional members to a given list, but you could define another list (somewhere else). You need to be able to constrain the value of s:students to being a single node. Can't do this in RDF.@@

4.3. RDF Reification

RDF applications sometimes need to make statements about statements, for instance, to record information about when a statement was made, who made it, or other similar information. For example, consider a statement about the tent we discussed in Section 3:

product item10245 has a weight whose value is 2.4

with a triple representation of:

exproducts:item10245  exterms:weight  "2.4" .

Now, suppose we wanted to say in RDF that this statement was made by John Smith. Since in RDF we can only make statements about resources, what we would like to be able to do is write something like:

 [[exproducts:item10245  exterms:weight  "2.4" .]] dc:creator  exstaff:85740 .

That is, we want to be able to turn the original statement into a resource, so that we can make it the subject of another RDF statement that talks about it. RDF provides a built-in vocabulary for modeling statements as resources. This modeling is called reification in RDF, and a model of a statement is called the reification of the statement.

The RDF reification vocabulary consists of the type rdf:Statement, and the properties rdf:subject, rdf:predicate, and rdf:object. In this vocabulary, a triple of the form:

foo  rdf:type  rdf:Statement .

is a statement that the resource foo is an RDF triple in some RDF document. The three properties rdf:subject, rdf:predicate, and rdf:object, when applied to foo, then specify the subject, predicate, and object components of that triple foo.

Using this vocabulary, a reification of our original triple:

exproducts:item10245  exterms:weight  "2.4" .

is given by the graph:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 .
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .

(The node that is intended to refer to the first triple, the blank node _:xxx in the reification, could be either a blank node or a URIref.)

The intended interpretation of a reification like this is that _:xxx should be understood as referring to the original triple (as a whole), which is described by the subject, predicate, and object triples in the reification. So, using the reification, we would express the fact that the original statement was made by John Smith using the graph:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 . 
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .
_:xxx dc:creator exstaff:85740 . 

Note that the intended interpretation is that the triple that _:xxx refers to is a particular instance of a triple in a particular RDF document, rather than some arbitrary triple having the same subject, predicate, and object. There could be several such triples that have the same subject, predicate and object properties. Although a graph is defined as a set of triples, several instances with the same triple structure might occur in different documents. Thus, without this understanding, it would be meaningful to claim that _:xxx does not refer to the triple in the first graph, but to some other triple with the same structure. This particular interpretation of reification is used because reification is intended to be used to express properties such as dates of composition and source information, as in our example, and these properties need to be applied to specific instances of triples.

Note also that the assertion of the reified statement is not the same as the assertion of the original statement, and neither implies the other. That is, when someone asserts that John said foo, they are not asserting foo themselves, just that John said it. Conversely, when someone asserts foo, they are not also asserting its reification, since by asserting foo they are not also saying that there are such things as statements that they intend to talk about.

We have referred to the intended interpretation of reification in the discussion above because, while this may be the interpretation that is generally intended when reification is used, RDF reification does not actually capture all this meaning. Specifically, RDF syntax by itself provides no way to "connect" an RDF triple to its reification. All that the graph:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 . 
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .
_:xxx dc:creator exstaff:85740 . 

actually says is, "there is a statement that has a subject exproducts:item10245, a predicate exterms:weight, and an object 2.4, and John made it". It does not say that that statement (referred to by _:xxx) is the same as some particular statement in some particular RDF document.

@@Brian couldn't understand whether the above para was getting at the statements/stating distinction. He also couldn't understand the para below. Perhaps what I need to do is add something like the following:

To see this, given the original triple:

exproducts:item10245  exterms:weight  "2.4" .

and a reification of it, together with an additional triple that says that associates John with the reification:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 .
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .
_:xxx dc:creator exstaff:85740 .

note that there is nothing that explicitly associates _:xxx with the original triple.

@@end new example@@

This does not mean that such "provenance" information cannot be expressed in RDF, just that it cannot be done using only the meaning RDF associates with the reification vocabulary. For example, if an RDF document (say, a Web page) has a URI, you could make statements about the resource identified by that URI and, based on some application-dependent understanding of how those statements should be interpreted, act as if those statements "distribute" over (apply equally to) all the statements in the document. Also, if some mechanism exists (outside of RDF) to assign URIs to individual RDF statements, then you could certainly make statements about those individual statements, using their URIs to identify them. In these cases, you would not need to use the reification vocabulary at all.

@@new example: To see this, if our original triple had a URI, say ex:statementfoo, then you could attribute that statement to John simply by the statement:

ex:statementfoo dc:creator exstaff:85740 .

with no use of the reification vocabulary.

In addition, you could use the reification vocabulary directly according to the intended interpretation described above, and have an application-dependent understanding as to how to associate specific triples with their intended reifications. However, other applications receiving this RDF would not necessarily share this application-dependent understanding, and thus would not necessarily interpret the graphs appropriately.

Finally, since the relation between triples and reifications of triples in any RDF graph or graphs need not be one-to-one, asserting a property about some resource described by a reification does not necessarily mean that the same property holds of another such resource, even if it has the same components. For example, given the following graph:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 . 
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .

_:yyy rdf:type rdf:Statement .
_:yyy rdf:subject exproducts:item10245 . 
_:yyy rdf:predicate exterms:weight . 
_:yyy rdf:object "2.4" .

_:xxx dc:creator exstaff:85740 .

it does not follow that:

_:yyy dc:creator exstaff:85740 .

4.4. Miscellaneous RDF Facilities

In addition to the RDF capabilities we've already described, RDF provides a number of other miscellaneous facilities. We cover these facilities in this section, along with some other topics which don't fit naturally into the other sections.

@@flatten this, making rdf:value into 4.4; can I have multiple anchors to preserve both miscellaneous and rdfvalue?@@

4.4.1 More on Structured Values: rdf:value

In Section 2.4, we noted that the RDF data model intrinsically supports only binary relations; that is, a statement specifies a relation between two resources. For example, the statement:

exstaff:85740  exterms:manager  exstaff:62345 .

states that the relation "manager" holds between two employees (presumably one manages the other).

However, in some cases we need to be able to represent information involving higher arity relations (relations between more than two resources) in RDF. We discussed one example of this in Section 2.4, where the problem was to represent the relationship between John Smith and his address information, and the value of John's address was a structured value of his street, city, state, and Zip. If we had tried to write this as a relation, we'd have seen that address was 5-ary relation of the form:

address(exstaff:85740, "1501 Grant Avenue", "Bedford", "Massachusetts", "01730")

We indicated that we can represent such structured information in RDF by considering the aggregate thing we want to talk about (here, the group of components representing John's address) as a separate resource, and then making separate statements about that new resource, as in the triples:

exstaff:85740  exterms:address  _:johnaddress .
_:johnaddress  exterms:street   "1501 Grant Avenue" .
_:johnaddress  exterms:city     "Bedford" .
_:johnaddress  exterms:state    "Massachusetts" .
_:johnaddress  exterms:Zip      "01730" .

(where _:johnaddress is the node identifier of the blank node resource representing John's address.)

This is a general way to represent any n-ary relation in RDF: you select one of the participants (John in this case) to serve as the subject of the main relation (address in this case). You then specify an intermediate resource to represent the rest of the relation (either with or without assigning it a URI), and then give that new resource properties representing the remaining components of the relation.

In the case of John's address, none of the individual parts of the structured value could be considered the "primary" value of the exterms:address property; all of the parts contribute equally to the value. However, in some cases one of the parts of the structured value is often thought of as the "primary" value, with the other parts of the relation providing additional contextual or other information that qualifies the primary value. For example, in our tent example in Section 3, we gave the weight of the particular tent we were describing as "2.4", i.e.,

exproduct:item10245  exterms:weight  "2.4" .

In fact, a more complete description of the weight would have been "2.4 kilograms" rather than just "2.4". To state this, the value of the exterms:weight property would need to have two components, the literal "2.4" and an indication of the unit of measure (kilograms). In this situation the literal "2.4" could be considered the "primary" value of the exterms:weight property, because frequently the value would be recorded simply as the value "2.4" (as we did in the triple above), relying on an understanding of the context to fill in the unstated units information.

In the RDF model a qualified property value of this kind is considered as simply another kind of structured value. To represent this, we create a new resource to represent the structured value as a whole (the weight, in this case), and to serve as the object of the original statement. We then give that new resource properties representing the individual parts of the structured value. In this case, we need a property for the literal "2.4", and a property for the unit "kilograms". RDF provides a pre-defined rdf:value property to denote the primary value (if there is one) of a structured value. So in this case, we would give the literal "2.4" as the value of the rdf:value property, and assign the resource exunits:kilograms as the value of an exterms:units property (assuming the resource exunits:kilograms is defined in a example.org schema with the URIref http://www.example.org/units/kilograms). The resulting triples would be:

exproduct:item10245  exterms:weight  _:weight10245 .
_:weight10245    rdf:value  "2.4" .
_:weight10245    exterms:units   exunits:kilograms .

which can be exchanged using the RDF/XML:

 <?xml version="1.0"?>
 <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

   <rdf:Description rdf:about="http://www.example.com/2002/04/products#item10245">
      <exterms:weight rdf:parseType="Resource">
        <exterms:units rdf:resource="http://www.example.org/units/kilograms" />

@@Above example uses a nested Bnode and parseType=Resource; need to rewrite with a node id if I remove that abbreviation from the syntax section?@@

The same approach can be used to represent quantities using any units of measure, as well as values taken from different classification schemes or rating systems, by using the rdf:value property to give the primary value, and using additional properties to identify the classification scheme or other information that further describes the value.

You need not use rdf:value for these purposes, and RDF does not associate any particular meaning with it. rdf:value is simply provided as a convenience for use in these commonly-occurring situations.

5. Defining RDF Vocabularies: RDF Schema

@@This section currently does not include a discussion of rdfs:Datatype, and the declaration of specific datatypes in schemas, and requires further synchronization in a number of other areas with the RDF Vocabulary Description Language specification.@@

@@Look at discussions with Patrick for rdfs:Datatype usage.@@

RDF provides a way to express simple statements about resources, using named properties and values. However, RDF user communities also need the ability to indicate that they are describing specific kinds or classes of resources, and will use specific properties in describing those resources. For example, the company example.com from our examples in Section 3 would want to define classes such as exterms:Tent, and properties such as exterms:model, exterms:weightInKg, and exterms:packedSize to describe them (we use QNames with various "example" namespace prefixes as the names of classes and properties here as a reminder that in RDF these names are actually URI references, as discussed in Section 2). Similarly, people interested in describing bibliographic resources would want to define classes such as ex2:Book or ex2:MagazineArticle, and define properties such as ex2:author, ex2:title, and ex2:subject to describe them. Other applications might need to define classes such as ex3:Person and ex3:Company, and properties such as ex3:age, ex3:jobTitle, ex3:stockSymbol, and ex3:numberOfEmployees. RDF itself provides no vocabulary for specifying these things. Instead, such classes and properties are defined as an RDF vocabulary. The facilities for defining RDF vocabularies are specified in RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY].

RDF Schema does not provide a specific vocabulary of application-oriented classes like exterms:Tent, ex2:Book, or ex3:Person, and properties like exterms:weightInKg, ex2:author or ex3:JobTitle. Instead, it provides the mechanisms needed to specify such classes and properties as part of a vocabulary, and to indicate which classes and properties are expected to be used together (for example, you might expect the property ex3:jobTitle to be used in the description of a ex3:Person). In other words, RDF Schema provides a basic type system for use in RDF models. The RDF Schema type system is similar in some respects to the type systems of object-oriented programming languages such as Java. For example, RDF Schema allows resources to be defined as instances of one or more classes. In addition, it allows classes to be organized in a hierarchical fashion; for example a class ex:Dog might be defined as a subclass of ex:Mammal which is a subclass of ex:Animal, meaning that any resource which is in class ex:Dog is also considered to be in class ex:Animal. However, RDF classes and properties are in some respects very different from programming language types. RDF class and property definitions do not create a straightjacket into which information must be forced, but instead provide additional information about the RDF resources they describe. This information can be used in a variety of ways. We will say more about this point in Section 5.3.

RDF Schema uses RDF itself to specify the RDF type system, by providing a set of pre-defined RDF resources and properties, together with their meanings, that can be used to define user-specific classes and properties. These additional RDF Schema resources extend RDF to include a larger reserved vocabulary with additional meaning. The RDF Schema (RDFS) vocabulary is defined in a namespace identified by the URI reference http://www.w3.org/2000/01/rdf-schema#" (in the examples, we will use the prefix rdfs: to refer to this namespace). We will illustrate RDF Schema's basic resources and properties in the following sections.

5.1. Defining Classes

A basic step in any kind of description process is identifying the various kinds of things to be described. RDF Schema refers to these "kinds of things" as classes. A class in RDF Schema corresponds to the generic concept of a Type or Category, somewhat like the notion of a class in object-oriented programming languages such as Java. RDF classes can be defined to represent almost anything, such as web pages, people, document types, databases or abstract concepts. Classes are defined using the RDFS-defined resources rdfs:Class and rdfs:Resource, and the properties rdf:type and rdfs:subClassOf.

For example, suppose we wanted to use RDF to provide information about different kinds of motor vehicles. In RDF Schema, we would first define a class called xyz:MotorVehicle (using xyz: to stand for the namespace we will use in this example). A class represents the group of resources that belong to the class, called its instances. In this case, we intend the class xyz:MotorVehicle to represent the group of resources that we intend to represent motor vehicles.

In RDF Schema, a class is any resource having an rdf:type property whose value is the RDFS-defined resource rdfs:Class. So a new class, such as xyz:MotorVehicle, is defined by creating an RDF resource to represent the new class, and giving it an rdf:type property whose value is the RDFS-defined resource rdfs:Class. (The resource rdfs:Class itself has an rdf:type of rdfs:Class.)

As we've already seen, the property rdf:type is used to indicate that a resource is an instance of a class. In our example, if we wanted to define a resource, say xyz:companyCar, to represent a particular motor vehicle, we would define the resource xyz:companyCar with an rdf:type property whose value is xyz:MotorVehicle, indicating that xyz:companyCar is an instance of the class resource we've just defined. A resource may be an instance of more than one class.

After defining class xyz:MotorVehicle, we might want to define additional classes representing various specialized kinds of motor vehicle, e.g., passenger vehicles, vans, minivans, and so on. We can define these classes in the same way as we defined class xyz:MotorVehicle, by defining a resource to represent each new class, and giving it an rdf:type property whose value is rdfs:Class. However, we want to do more than just define the separate classes; we also want to indicate their special relationship to class xyz:MotorVehicle, i.e., that they are specialized kinds of MotorVehicle. To do this, we use the RDFS concept of subclass.

An RDF subclass represents a subset/superset relationship between two classes. We define this relationship using the pre-defined rdfs:subClassOf property to relate the two classes. For example, to state that xyz:Van is a subclass of xyz:MotorVehicle, we would define the class xyz:Van, and give it an rdfs:subClassOf property whose value is the (class) resource xyz:MotorVehicle. The meaning of this subclass relationship is that if xyz:Van is a subclass of xyz:MotorVehicle, and resource xyz:mycar is an instance of xyz:Van, then xyz:mycar is also implicitly considered an instance of xyz:Motorvehicle (that is, you can "infer" or act as if xyz:mycar is an instance of xyz:MotorVehicle even if this is not explicitly stated).

The rdfs:subClassOf property is transitive. This means, for example, that if class xyz:MiniVan is a subclass of class xyz:Van, and xyz:Van is a subclass of xyz:Motorvehicle, then xyz:MiniVan is also implicitly a subclass of xyz:Motorvehicle. As a result, resources that are instances of class xyz:MiniVan are also considered instances of class xyz:Motorvehicle (as well as of class xyz:Van). A class may be a subclass of more than one class (for example, xyz:MiniVan may be a subclass of both xyz:Van and xyz:PassengerVehicle). All classes are implicitly subclasses of class rdfs:Resource (since the instances belonging to all classes are resources).

The example in Figure 17 shows the simple class hierarchy we have been discussing. At the bottom, we show a class MotorVehicle. We then show three subclasses of MotorVehicle, namely PassengerVehicle, Truck and Van, related to class MotorVehicle by rdfs:subClassOf properties. At the top, we show class Minivan, which is a subclass of both Van and PassengerVehicle.

A Simple Class Hierarchy
Figure 17: A Simple Class Hierarchy

Some corresponding RDF/XML syntax for this schema, defining the new classes using the techniques for defining new resources described in Section 3, is shown below.


<rdf:Description rdf:ID="MotorVehicle">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="http://www.w3.org/2000/01/rdf-schema#Resource"/>

<rdf:Description rdf:ID="PassengerVehicle">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="#MotorVehicle"/>

<rdf:Description rdf:ID="Truck">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="#MotorVehicle"/>

<rdf:Description rdf:ID="Van">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="#MotorVehicle"/>

<rdf:Description rdf:ID="MiniVan">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="#Van"/>
  <rdfs:subClassOf rdf:resource="#PassengerVehicle"/>


This schema uses rdf:ID to assign names, such as MotorVehicle, to the new resources (classes) that it defines. These names can then be referred to in other class definitions within the same schema. To refer to this RDF schema in RDF instance data (e.g., data defining individual vehicles of these classes), which might be located elsewhere, the instance data would typically include an XML namespace declaration referencing the schema, for example (assuming that the schema was the resource http://example.org/schemas/vehicles.rdfs), the namespace declaration xmlns:xyz="http://example.org/schemas/vehicles#". This would allow the instance data to use abbreviations such as xyz:MotorVehicle to refer unambiguously to the class MotorVehicle from this RDF Schema. As noted in Section 3, to ensure that these references would be consistently maintained even if the schema were relocated, the schema could also include an explicit xml:base="http://example.org/schemas/vehicles" declaration.

5.2. Defining Properties

In addition to defining the specific classes of things they want to describe, user communities also need to be able to define specific properties that characterize those classes of things (such as rearSeatLegRoom to describe a passenger vehicle). In RDF Schema, properties are defined using the RDF-defined class rdf:Property, and the RDFS-defined properties rdfs:domain, rdfs:range, and rdfs:subPropertyOf.

All properties in RDF are defined as instances of class rdf:Property. So a new property, such as exterms:weightInKg, is defined by creating an RDF resource to represent the new property, and giving it an rdf:type property whose value is the resource rdf:Property.

RDF Schema also provides a mechanism for specifying additional information that describes how properties and classes are intended to be used together in RDF data. The most important information of this kind is supplied by using the RDFS-defined properties rdfs:range and rdfs:domain as further descriptions of individual properties.

The rdfs:range property is used to indicate that the values of a particular property are intended to be instances of a designated class. For example, if we wanted to indicate that the property ex:author was intended to have values that are instances of a class ex:Person, we would give the (property) resource ex:author an rdfs:range property whose value is the (class) resource ex:Person.

The rdfs:range property can also be used to indicate that the value of a property is intended to be a literal of a particular datatype. For example, if we wanted to indicate that the property ex:age was intended to be a literal of the XML Schema datatype xsd:integer, we would give the property) resource ex:age an rdfs:range property whose value is the resource xsd:integer.

A property, say ex:hasMother, can have zero, one, or more than one range property. If ex:hasMother has no range property, then we are saying nothing about the intended values of the ex:hasMother property. If ex:hasMother has one range property, say one specifying ex:Person as the range, this says that the values of the ex:hasMother property are intended to be instances of class ex:Person. If ex:hasMother has more than one range property, say one specifying ex:Person as its range, and another specifying ex:Female as its range, this says that the values of the ex:hasMother property are intended to be instances of all of the classes specified as the ranges, i.e., that any value of ex:hasMother should be both a ex:Female and a ex:Person.

The rdfs:domain property is used to indicate that a particular property is intended to be applied to a designated class. For example, if we wanted to indicate that the property ex:author was intended to apply to instances of class ex:Book, we would give the (property) resource ex:author an rdfs:domain property whose value is the (class) resource ex:Book.

A given property, say exterms:weight, may have zero, one, or more than one domain property. If exterms:weight has no domain property, then we are saying nothing about the resources that exterms:weight properties may be used with (any resource could have a exterms:weight property). If exterms:weight has one domain property, say one specifying ex:Book as the domain, this says that the exterms:weight property is intended to be applied to instances of class ex:Book. If exterms:weight has more than one domain property, say one specifying ex:Book as the domain and another one specifying ex:MotorVehicle as the domain, this says that any resource that has a exterms:weight property is intended to be an instance of all of the classes specified as the domains, i.e., that any resource that has a exterms:weight property should be both a ex:Book and a ex:MotorVehicle (illustrating the need for care in specifying domains and ranges).

We can illustrate the use of these range and domain specifications by continuing with our earlier example of xyz:MotorVehicle. In this example, we define two properties: xyz:registeredTo and xyz:rearSeatLegRoom. The xyz:registeredTo property is intended to apply to any xyz:MotorVehicle and its value is intended to be a xyz:Person (a class we assume has been defined elsewhere). For the sake of this example, xyz:rearSeatLegRoom is intended to apply only to instances of class xyz:PassengerVehicle. The value is intended to be an xsd:integer, which is the number of centimeters of rear seat legroom. These definitions are shown in the RDF/XML below (we assume we are adding this RDF/XML to the RDF/XML defining the classes that we gave earlier):


<rdf:Description rdf:ID="registeredTo">
  <rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>
  <rdfs:domain rdf:resource="#MotorVehicle"/>
  <rdfs:range rdf:resource="http://www.example.org/classes#Person"/>

<rdf:Description rdf:ID="rearSeatLegRoom">
  <rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>
  <rdfs:domain rdf:resource="#PassengerVehicle"/> 
  <rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#integer"/>


RDF Schema provides a way to specialize properties as well as classes. We define this specialization relationship between two properties using the pre-defined rdfs:subPropertyOf property. For example, if ex:biologicalFather and ex:biologicalParent are both properties, we can define ex:biologicalFather as a subproperty of ex:biologicalParent by giving ex:biologicalFather an rdfs:subPropertyOf property whose value is ex:biologicalParent.

The meaning of this relationship is that if ex:biologicalFather is a subproperty of the broader property ex:biologicalParent, and if an instance ex:fred is the ex:biologicalFather of another instance ex:john, then ex:fred is implicitly considered to also be the ex:biologicalParent of ex:john. The RDF/XML corresponding to these examples is shown below.


<rdf:Description rdf:ID="biologicalParent">
  <rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>

<rdf:Description rdf:ID="biologicalFather">
  <rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>
  <rdfs:subPropertyOf rdf:resource="#biologicalParent"/>


A property may be a subproperty of zero, one or more properties. All RDF rdfs:range and rdfs:domain properties that apply to an RDF property also apply to each of its sub-properties.

Now that we've shown how to define classes and properties using RDF Schema, we can see what instances corresponding to those definitions might look like. For example, the following is an instance of the xyz:PassengerVehicle class we defined above (which we assume is being defined in the same document as the schema), together with some hypothetical values for its xyz:registeredTo and xyz:rearSeatLegRoom properties. Note the use of the rdf:type property to indicate its class membership. Also note how we can apply a xyz:registeredTo property to this instance of xyz:PassengerVehicle, because xyz:PassengerVehicle is a subclass of xyz:MotorVehicle.

@@Be more explicit about fact that the instances are in the same document as the schema? Show how the schema is referenced if they aren't (e.g., by URIrefs of the classes and properties, or by using isDefinedBy if you want a link to the actual document?@@

  <?xml version="1.0"?>
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

    <rdf:Description rdf:ID="johnSmithsCar">
         <rdf:type rdf:resource="#PassengerVehicle"/>
         <xyz:registeredTo rdf:resource="http://www.example.org/staffid/85740"/>

As we discussed in Section 3, the RDF/XML syntax provides an abbreviation for instances defined as members of classes using the rdf:type property. Using this abbreviation, we could define this same instance as:

  <?xml version="1.0"?>
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

    <xyz:PassengerVehicle rdf:ID="johnSmithsCar">
         <xyz:registeredTo rdf:resource="http://www.example.org/staffid/85740"/>

5.3. Interpreting RDF Schema Declarations

As noted earlier, the RDF Schema type system is similar in some respects to the type systems of object-oriented programming languages such as Java. However, RDF differs from most programming language type systems in several important respects.

One important difference is that instead of defining a class in terms of the properties its instances may have, an RDF schema defines properties in terms of the classes of resource to which they are intended to apply, using domain and range properties. For example, a classical object-oriented programming language might define a class Book with an attribute called author having values of type Person. A corresponding RDF schema would define a class ex:Book, and, in a separate definition, a property ex:author having a domain of ex:Book and a range of ex:Person.

The difference between these approaches may seem to be only syntactic, but in fact there is an important difference. In the programming language class definition, the attribute author is part of the definition of class Book, and applies only to instances of class Book. Another class (say, softwareModule) might also have an attribute called author, but this would be considered a different attribute. In other words, the scope of an attribute definition in most programming languages is restricted to the class or type in which it is defined. In RDF, on the other hand, property definitions are, by default, independent of class definitions, and have, by default, global scope (although they may optionally be declared to apply only to certain classes using domain specifications). So, for example, an RDF schema could define a property exterms:weight without a domain being specified. This property could then be used to describe instances of any class that might be considered to have a weight. One benefit of the RDF property-based approach is that it becomes easier to extend the use of property definitions to situations that might not have been anticipated by the original definer, provided the properties have not been made overly specific by domain specifications. (Of course, this is a "benefit" which must be used with care, to insure that properties are not mis-applied in inappropriate situations.)

Another important difference is that RDF Schema declarations are not necessarily prescriptive in the way programming language type declarations typically are. For example, if a programming language declares a class Book with an author attribute having values of type Person, this is usually interpreted as a group of constraints. The language will not allow the creation of an instance of Book without an author attribute, and it will not allow an instance of Book with an author attribute that does not have a Person as its value. Moreover, if author is the only attribute defined for class Book, the language will not allow an instance of Book with some other attribute.

RDF Schema, on the other hand, provides schema declarations as additional descriptions of RDF data, but does not prescribe how these descriptions should be used by an application. For example, suppose an RDF schema specifies an ex:author property with an rdfs:range of class ex:Person. This is simply an RDF statement that RDF statements containing ex:author properties have instances of ex:Person as objects. This statement must be combined with the RDF statements represented by the instance data in determining what a given group of RDF statements means.

This schema-supplied information might be used in different ways. One application might interpret this information as specifying part of a template for RDF data it is creating, and use it to ensure that any ex:author property has a value of the indicated (ex:Person) class. That is, this application interprets the schema declaration as a constraint in the same way that a programming language might. However, another application might interpret this schema information as providing additional information about data it is receiving, information which may not be provided explicitly in the original data. For example, this second application might receive some RDF data that includes an ex:author property whose value is a resource of unspecified class, and use this schema-provided statement to conclude that the resource must be of class ex:Person. A third application might receive some RDF data that includes an ex:author property whose value is a resource of class ex:Corporation, and use this schema information as the basis of a warning that "there may be an inconsistency here, but on the other hand there may not be". Somewhere else there may be a declaration that resolves the apparent inconsistency (e.g., a declaration to the effect that "a Corporation is a (legal) Person").

Moreover, depending on how the application interprets the property declarations, an instance might be allowed to exist either without some of the declared properties (e.g., you might have an instance of ex:Book without an ex:author property, even if ex:author is declared as having a domain of ex:Book), or with additional properties (you might create an instance of ex:Book with a xyz:technicalEditor property, even though you haven't defined such a property in your particular schema.)

In other words, RDF Schema declarations are always descriptions of RDF instance data. They may also be prescriptive (introduce constraints), but only if the application interpreting those statements wants to treat them that way. All RDF Schema does is provide a way of stating this additional information. Whether this information conflicts with explicitly specified instance data is up to the application to determine and act upon.

5.4. Other Schema Information

RDF Schema also defines a number of other properties, which can be used to provide documentation and other information about an RDF schema or about instances. For example the rdfs:comment property can be used to provide a human-readable description of a resource. The rdfs:label property can be used to provide a more human-readable version of a resource's name. The rdfs:seeAlso property can be used to indicate a resource that might provide additional information about the subject resource. The rdfs:isDefinedBy property is a subproperty of rdfs:seeAlso, and can be used to indicate the resource that defines the subject resource. For further discussion of the use of these properties, you should consult RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY].

5.5. Richer Schema Languages

RDF Schema provides basic capabilities for defining RDF vocabularies, but additional capabilities are also possible, and can be useful. These capabilities may be provided through further development of RDF Schema, or in other languages. Other richer schema capabilities that have been identified as useful (but that are not provided by RDF Schema) include:

  • cardinality constraints on properties, e.g., that a Person has exactly one biological father.
  • specifying that a given property (such as hasAncestor) is transitive, e.g., that if A hasAncestor B, and B hasAncestor C, then A hasAncestor C.
  • specifying that two different classes, defined in separate schemas, actually represent the same concept.
  • specifying that two different instances, defined separately, actually represent the same individual.
  • the ability to define new classes in terms of combinations (e.g., unions and intersections) of other classes.

The additional capabilities mentioned above, in addition to others, are the targets of ontology languages such as DAML+OIL [DAML+OIL] and OWL [OWL]. Both these languages are based on RDF and RDF Schema (and both currently provide all the additional capabilities mentioned above). The intent of such languages is to provide additional machine-processable semantics for resources, that is, to make the machine representations of resources more closely resemble their intended real world counterparts. While such capabilities are not necessarily needed to build useful applications using RDF (see Section 6 for a description of a number of RDF applications), the development of such languages is a very active subject of work as part of the development of the Semantic Web.

6. Some RDF Applications: RDF in the Field

In the previous sections, we have described the general capabilities of RDF and RDF Schema. While we have used examples within those sections to illustrate those capabilities, and some of those examples may have suggested potential RDF applications, we have not yet discussed any real ones. In this section, we will describe some actual deployed RDF applications, showing how RDF supports various real-world requirements to represent and manipulate information about a wide variety of things.

@@Sections could be more "tutorial"; e.g., pointing out how the examples use various RDF features to do various important things.@@

6.1 Dublin Core Metadata Initiative

Metadata is data about data. Specifically, the term refers to data used to identify, describe, or locate information resources, whether these resources are physical or electronic. While structured metadata processed by computers is relatively new, the basic concept of metadata has been used for many years in helping manage and use large collections of information. Library card catalogs are a familiar example of such metadata.

The Dublin Core is a set of "elements" (properties) for describing documents (and hence, for recording metadata). The element set was originally developed at the March 1995 Metadata Workshop in Dublin, Ohio. The Dublin Core has subsequently been modified on the basis of later Dublin Core Metadata workshops, and is currently maintained by the Dublin Core Metadata Initiative. The goal of the Dublin Core is to provide a minimal set of descriptive elements that facilitate the description and the automated indexing of document-like networked objects, in a manner similar to a library card catalog. The Dublin Core metadata set is intended to be suitable for use by resource discovery tools on the Internet, such as the "webcrawlers" employed by popular World Wide Web search engines. In addition, the Dublin Core is meant to be sufficiently simple to be understood and used by the wide range of authors and casual publishers who contribute information to the Internet. Dublin Core elements have become widely used in documenting Internet resources (we have already used the Dublin Core creator element in earlier examples). The current elements of the Dublin Core are defined in the Dublin Core Metadata Element Set, Version 1.1: Reference Description [DC], and contain definitions for the following properties:

  • Title: A name given to the resource.
  • Creator: An entity primarily responsible for making the content of the resource.
  • Subject: The topic of the content of the resource.
  • Description: An account of the content of the resource.
  • Publisher: An entity responsible for making the resource available
  • Contributor: An entity responsible for making contributions to the content of the resource.
  • Date: A date associated with an event in the life cycle of the resource.
  • Type: The nature or genre of the content of the resource.
  • Format: The physical or digital manifestation of the resource.
  • Identifier: An unambiguous reference to the resource within a given context.
  • Source: A Reference to a resource from which the present resource is derived.
  • Language: A language of the intellectual content of the resource.
  • Relation: A reference to a related resource.
  • Coverage: The extent or scope of the content of the resource.
  • Rights: Information about rights held in and over the resource.

Information using the Dublin Core elements may be represented in any suitable language (e.g., in HTML Meta elements). However, RDF is an ideal representation for Dublin Core information. The examples below represent the simple description of a set of resources in RDF using the Dublin Core vocabulary. Note that the specific Dublin Core RDF vocabulary shown here is not intended to be authoritative. The Dublin Core Reference Description [DC] is the authoritative reference.

Here is a description of a Web site home page using Dublin Core properties:

    <rdf:Description rdf:about="http://www.dlib.org">
      <dc:title>D-Lib Program - Research in Digital Libraries</dc:title>
      <dc:description>The D-Lib program supports the community of people
       with research interests in digital libraries and electronic
      <dc:publisher>Corporation For National Research Initiatives</dc:publisher>
          <rdf:li>Research; statistical methods</rdf:li>
          <rdf:li>Education, research, related topics</rdf:li>
          <rdf:li>Library use Studies</rdf:li>
      <dc:type>World Wide Web Home Page</dc:type>

Note that both RDF and the Dublin Core define an (XML) element called "Description" (although here we've written the Dublin Core element name in lower case). Even if the initial letter were identically uppercase, the XML namespace mechanism enables us to distinguish between these two elements (one is rdf:Description, and the other is dc:description). Also, as a matter of interest, if you access "http://purl.org/dc/elements/1.1/" in a Web browser (as of the current writing), you will get an RDF Schema declaration for the Dublin Core Element Set 1.1.]

The second example describes a published magazine.

    <rdf:Description rdf:about="http://www.dlib.org/dlib/may98/05contents.html">
      <dc:title>DLIB Magazine - The Magazine for Digital Library Research
        - May 1998</dc:title>
      <dc:description>D-LIB magazine is a monthly compilation of
       contributed stories, commentary, and briefings.</dc:description>
      <dc:contributor>Amy Friedlander</dc:contributor>
      <dc:publisher>Corporation for National Research Initiatives</dc:publisher>
      <dc:type>electronic journal</dc:type>
          <rdf:li>library use studies</rdf:li>
          <rdf:li>magazines and newspapers</rdf:li>
      <dcterms:isPartOf rdf:resource="http://www.dlib.org"/>

In this example, we've used (in the third line from the bottom) the Dublin Core qualifier isPartOf (from a separate namespace) to indicate that this magazine is "part of" the previously-described web site.

The third example is of a specific article in the magazine referred to in the previous example.

    <rdf:Description rdf:about="http://www.dlib.org/dlib/may98/miller/05miller.html">
      <dc:title>An Introduction to the Resource Description Framework</dc:title>
      <dc:creator>Eric J. Miller</dc:creator>
      <dc:description>The Resource Description Framework (RDF) is an
       infrastructure that enables the encoding, exchange and reuse of
       structured metadata. rdf is an application of xml that imposes needed
       structural constraints to provide unambiguous methods of expressing
       semantics. rdf additionally provides a means for publishing both
       human-readable and machine-processable vocabularies designed to
       encourage the reuse and extension of metadata semantics among
       disparate information communities. the structural constraints rdf
       imposes to support the consistent encoding and exchange of
       standardized metadata provides for the interchangeability of separate
       packages of metadata defined by different resource description
       communities. </dc:description>
      <dc:publisher>Corporation for National Research Initiatives</dc:publisher>
          <rdf:li>machine-readable catalog record formats</rdf:li>
          <rdf:li>applications of computer file organization and
           access methods</rdf:li>
      <dc:rights>Copyright @ 1998 Eric Miller</dc:rights>
      <dc:type>Electronic Document</dc:type>
      <dcterms:isPartOf rdf:resource="http://www.dlib.org/dlib/may98/05contents.html"/>

In this final example, we've also used the qualifier isPartOf, this time to indicate that this article is "part of" the previously-described magazine.


PRISM: Publishing Requirements for Industry Standard Metadata [PRISM] is a metadata specification developed in the publishing industry. Magazine publishers and their vendors formed the PRISM Working Group to identify the industry's needs for metadata and define a specification to meet them. Publishers want to use existing content in many ways in order to get a greater return on the investment made in creating it. Converting magazine articles to HTML for posting on the web is one example. Licensing it to aggregators like LexisNexis is another. All of these are "first uses" of the content; typically they all go live at the time the magazine hits the stands. The publishers also want their content to be "evergreen". It might be used in new issues, such as in a retrospective article. It could be used by other divisions in the company, such as in a book compiled from the magazine's photos, recipes, etc. Another use is to license it to outsiders, such as in a reprint of a product review, or in a retrospective produced by a different publisher. This overall goal requires a metadata approach which emphasizes discovery, rights tracking, and end-to-end metadata.

Discovery: Discovery is a general term for finding content which encompasses searching, browsing, content routing, and other techniques. Discussions of discovery frequently center on a consumer searching a public web site. However, discovering content is much broader than that. The audience may consist of consumers, or it may consist of internal users such as researchers, designers, photo editors, licensing agents, etc. To assist discovery, PRISM provides properties to describe the topics, formats, genre, origin, and contexts of a resource. It also provides means for categorizing resources using multiple subject description taxonomies.

Rights Tracking: Magazines frequently contain material licensed from others. Photos from a stock photo agency are the most common type of licensed material, but articles, sidebars, and all other types of content may be licensed. Simply knowing if content was licensed for one-time use, requires royalty payments, or is wholly-owned by the publisher is a struggle. PRISM provides elements for basic tracking of such rights. A separate namespace (vocabulary) defined in the PRISM specification supports description of places, times, and industries where content may or may not be used.

End-to-end metadata: Most published content already has metadata created for it. Unfortunately, when content moves between systems, the metadata is frequently discarded, only to be re-created later in the production process at considerable expense. PRISM aims to reduce this problem by providing a specification that can be used in multiple stages in the content production pipeline. An important feature of the PRISM specification is its use of other existing specifications. Rather than create an entirely new thing, the group decided to use existing specifications as much as possible, and only define new things where needed. For this reason, the PRISM specification uses XML, RDF, Dublin Core, and well as various ISO formats and vocabularies.

A PRISM description may be as simple as a few Dublin Core properties with literal values. The example below describes a photograph, giving basic information on its title, photographer, format, etc.

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

 <rdf:Description rdf:about="http://wanderlust.com/2000/08/Corfu.jpg">
  <dc:title>Walking on the Beach in Corfu</dc:title>
  <dc:description>Photograph taken at 6:00 am on Corfu with two models
  <dc:creator>John Peterson</dc:creator>
  <dc:contributor>Sally Smith, lighting</dc:contributor>

PRISM also augments the Dublin Core to allow more detailed descriptions. The augmentations are defined in three new namespaces, generally cited using the prefixes prism:, pcv:, and prl:.

prism: This prefix refers to the main PRISM namespace, whose URI is http://prismstandard.org/namespaces/basic/1.0/. Most of its properties are more specific versions of properties from the Dublin Core. For example, dc:date is extended by properties like prism:publicationTime, prism:releaseTime, prism:expirationTime, etc.

pcv: This prefix refers to the PRISM Controlled Vocabulary namespace, whose URI is http://prismstandard.org/namespaces/pcv/1.0/. Currently, common practice for describing the subject(s) of an article is by supplying appropriate-seeming keywords. Unfortunately, simple keywords do not make a great difference in retrieval performance, due to the fact that different people will use different keywords [BATES96]. Best practice is to code the articles with subject terms from a "controlled vocabulary". The vocabulary should provide as many synonyms as possible for its terms in the vocabulary. This way the controlled terms provide a meeting ground for the keywords supplied by the searcher and the indexer. The PRISM Controlled Vocabulary (pcv) namespace provides properties for specifying terms in a vocabulary, the relations between terms, and alternate names for the terms.

prl: This prefix refers to the PRISM Rights Language namespace, whose URI is http://prismstandard.org/namespaces/prl/1.0/. Digital Rights Management is an area undergoing considerable upheaval. There are a number of proposals for rights management languages, but none are clearly favored throughout the industry. Because there was no clear choice to recommend, the PRISM Rights Language (PRL) was defined as an interim measure. It provides properties which let people say if an item can or can't be 'used', depending on conditions of time, geography, and industry. This is believed to be an 80/20 tradeoff which will help publishers begin to save money when tracking rights. It is not intended to be a general rights language, or allow publishers to automatically enforce limits on consumer uses of the content.

PRISM uses RDF because of its abilities for dealing with descriptions of varying complexity. Currently, a great deal of metadata uses simple character string values, such as


Over time the developers of PRISM expect uses of the PRISM specification to become more sophisticated, moving from simple literal values to more structured values. In fact, that range of values is a situation being faced now. Some publishers already use sophisticated controlled vocabularies, others are barely using manually-supplied keywords. Some examples of the different kinds of values that can be given are:


<dc:coverage rdf:resource="http://prismstandard.org/vocabs/ISO-3166/GR"/>


  <pcv:Descriptor rdf:about="http://prismstandard.org/vocabs/ISO-3166/GR">
    <pcv:label xml:lang="en">Greece</pcv:label>
    <pcv:label xml:lang="fr">Grece</pcv:label>

Note also that there are properties whose meanings are similar, or subsets of other properties. For example, the geographic subject of a resource could be given with




Any of those properties might use the simple literal value, or a more complex structured value. Such a range of possibilities cannot be adequately described by DTDs, or even by the newer XML Schemas. While there is a wide range of syntax to deal with, RDF's graph model has a simple structure - a list of 'triples'. Dealing with the metadata in the triples domain makes it much easier for older software to accommodate content with new extensions.

@@Above is not very clear; it concludes the structured value stuff, so presumably could be made clearer; e.g., structured value a reference to the ISO code example.@@

We will close this section with two final examples. The first example says that the image (.../Corfu.jpg) cannot be used (#none) in the tobacco industry (code 21 in SIC, the Standard Industrial Classifications).

<rdf:RDF xmlns:prism="http://prismstandard.org/namespaces/basic/1.0/"

 <rdf:Description rdf:about="http://wanderlust.com/2000/08/Corfu.jpg">
  <dc:rights rdf:parseType="Resource"
     <prl:usage rdf:resource="#none"/>
     <prl:industry rdf:resource="http://prismstandard.org/vocabs/SIC/21"/>

The second says that the photographer for the Corfu image was employee 3845, better known as John Peterson. It also says that the geographic coverage of the photo is Greece. It does so by providing, not just a code from a controlled vocabulary, but a cached version of the information for that term in the vocabulary.

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

  <rdf:Description rdf:about="/2000/08/Corfu.jpg">
    <dc:identifier rdf:resource="/content/2357845" />
      <pcv:Descriptor rdf:about="/emp3845">
        <pcv:label>John Peterson</pcv:label>
        <pcv:label xml:lang="en">Greece</pcv:label>
        <pcv:label xml:lang="fr">Grece</pcv:label>

6.3 XPackage

Many situations involve the need to maintain information about structured groupings of resources and their associations that are, or may be, used as a unit. The XML Package (XPackage) specification [XPACKAGE] provides a framework for defining such groupings, called packages. XPackage specifies a framework for describing the resources included in such packages, the properties of those resources, their method of inclusion, and their relationships with each other. XPackage applications include specifying the stylesheets used by a document, declaring the images shared by multiple documents, indicating the author and other metadata of a document, describing how namespaces are used by XML resources, and providing a manifest for bundling resources into a single archive file.

The XPackage framework is based upon XML, RDF, and the XML Linking Language [XLINK], and provides two RDF vocabularies: one for general packaging descriptions, and another specifically for describing XML resources. The XPackage framework also allows customization through extension and/or restriction.

One application of XPackage is the description of XHTML documents and their supporting resources. An XHTML document retrieved from a web site may rely on other resources such as stylesheets and image files that also need to be retrieved. However, the identities of these supporting resources may not be obvious without processing the entire document. Other information about the document, such as the name of its author, may also not be available without processing the document. XPackage allows such descriptive information to be stored in a standard way in a package description document containing RDF. The outer elements of a package description document describing such an XHTML document might look like the following example (with namespace declarations removed for simplicity):

<?xml version="1.0"?>

    (description of individual resources go here)


Resources (such as the XHTML document, stylesheets, and images) are described within this package description document. The XHTML document resource itself is described using an RDF resource description element <xpackage:resource> from the XPackage ontology (the term XPackage uses for a vocabulary). Each resource description element may include RDF properties from various ontologies. In the example below, the document's MIME content type ("application/xhtml+xml") is defined using a standard XPackage property from the XPackage ontology, xpackage:contentType. Another property, the document's author (in this case, "Garret Wilson"), is described using a property from the Dublin Core (which is considered a custom ontology in XPackage), resulting in a dc:creator property. XPackage itself specifies an extension property set specifically for XML-based resources, the XML ontology, including specifying XML namespaces and stylesheets used with the xmlprop:namespace and xmlprop:style properties, respectively.

    <xpackage:resource rdf:about="urn:examples:xhtmldocument-doc">
      <rdfs:comment>The XHTML document.</rdfs:comment>
      <xpackage:location xlink:href="doc.html"/>
      <xmlprop:namespace rdf:resource="http://www.w3.org/1999/xhtml"/>
      <xmlprop:style rdf:resource="urn:examples:xhtmldocument-stylesheet"/>
      <xmlprop:annotation rdf:resource="urn:examples:xhtmldocument-annotation"/>
      <dc:creator>Garret Wilson</dc:creator>
          <rdf:li rdf:resource="urn:examples:xhtmldocument-stylesheet"/>
          <rdf:li rdf:resource="urn:examples:xhtmldocument-image"/>

The xpackage:manifest property indicates that both the stylesheet and image resources are necessary for processing; those resources are described separately within the package description document. The example stylesheet resource description below lists its location ("stylesheet.css") using the XPackage ontology xpackage:location property (which is compatible with XLink), and shows through use of the XPackage ontology xpackage:contentType property that it is a CSS stylesheet ("text/css").

    <xpackage:resource rdf:about="urn:examples:xhtmldocument-css">
      <rdfs:comment>The document stylesheet.</rdfs:comment>
      <xpackage:location xlink:href="stylesheet.css"/>

The full version of this example may be found in [XPACKAGE].

6.4 RSS 1.0: RDF Site Summary

@@Following paras too "hypey" and also a lot of redundant stuff. Too many "very"s.@@

RSS 1.0 ("RDF Site Summary") is an RDF Vocabulary that provides a lightweight multipurpose extensible metadata description and syndication format. In short, RSS 1.0 is a very powerful and extensible way of describing, managing and making available to very broad audiences relevant and timely information. It allows information to be made available in a very rich and reusable way and it is also perhaps the most widely deployed RDF application on the web.

When you consider all of the information that that you access on the Web on a day-to-day basis; your schedule, to-do lists, news headlines, search results, "What's New", etc. the challenges for managing this information become daunting. It becomes increasingly difficult to manage this information and integrate this into a coherent whole as the sources of and the diversity of this information increase. RSS 1.0 is an RDF vocabulary that allows the syndication of information, or metadata, and so facilities the distribution and re-purposing of this data.

A Simple Example

The W3C home page, shown below, is a primary point of contact with the public and serves in part to disseminate information about the deliverables of the Consortium. The center column of news items changes frequently. To support the timely dissemination of this information, the W3C Team has implemented an RDF Site Summary (RSS 1.0) news feed that makes the content in the center column available to others to repurpose as they will. News syndication sites may merge the headlines into a summary of the day's latest news, others may display the headlines as links as a service to their readers, and, increasingly, individuals may subscribe to this feed with a desktop application. These desktop RSS readers allow their users to keep track of potentially hundreds of sites, without having to visit each one in their browser.

w3c rss

Countless sites all over the Web provide RSS 1.0 feeds. Here is an example of the W3C feed:

<?xml version="1.0" encoding="utf-8"?>

<rdf:RDF xmlns="http://purl.org/rss/1.0/"

<channel rdf:about="http://www.w3.org/2000/08/w3c-synd/home.rss">
<title>The World Wide Web Consortium</title>
<description>Leading the Web to its Full Potential...</description>


<rdf:li rdf:resource="http://www.w3.org/News/2002#item164"/>
<rdf:li rdf:resource="http://www.w3.org/News/2002#item168"/>
<rdf:li rdf:resource="http://www.w3.org/News/2002#item167"/>


<item rdf:about="http://www.w3.org/News/2002#item164">
<title>User Agent Accessibility Guidelines Become a W3C 
Proposed Recommendation</title>
<description>17 October 2002: W3C is pleased to announce the 
advancement of User Agent Accessibility Guidelines 1.0 to 
Proposed Recommendation. Comments are welcome through 14 November. 
Written for developers of user agents, the guidelines lower 
barriers to Web accessibility for people with disabilities 
(visual, hearing, physical, cognitive, and neurological). 
The companion Techniques Working Draft is updated. Read about 
the Web Accessibility Initiative. (News archive)</description>

<item rdf:about="http://www.w3.org/News/2002#item168">
<title>Working Draft of Authoring Challenges for Device 
Independence Published</title>
<description>25 October 2002: The Device Independence 
Working Group has released the first public Working Draft of 
Authoring Challenges for Device Independence. The draft describes 
the considerations that Web authors face in supporting access to 
their sites from a variety of different devices. It is written 
for authors, language developers, device experts and developers 
of Web applications and authoring systems. Read about the Device 
Independence Activity (News archive)</description>

<item rdf:about="http://www.w3.org/News/2002#item167">
<title>CSS3 Last Call Working Drafts Published</title>
<description>24 October 2002: The CSS Working Group has 
released two Last Call Working Drafts and welcomes comments 
on them through 27 November. CSS3 module: text is a set of 
text formatting properties and addresses international contexts. 
CSS3 module: Ruby is properties for ruby, a short run of text 
alongside base text typically used in East Asia. CSS3 module: 
The box model for the layout of textual documents in visual 
media is also updated. Cascading Style Sheets (CSS) is a 
language used to render structured documents like HTML and 
XML on screen, on paper, and in speech. Visit the CSS home 
page. (News archive)</description>


As you can see, the format is designed for content that can be packaged into easily disinguishable sections. News sites, web logs, sports scores, stock quotes, and the like are all use-cases for RSS 1.0.

The feed can be requested by any application able to speak HTTP. More recently, however, RSS 1.0 applications are splitting into three different catagories:

  • Online aggregators - Sites such as Meerkat and NewsIsFree, as pictured below (each mirroring W3C's column of news). These gather feeds from thousands of sources, and seperate each of the <item>s out, adding them together again into one large group. The whole group is then made searchable. In this way, one can search for the latest news on, for example, 'Java' from perhaps thousands of sites, without having to search them all.
  • Desktop Readers - Utilities such as Amphetadesk and NetNewsWire Lite allow their users to subscribe to hundreds of feeds from their desktop. Readers customarily refresh each feed once an hour, allowing users to stay up to date.
  • Scripts - RSS's original purpose was to allow webmasters to include the content of another's site within their own. RSS 1.0 is still used in this way, with many sites (Slashdot for example) incorporating RSS feeds on their front page.

meerkat rss news-is-free rss


RSS 1.0 is, by design, extremely extensible. By importing additional RDF vocabularies, or modules as they are known within the RSS development community, the RSS 1.0 author can provide large amounts of metadata and handling instructions to the recipient of the file. Modules can, as with more general RDF vocabularies, be written by anyone. Currently there are 3 official modules and 19 proposed modules readily recognised by the community at large. These modules range from the complete Dublin Core module to more specialised RSS-centric modules such as the Aggregation module.

The Name

Care should be taken when discussing RSS is the scope of RDF. There are currently two RSS specification strands. One strand (RSS 0.91,0.92,0.93,0.94 and 2.0) does not use RDF. The other strand (RSS 0.9 and 1.0) does.


Electric utilities use power system models for a number of different purposes. For example, simulations of power systems are necessary for planning and security analysis. Power system models are also used in actual operations, e.g., by the Energy Management Systems (EMS) used in energy control centers. An operational power system model can consist of thousands of classes of information. In addition to using these models in-house, utilities need to exchange system modeling information, both in planning, and for operational purposes, e.g., for coordinating transmission and ensuring reliable operations. However, individual utilities use different software for these purposes, and as a result the system models are stored in different formats, making the exchange of these models difficult.

In order to support the exchange of power system models, utilities needed to agree on common definitions of power system entities and relationships. To support this, the Electric Power Research Institute (EPRI) a non-profit energy research consortium, developed a Common Information Model (CIM). The CIM specifies common semantics for power system resources, their attributes, and relationships. In addition, to further support the ability to electronically exchange CIM models, the power industry has developed CIM/XML, a language for expressing CIM models in XML. CIM/XML is an RDF application, using RDF and RDF Schema to organize its XML structures. The North American Electric Reliability Council (NERC) (an industry-supported organization formed to promote the reliability of electricity delivery in North America) has adopted CIM/XML as the standard for exchanging models between power transmission system operators. The CIM/XML format is also going through an IEC international standardization process. An excellent discussion of CIM/XML can be found in [DWZ01]. [NB: This power industry CIM should not be confused with the CIM developed by the Distributed Management Task Force for defining management information for distributed software, network, and enterprise environments. The DMTF CIM also has an XML representation, but does not use RDF.]

The CIM can represent all of the major objects of an electric utility as object classes and attributes, as well as their relationships. CIM uses these object classes and attributes to support the integration of independently developed applications between vendor specific EMS systems, or between an EMS system and other systems that are concerned with different aspects of power system operations, such as generation or distribution management.

The CIM is specified as a set of class diagrams using the Unified Modeling Language (UML). The base class of the CIM is the PowerSystemResource class, with other more specialized classes such as Substation, Switch, and Breaker being defined as subclasses. CIM/XML represents the CIM as an RDF schema vocabulary, and uses RDF/XML as the language for exchanging specific system models. The following are examples of class and property definitions from CIM/XML:

<rdfs:Class rdf:ID="PowerSystemResource"> 
  <rdfs:label xml:lang="en">PowerSystemResource</rdfs:label> 
  <rdfs:subClassOf rdf:resource="rdfs:Resource" />
  <rdfs:comment>"A power system component that can be either an
    individual element such as a switch or a set of elements 
    such as an substation. PowerSystemResources that are sets 
    could be members of other sets. For example a Switch is a 
    member of a Substation and a Substation could be a member 
    of a division of a Company"</rdfs:comment> 

<rdfs:Class rdf:ID="Breaker"> 
  <rdfs:label xml:lang="en">Breaker</rdfs:label> 
  <rdfs:subClassOf rdf:resource="#Switch" /> 
  <rdfs:comment>"A mechanical switching device capable of making, 
     carrying, and breaking currents under normal circuit conditions 
     and also making, carrying for a specified time, and breaking 
     currents under specified abnormal circuit conditions e.g. those 
     of short circuit. The typeName is the type of breaker, e.g., 
     oil, air blast, vacuum, SF6."</rdfs:comment> 

<rdf:Property rdf:ID="Breaker.ampRating"> 
   <rdfs:label xml:lang="en">ampRating</rdfs:label> 
   <rdfs:domain rdf:resource="#Breaker" /> 
   <rdfs:range rdf:resource="#CurrentFlow" /> 
   <rdfs:comment>"Fault interrupting rating in amperes"</rdfs:comment> 

CIM/XML uses only a subset of the complete RDF/XML syntax, in order to simplify serialization of models. In addition, CIM/XML implements some extensions to the RDF Schema vocabulary (defined in the cims: namespace) to support inverse roles and multiplicity (cardinality) constraints describing how many instances of a given property are allowed for a given resource from the CIM UML diagrams (allowable values for a multiplicity declaration are zero-or-one, exactly-one, zero-or-more, one-or-more). The following properties illustrate these extensions:

<rdf:Property rdf:ID="Breaker.OperatedBy"> 
   <rdfs:label xml:lang="en">OperatedBy</rdfs:label> 
   <rdfs:domain rdf:resource="#Breaker" /> 
   <rdfs:range rdf:resource="#ProtectionEquipment" /> 
   <cims:inverseRoleName rdf:resource="#ProtectionEquipment.Operates" /> 
   <cims:multiplicity rdf:resource="http://www.cim-logic.com/schema/990530#M:0..n" />
   <rdfs:comment>"Circuit breakers may be operated by 
       protection relays."</rdfs:comment>

<rdf:Property rdf:ID="ProtectionEquipment.Operates"> 
   <rdfs:label xml:lang="en">Operates</rdfs:label> 
   <rdfs:domain rdf:resource="#ProtectionEquipment" /> 
   <rdfs:range rdf:resource="#Breaker" /> 
   <cims:inverseRoleName rdf:resource="#Breaker.OperatedBy" /> 
   <cims:multiplicity rdf:resource="http://www.cim-logic.com/schema/990530#M:0..n" />
   <rdfs:comment>"Circuit breakers may be operated by 
       protection relays."</rdfs:comment>

EPRI has conducted successful interoperability tests using CIM/XML to exchange real-life, large-scale models (involving, in the case of one test, data describing over 2000 substations) between a variety of vendor products, and validating that these models would be correctly interpreted by typical utility applications. Although the CIM was originally intended for EMS systems, it is also being extended to support power distribution and other applications as well.

The Object Management Group has adopted an object interface standard to access CIM power system models called the Data Access Facility [DAF]. Like the CIM/XML language, the DAF is based on the RDF model and shares the same RDFS CIM schema. However, while CIM/XML enables a model to be exchanged as a document, DAF enables an application to access the model as a set of objects.

CIM/XML illustrates the useful role RDF can play in supporting XML-based exchange of information that is naturally expressed as entity-relationship or object-oriented classes, attributes, and relationships (even when that information will not necessarily be Web-accessable). In these cases, RDF provides a basic structure for the XML in support of identifying objects, and using them in structured relationships. This connection is illustrated by a number of applications using RDF/XML for information interchange, as well as a number of projects investigating linkages between RDF (or ontology languages such as DAML+OIL) and UML (and its XML representations).

The need for additional declarative power illustrated by the need to add cardinality constaints to CIM/XML illustrates the type of requirement leading to the development of more powerful RDF-based schema/ontology languages such as DAML+OIL or OWL described in Section 5.5. Such languages may be appropriate in supporting many similar modeling applications in the future.

Finally, CIM/XML also illustrates an important fact for those looking for additional examples of "RDF in the Field": sometimes languages are described as "XML" languages, or systems are described as using "XML", and the "XML" they are actually using is RDF/XML, i.e., they are RDF applications. Sometimes it is necessary to go fairly far into the description of the language or system in order to find this out (in some examples that have been found, RDF is never explicitly mentioned at all, but sample data clearly shows it is RDF/XML). Moreover, in applications such as CIM/XML, the RDF that is created will not be readily found on the Web, since it is intended for information exchange between software components rather than for general access (although future scenarios could be imagined in which more of this type of RDF would become Web-accessible).

6.6 Gene Ontology Consortium

As Section 6.4 suggests, structured metadata using controlled vocabularies plays an important role in medicine, enabling efficient literature searches and aiding in the distribution and exchange of medical knowledge. At the same time, the field of medicine is rapidly changing, and with that comes the need to develop additional vocabularies.

The objective of the Gene Ontology (GO) Consortium is to provide controlled vocabularies to describe specific aspects of gene products. Collaborating databases annotate their gene products (or genes) with GO terms, providing references and indicating what kind of evidence is available to support the annotations. The use of common GO terms by these databases facilitates uniform queries across them. The GO ontologies are structured to allow both attribution and querying to be performed at different levels of granularity. The GO vocabularies are dynamic, since knowledge of gene and protein roles in cells is accumulating and changing.

The three organizing principles of the GO are molecular function, biological process and cellular component. A gene product has one or more molecular functions and is used in one or more biological processes; it may be, or may be associated with, one or more cellular components. Definitions of the terms within all three of these ontologies are contained in a single (text) definition file. XML (actually, RDF/XML) formatted versions, containing all three ontology files and all available definitions, are generated monthly.

Function, process and component are represented as directed acyclic graphs (DAGs) or networks. A child term may be an "instance" of its parent term (isa relationship) or a component of its parent term (part-of relationship). A child term may have more than one parent term and may have a different class of relationship with its different parents. Synonyms and cross-references to external databases are also represented in the ontologies. RDF was chosen for use in the XML versions of the ontologies because of its flexibility in representing these graph structures, as well as its widespread tool support.

The following is a sample from the GO documentation:

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE go:go> 
<go:go xmlns:go="http://www.geneontology.org/xml-dtd/go.dtd#" 
  <go:version timestamp="Wed May 9 23:55:02 2001" /> 

     <go:term rdf:about="http://www.geneontology.org/go#GO:0003673"> 

     <go:term rdf:about="http://www.geneontology.org/go#GO:0003674"> 
        <go:definition>The action characteristic of a gene product.</go:definition> 
        <go:part-of rdf:resource="http://www.geneontology.org/go#GO:0003673" /> 

     <go:term rdf:about="http://www.geneontology.org/go#GO:0016209"> 
        <go:isa rdf:resource="http://www.geneontology.org/go#GO:0003674" /> 
           <go:evidence evidence_code="ISS"> 
           <go:evidence evidence_code="ISS"> 

The example illustrates that go:term is the basic element. The GO has added its own extensions to the RDF vocabulary (they do not use RDFS). For example, term GO:0016209 has the element <go:isa rdf:resource="http://www.geneontology.org/go#GO:0003674" />. This tag represents the relationship "GO:0016209 isa GO:0003674", or, in English, "Antioxidant is a molecular function." Another specialized relationship is go:part-of. For example, GO:0003674 has the element <go:part-of rdf:resource="http://www.geneontology.org/go#GO:0003673" />. This says that "Molecular function is part of the Gene Ontology".

Every annotation must be attributed to a source, which may be a literature reference, another database or a computational analysis. The annotation must indicate what kind of evidence is found in the cited source to support the association between the gene product and the GO term. A simple controlled vocabulary is used to record evidence. Examples include:

  • ISS means "inferred from sequence similarity [with <database:sequence_id>]"
  • IDA means "inferred from direct assay"
  • TAS means "traceable author statement"

The go:dbxref element represents the term in an external database, and go:association represents the gene associations of each term. go:association can have both go:evidence, which holds a go:dbxref to the evidence supporting the association, and a go:gene_product, which contains the gene symbol and go:dbxref.

The GO illustrates a number of interesting points. First, it shows that the value of using XML for information exchange can be enhanced by structuring that XML using RDF. This is particularly true for data that has a graph or network structure, rather than being a strict hierarchy. The GO is also another example in which the RDF will not necessarily appear for direct use on the Web (although the files are Web-accessible). It is also another example of data which is, on the surface, described as "XML", but on closer examination is RDF/XML. In addition, the GO illustrates the role RDF can play as a basis for representing ontologies. This role will be further enhanced once richer RDF-based languages for specifying ontologies, such as DAML+OIL or OWL, become more widely used.

7. Other Parts of the RDF Specification

In Section 1, we indicated that the RDF Specification consists of a number of documents (in addition to this Primer):

We have already discussed the subjects of the first three of these documents, basic RDF concepts (in Section 2), the RDF/XML syntax (in Section 3) and RDF Schema (in Section 5). In this section, we briefly describe the remaining documents, in order to explain their role in the complete specification of RDF.

7.1 RDF Semantics

@@Fair overlap now with Concepts doc description of MT. Reduce?@@

RDF is being developed as part of the W3C's Semantic Web Activity . As described in the Semantic Web Activity Statement,

The Semantic Web is an extension of the current Web in which information is given well-defined meaning, better enabling computers and people to work in cooperation. It is the idea of having data on the Web defined and linked in a way that it can be used for more effective discovery, automation, integration, and reuse across various applications. The Web can reach its full potential if it becomes a place where data can be shared and processed by automated tools as well as by people.

RDF is a language designed to support the Semantic Web, in much the same way that HTML is the language that helped initiate the original Web. In order to serve this purpose, the meaning of RDF statements must be defined in a very precise manner.

The RDF Semantics [RDF-SEMANTICS] provides this precise definition, using a technique known to logicians as a "model-theoretic semantics". A model-theoretic semantics for a language assumes that the language refers to a 'world', and describes the minimal conditions that a world must satisfy in order to assign an appropriate meaning for every expression in the language. A particular world is called an interpretation, so that model theory might be better called 'interpretation theory'. The idea is to provide an abstract, mathematical account of the properties that any such interpretation must have, making as few assumptions as possible about its actual nature or intrinsic structure. The RDF model theory is couched in the language of set theory because that is the normal language of mathematics - for example, the model theory assumes that names denote things in a set IR called the 'universe' - but the use of set-theoretic language in the RDF model theory is not supposed to imply that the things in the universe are set-theoretic in nature.

The chief utility of such a semantic theory is not to suggest any particular processing model, or to provide any deep analysis of the nature of the things being described by the language (in the case of RDF, the nature of resources), but rather to provide a technical tool to analyze the semantic properties of proposed operations on the language; in particular, to provide a way to determine when they preserve meaning.

The RDF model theory treats RDF as a simple assertional language, in which each triple makes a distinct assertion, and the meaning of any triple is not changed by adding other triples. Based on the semantics defined in the model theory, it is simple to translate an RDF graph into a logical expression with essentially the same meaning.

In other words, the RDF model theory provides the formal underpinnings for all of the concepts we have described.

7.2 Test Cases

@@Cover Brian's comments about this sect.@@

The RDF Test Cases [RDF-TESTS] supplement the textual RDF specifications with specific examples of RDF/XML syntax and the corresponding RDF graph triples. To describe these examples, it introduces a notation called N-triples, which provides the basis for the triples notation used throughout this Primer. The test cases themselves are also published in machine-readable form at Web locations referenced by the Test Cases document, so developers can use these as the basis for automated testing of RDF software.

The Test Cases document also contains a number of "entailment tests", which indicate entailments (conclusions) that applications are allowed by the RDF specifications to draw from RDF data.

The test cases are not a complete specification of RDF, and are not intended to take precedence over the normative specification documents. However, they are intended to illustrate the intent of the RDF Core Working Group with respect to the design of RDF, and developers may find these test cases helpful should the wording of the specifications be unclear on any point of detail.

8. References

8.1 Normative References

Resource Description Framework (RDF): Concepts and Abstract Syntax, Klyne G., Carroll J. (Editors), World Wide Web Consortium, 08 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/. The latest version is http://www.w3.org/TR/rdf-concepts/.
Application/rdf+xml Media Type Registration, Swartz A., IETF Internet Draft, August 2002 (work in progress). Version available at http://www.ietf.org/internet-drafts/draft-swartz-rdfcore-rdfxml-mediatype-01.txt .
Resource Description Framework (RDF) Model and Syntax Specification, Lassila O., Swick R. (Editors), World Wide Web Consortium. 22 February 1999. This version is http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/. The latest version is http://www.w3.org/TR/REC-rdf-syntax/.
RDF Semantics, Hayes P. (Editor), World Wide Web Consortium, 12 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-rdf-mt-20021112/. The latest version is http://www.w3.org/TR/rdf-mt/.
RDF/XML Syntax Specification (Revised), Beckett D. (Editor), World Wide Web Consortium, 8 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/. The latest version is http://www.w3.org/TR/rdf-syntax-grammar.
RDF Test Cases, Grant J., Beckett D. (Editors), World Wide Web Consortium, 12 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-rdf-testcases-20021112/. The latest version is http://www.w3.org/TR/rdf-testcases/.
RDF Vocabulary Description Language 1.0: RDF Schema, Brickley D., Guha R.V. (Editors), World Wide Web Consortium, 12 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-rdf-schema-20021112/. The latest version is http://www.w3.org/TR/rdf-schema/.
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax, Berners-Lee T., Fielding R., Masinter L., IETF, August 1998. This document is http://www.isi.edu/in-notes/rfc2396.txt.
Extensible Markup Language (XML) 1.0, Second Edition, Bray T., Paoli J., Sperberg-McQueen C.M, Maler E. (Editors), World Wide Web Consortium, 6 October 2000. This version is http://www.w3.org/TR/2000/REC-xml-20001006. The latest version is http://www.w3.org/TR/REC-xml.
XML Base, Marsh J. (Editor), W3C Recommendation. World Wide Web Consortium, 27 June 2001. This version is http://www.w3.org/TR/2001/REC-xmlbase-20010627/. The latest version is http://www.w3.org/TR/xmlbase/.
Namespaces in XML, Bray T., Hollander D., Layman A. (Editors), World Wide Web Consortium, 14 January 1999. This version is http://www.w3.org/TR/1999/REC-xml-names-19990114/. The latest version is http://www.w3.org/TR/REC-xml-names/.

8.2 Informational References

Addressing Schemes, Connolly D., 2001. This document is http://www.w3.org/Addressing/schemes.html.
Indexing and Access for Digital Libraries and the Internet: Human, Database, and Domain Factors, Bates M.J., 1996. This document is http://is.gseis.ucla.edu/research/mjbates.html.
What the Semantic Web can represent, Berners-Lee T., 1998. This document is http://www.w3.org/DesignIssues/RDFnot.html.
Conceptual Graphs, Sowa J., ISO working document ISO/JTC1/SC32/WG2 N 000, 2 April 2001 (work in progress). Available at http://users.bestweb.net/~sowa/cg/cgstand.htm.
Utility Management System (UMS) Data Access Facility, Object Management Group, OMG document formal/01-06-01, June 2001. This document is http://cgi.omg.org/docs/formal/01-06-01.pdf.
DAML+OIL (March 2001) Reference Description, Connolly D., van Harmelen F., Horrocks I., McGuinness D.L., Patel-Schneider P.F., Stein L.A., W3C Note 18 December 2001. This document is http://www.w3.org/TR/daml+oil-reference.
Dublin Core Metadata Element Set, Version 1.1: Reference Description, 02 July 1999. This document is http://dublincore.org/documents/dces/.
XML for CIM Model Exchange , deVos A., Widergreen S.E., Zhu J., Proc. IEEE Conference on Power Industry Computer Systems, Sydney, Australia, 2001. This document is http://www.langdale.com.au/PICA/.
Logic, Algebra and Databases, Gray P., Ellis Horwood Ltd., 1984. ISBN 0-85312-709-3, 0-85312-803-0, 0-470-20103-7, 0-470-20259-9.
In Defense of Logic, Hayes P., Proceedings from the International Joint Conference on Artificial Intelligence, 1975, San Francisco. Morgan Kaufmann Inc., 1977. Also in Computation and Intelligence: Collected Readings, Luger G. (ed), AAAI press/MIT press, 1995. ISBN 0-262-62101-0.
Knowledge Interchange Format, Genesereth M., draft proposed American National Standard NCITS.T2/98-004. Available at http://logic.stanford.edu/kif/dpans.html.
Artificial Intelligence: Structures and Strategies for Complex Problem Solving (3rd ed.), Luger G., Stubblefield W., Addison Wesley Longman, 1998. ISBN 0-805-31196-3.
Naming and Addressing: URIs, URLs, ..., Connolly D., 2002. This document is http://www.w3.org/Addressing/.
OWL Web Ontology Language 1.0 Reference, Dean M., Connolly D., van Harmelen F., Hendler J., Horrocks I., McGuinness D.L., Patel-Schneider P.F., Stein L.A. (Editors), World Wide Web Consortium, 12 November 2002 (work in progress). This version is http://www.w3.org/TR/2002/WD-owl-ref-20021112/. The latest version is http://www.w3.org/TR/owl-ref/.
PRISM: Publishing Requirements for Industry Standard Metadata, Version 1.1, 19 February 2002. This document is http://www.prismstandard.org/techdev/prismspec11.asp.
RDF Issue Tracking, McBride B., 2002. This document is http://www.w3.org/2000/03/rdf-tracking/.
RDF Site Summary (RSS) 1.0, Beged-Dov G., Brickley D., Dornfest R., Davis I., Dodds L., Eisenzopf J., Galbraith D., Guha R.V., MacLeod K., Miller E., Swartz A., van der Vlist E., 2000. This document is http://purl.org/rss/1.0/spec.
Knowledge Representation: Logical, Philosophical and Computational Foundations, Sowa J., Brookes/Cole, 2000. ISBN 0-534-94965-7.
Web Architecture: Describing and Exchanging Data, Berners-Lee T., Connolly D., Swick R., W3C Note, 7 June 1999. This document is http://www.w3.org/1999/04/WebData.
XML Linking Language (XLink) Version 1.0, DeRose S., Maler E., Orchard D. (Editors), World Wide Web Consortium, 27 June 2001. This version is http://www.w3.org/TR/2001/REC-xlink-20010627/. The latest version is http://www.w3.org/TR/xlink/.
XML Schema Part 2: Datatypes, Biron P., A. Malhotra (Editors), World Wide Web Consortium. 2 May 2001. This version is http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/. The latest version is http://www.w3.org/TR/xmlschema-2/.
XML Package (XPackage) 1.0 , Wilson G., Open eBook Forum Editor's Working Draft, 23 October 2002. This version is http://www.xpackage.org/specification/xpackage-draft-20021023.html. The latest version is http://www.xpackage.org/specification/.

9. Acknowledgments

This document has benefited from inputs from many members of the RDF Core Working Group. Specific thanks to Dave Beckett, Dan Brickley, Ron Daniel, Ben Hammersley, Martyn Horner, Graham Klyne, Sean Palmer, Patrick Stickler, Aaron Swartz, Ralph Swick, and Garret Wilson, who provided valuable contributions to this document.

In addition, this document contains a significant contribution from Pat Hayes, Sergey Melnik, and Patrick Stickler, who led the development of the RDF datatype facilities described in the RDF family of specifications.

Appendix A: Uniform Resource Identifiers (URIs) Survival Guide

@@This section needs editing now its an appendix.@@

If we want to discuss something, we must first identify it. How else will we know what we are referring to? In everyday communication, we use references such as "Bob", "The Moon", "373 Whitaker Ave.", "California", "VIN 2745534", "today's weather", etc., to identify things. Ambiguities in these identifiers are generally resolved in terms of a shared semantic context between the sender and the receiver. To refer to "things" on the Web, we also use identifiers.

As we've seen, the Web already provides one form of identifier, the Uniform Resource Locator (URL). We used a URL in our original example to identify the Web page that John Smith created. A URL is a character string that identifies a Web resource by representing its primary access mechanism (essentially, its network "location"). However, we would like to be able to record information about many things in addition to Web pages. In particular, we'd like to record information about lots of things that don't have network locations or URLs. For example, I (a human being) don't have a network location or URL, and yet my employer needs to record all sorts of things about me in order to pay my salary, keep track of the work that I've been doing, and so on. My doctor needs to record other sorts of things about me in order to keep track of my medical history, tests that have been performed (and the results, who performed them, and when), inoculations I've received, etc.

We've recorded information about lots of things that don't have URLs in files (both manual and automated) for many years, and the way we identify those things is by assigning them identifiers: values that we uniquely associate with the individual things. The identifiers we use to identify various kinds of things go by names like "Social Security Number", "Part Number", "license number", "employee number", "user-id", etc. In some cases, these identifiers (such as Social Security Numbers) are assigned by a recognized authority of some kind. In other cases, these identifiers are generated by a private organization or individual. In some cases, these identifiers have a national or international scope within which they are unique (a Social Security Number has national scope), while in other cases they may only be unique within a very limited scope (my employee number is only unique among the numbers assigned by my specific employer). Nevertheless, these identifiers serve, if used properly, to identify the things we want to talk about.

The Web provides its own form of identifier for these purposes, called the Uniform Resource Identifier (URI). The URLs we've already discussed are a particular kind of URI. All URIs share the property that different persons or organizations can independently create them, and use them to identify things. However, URIs are not limited to identifying things that have network locations, or use other computer access mechanisms. In fact, we can create a URI to refer to anything we want to talk about, including

  • network-accessible things, such as an electronic document, an image, a service (e.g., "today's weather report for Los Angeles"), or a group of other resources.
  • things that are not network-accessible, such as human beings, corporations, and bound books in a library.
  • abstract concepts that don't physically exist, like the concept of a "creator".

URIs essentially constitute an infinite stock of names that can be used to identify things. A number of different URI schemes (URI forms) have been already been developed, and are being used, for various purposes. Examples include:

  • http: (Hypertext Transfer Protocol, primarily for Web pages)
  • mailto: (email addresses), e.g., mailto:em@w3.org
  • ftp: (File Transfer Protocol)
  • urn: (Uniform Resource Names, intended to be persistent location-independent resource identifiers), e.g., urn:isbn:0-520-02356-0 (for a book)

URIs are defined in RFC 2396 [URIS]. Some additional discussion of URIs can be found in Naming and Addressing: URIs, URLs, ... [NAMEADDRESS]. A list of existing URI schemes can be found in Addressing Schemes [ADDRESS-SCHEMES], and it is a good idea to consider adapting one of the existing schemes for any specialized identification purposes you may have, rather than trying to invent a new one.

No one person or organization controls who makes URIs or how they can be used. While some URI schemes, such as URL's http:, depend on centralized systems such as DNS, other schemes, such as freenet:, are completely decentralized. This means that, as with any other kind of name, you don't need special authority or permission to create a URI for something. Also, you can create URIs for things you don't own, just as in ordinary language you can use whatever name you like for things you don't own. The URI is the foundation of the Web. While nearly every other part of the Web can be replaced, the URI cannot: it holds the Web together.

Since the URI is such a general identification mechanism, capable of identifying anything, it should not be surprising that RDF uses URIs as the basis of its mechanism for identifying the subjects, predicates, and objects in statements. To be more precise, RDF uses URI references [URIS] to define its subjects, predicates, and objects. A URI reference (or URIref) is a URI, together with an optional fragment identifier at the end. For example, the URI reference http://www.example.org/index.html#section2 consists of the URI http://www.example.org/index.html and (separated by the "#" character) the fragment identifier Section2. RDF defines a resource as anything that is identifiable by a URI reference, and hence using URIrefs allows RDF to describe practically anything, and to state relationships between such things as well.

In order to make writing URIrefs easier, URIrefs may be either absolute or relative. An absolute URIref refers to a resource independently of the context in which the URIref appears, e.g., the URIref http://www.example.org/index.html. A relative URIref is a shorthand form of an absolute URIref, where some prefix of the URIref is missing, and information from the context in which the URIref appears is required to fill in the missing information. For example, the relative URIref otherpage.html, when appearing in a resource http://www.example.org/index.html, would be filled out to the absolute URIref http://www.example.org/otherpage.html. A URIref without a URI part is considered a reference to the current document (the document in which it appears). So, an empty URIref within a document is considered equivalent to the URIref of the document itself. A URIref consisting of just a fragment identifier is considered equivalent to the URIref of the document in which it appears, with the fragment identifier appended to it. For example, within http://www.example.org/index.html, if #section2 appeared as a URIref, it would be considered equivalent to the absolute URIref http://www.example.org/index.html#section2.

Both RDF and web browsers use URIrefs to identify things. However, RDF and browsers interpret URIrefs in slightly different ways. This is because RDF uses URIrefs only to identify things, while browsers also use URIrefs to retrieve things. Often there is no effective difference, but in some cases the difference can be significant. One obvious difference is when a URIref is used in a browser, there is the expectation that it identifies a resource that can actually be retrieved: that something is actually "at" the location identified by the URI. However, in RDF a URIref may be used to identify something, such as a person, that cannot be retrieved on the web. People sometimes use RDF together with a convention that, when a URIref is used to identify an RDF resource, a page containing descriptive information about that resource will be placed on the web "at" that URI, so that the URIref can be used in a browser to retrieve that information. This can be a useful convention in some circumstances (although it creates a difficulty in distinguishing the identity of the original resource from the identity of the web page describing it). However, this convention is not an explicit part of the definition of RDF, and RDF itself does not assume that a URIref identifies something that can be retrieved.

Another difference is in the way URIrefs with fragment identifiers are handled. Fragment identifiers are often seen in URLs that identify HTML documents, where they serve to identify a specific place within the document identified by the URL. In normal HTML usage, where URI references are used to retrieve the indicated resources, the two URIrefs:


are related (they both refer to the same document, the second one identifying a location within the first one). However, as noted already, RDF uses URI references purely to identify resources, not to retrieve them, and RDF assumes no particular relationship between these two URIrefs. As far as RDF is concerned, they are syntactically different URI references, and hence may refer to unrelated things. (This doesn't mean that the HTML-defined containment relationship might not exist, just that RDF doesn't assume that a relationship exists based only on the fact that the URI parts of the URI references are the same.)

In later sections, we'll see how RDF uses URIrefs for identifying the subjects, predicates, and objects in statements. But before we do that, we need to briefly introduce, in the next section, the basis of how RDF statements can be physically represented and exchanged.

Appendix B: Extensible Markup Language (XML) Survival Guide

@@This section needs editing now its an appendix.@@

The Extensible Markup Language [XML] was designed to allow anyone to design their own document format and then write a document in that format. Like HTML documents (Web pages), XML documents contain text. This text consists primarily of plain text content, and markup in the form of tags. This markup allows a processing program to interpret the various pieces of content (elements). In HTML, the set of permissible tags, and their interpretation, is defined by the HTML specification. However, XML allows users to define their own markup languages (tags and the structures in which they can appear) adapted to their own specific requirements. For example, the following is a simple passage marked up using an XML-based markup language:

<sentence><person href="http://example.com/#me">I</person> 
just got a new pet <animal>dog</animal>.</sentence>

Elements delimited by tags (<sentence>, <person>, etc.) are introduced to reflect a particular structure associated with the passage. These tags allow a program written with an understanding of these particular elements to properly interpret the passage.

This particular markup language uses the words "sentence," "person," and "animal" as tag names in an attempt to convey some of the meaning of the elements; and they would convey meaning to an English-speaking person reading it, or to a program specifically written to interpret this vocabulary. However, there is no built-in meaning here. For example, to non-English speakers, or to a program not written to understand this markup, the element <person> may mean absolutely nothing. Take the following passage, for example:

<dfgre><reghh bjhb="http://example.com/#me">I</reghh> 
just got a new pet <yudis>dog</yudis>.</dfgre>

To a machine, this passage has exactly the same structure as the previous example. However, it is no longer clear to an English-speaker what is being said, because the tags are no longer English words. Moreover, others may have used the same words as tags in their own markup languages, but with completely different intended meanings. For example, "sentence" in another markup language might refer to the amount of time that a convicted criminal must serve in a penal institution. So additional mechanisms must be provided to help keep XML vocabulary straight.

To prevent confusion, it is necessary to uniquely identify markup elements. This is done in XML using XML Namespaces [XML-NS]. A namespace is just a way of identifying a part of the Web (space) which acts as a qualifier for a specific set of names. A namespace is created for an XML markup language by creating a URI for it. By qualifying tag names with the URIs of their namespaces, anyone can create their own tags and properly distinguish them from tags with identical spellings created by others. A useful practice is to create a Web page to describe the markup language (and the intended meaning of the tags) and use the URL of that Web page as the URI for its namespace. The following example illustrates the use of an XML namespace.

<my:sentence xmlns:my="http://example.org/xml/documents/">
   <my:person my:href="http://example.com/#me">I</my:person> 
just got a new pet <my:animal>dog</my:animal>.

In this example, xmlns:my="http://example.org/xml/documents/ declares a namespace for use in this piece of XML. It maps the prefix my to the namespace URI http://example.org/xml/documents/. The XML content can then use qualified names (or QNames) like my:person as tags. A QName contains a prefix that identifies a namespace, followed by a colon, and then a local name for an XML tag (element) or attribute. By using namespace URIs to distinguish specific groups of names, and qualifying tags with the URIs of the namespaces they come from, as in this example, we don't have to worry about tag names conflicting. Two tags having the same spelling are considered the same only if they also have the same namespace URIs.

RDF defines a specific XML markup language, referred to as RDF/XML, for use in representing RDF information, and for exchanging it between machines. An example of RDF/XML was given in Section 1, and the language is described in more detail in Section 3.

Appendix C: Changes

Changes since the 26 April 2002 Working Draft:

  • Added Appendix A.
  • Miscellaneous formatting corrections.
  • Introduced abbreviated notation for triples and made corresponding changes to the examples.
  • Divided References into Normative and Informational References, added references, and corrected format.
  • Added additional material on URIs and discussion about fragment identifiers.
  • Changed many references from "URI" to "URIref" to clarify the distinction made in the above material.
  • Corrected material on rdf:ID and rdf:about, and on parseType="Resource", in Section 3.
  • Added material on xml:base and basic serialization syntax to Section 3.
  • Added material on rdf:type and RDF/XML abbreviations to Section 3.
  • Corrected material on RDF capabilities in the Abstract and Section 1.
  • Added material on reification, rdf:value, and Boolean valued properties, and added a new section to hold it.
  • Rewrote the RDF Schema material to emphasize its descriptive role, and to include how xml:base might be used, and changed its placement.
  • Added an example to Section 2 illustrating the use of blank nodes to model things that don't naturally have URIs.
  • Added a brief description of metadata to the Dublin Core section.
  • Added additional subsections of Section 6 describing further examples of RDF usage.
  • Added material on typed literals as Section 2.5 and in Section 3.1, and material on using datatypes in schemas in Section 5.2.
  • Added coverage of rdf:parseType="Literal" to Section 3.3.
  • Rewrote Section 4 to better describe RDF containers, and added coverage of RDF Lists.
  • Added the Concepts document to the list of RDF specifications, and added various references to it.

Changes since the 11 November 2002 Working Draft:

@@Fill in@@

  • Updated references.
  • Moved most of the URI and XML discussion to appendices.
  • Removed discussion of many RDF/XML abbreviations (pointed to RDF/XML Syntax Specification).
  • Rewrote Section 4.
  • Removed discussion of Boolean-valued properties.
  • Removed discussion of RDF datatype framework (pointed to RDF Concepts document).
  • Added figure for example in Section 1.
  • Miscellaneous edits.
  • Redrew all the figures.
  • Added background reading material from Concepts.
  • Substituted "group" for "collection" in generic uses (to distinguish from RDF collections).
  • .