RDF Primer

Editor's Working Draft 25 July 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, the availability schedule for some shared resource, or the description of a Web user's preferences for information delivery. 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 prices, publishers, and availability), and information about people referred to on the Web. 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. This Primer is designed to provide the reader with the basic fundamentals required to effectively use RDF in their particular applications.

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 decisions made by the Working Group designed to provide the reader the basic fundamentals 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. @@In particular, we are still processing comments received on the previous version, so this version does not reflect all the changes that might be made based on those comments.@@

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/.

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
      2.1 Uniform Resource Identifiers (URIs)
      2.2 Documents: Extensible Markup Language (XML)
      2.3 The RDF Model
      2.4 Structured Property Values and Blank Nodes
  3. An XML Syntax for RDF
  4. Other RDF Classes and Properties
      4.1 RDF Containers
      4.2 RDF Reification
      4.3 Miscellaneous RDF Facilities
  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 Intelligent Routing
      6.5 RSS: RDF Site Summary 1.0
  7. Other Parts of the RDF Specification
      7.1 Model Theory
      7.2 Test Cases
  8. RDF As a Data Model
  9. References
      9.1 Normative References
      9.2 Informational References
10. Acknowledgments


  A. 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, the availability schedule for some shared resource, or the description of a Web user's preferences for information delivery. 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 prices, publishers, and availability), and information about people referred to on the Web.

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.

To make this discussion somewhat more concrete as soon as possible, the following is a small chunk of RDF in its XML serialization format.

<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"/>
    <personalTitle>Semantic Web Activity Lead</personalTitle> 

This example roughly translates as a collection of statements "there is someone whose name is Eric Miller, whose email address is em@w3.org, and whose title is Semantic Web Activity Lead". Note that the example contains what seem to be Web addresses, as well as some properties like mailbox and fullName, and their respective values em@w3.org, and Eric Miller.

Like HTML, this form of information is machine processable, and links pieces of data across the Web. However, unlike conventional hypertext, RDF references can refer to any identifiable thing, including things that may or may not be Web-based data. The result is that in addition to describing Web pages, we can also convey information about cars, businesses, people, news events, etc. Further, RDF references themselves can be labeled, to indicate the kind of relationship that exists between the linked items.

The complete specification of RDF consists of a number of documents:

This Primer is intended to augment the other parts of the RDF specification, 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 (from the W3C's point of view) specification of RDF. The examples and other explanatory material in this document 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 (facts about) Web resources, e.g., Web pages. For example, imagine that we want to record the fact that someone named John Smith created a particular Web page. A straightforward way to state this fact in English would be in the form of a simple statement, e.g.:

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 (the creator) 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, object, or predicate 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 format 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.

2.1 Uniform Resource Identifiers (URIs)

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 collection 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 [URI]. 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 [URI] 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.

While both RDF and web browsers use URIrefs to identify things, 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, like a person, that has no physical existence on the web, and hence can't be retrieved. 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 URI references:


are related (they both refer to the same document, the second one being 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 http://www.example.org/index.html and http://www.example.org/index.html#Section2. As far as RDF is concerned, they are just 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 are physically represented and exchanged.

2.2 Documents: Extensible Markup Language (XML)

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" to attempt to convey meaning; 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 the exact 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 collections 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 writing down 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.

In RDF/XML, XML QName tags are used with namespace URIs to provide URIrefs for the names used in RDF. This is done by concatenating the namespace URI and the local (tag) name. For example, if the XML namespace assigned to prefix foo has the URI http://example.org/somewhere/, then the tag foo:bar would correspond to the RDF resource with URIref http://example.org/somewhere/bar. Similarly, in the previous example, the my:person tag would have the namespace URI http://example.org/xml/documents/ concatenated with local name person, giving it the URIref http://example.org/xml/documents/person. This simple method restricts the URIrefs that can be generated from the XML, and allows the same URIref to be constructed in multiple ways, but if care is taken, it works satisfactorily.

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
Figure 1: A Simple RDF Statement (SVG version)

Collections of statements are represented by corresponding collections 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 (SVG version)

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

Sometimes it is not convenient to draw graphs, so an alternative way of writing down the statements, called N-Triples, can also be used. In the N-Triples notation, each statement in the graph is written as a simple triple of subject, predicate, and object node labels (either URIref or character string), in that order. The N-Triples representing the above three statements would be written:

<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 N-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 N-Triples representation of the graph, but only once in the drawn graph. However, the triples represent exactly the same information as the graph.

The N-triples syntax requires that URI references be written out in full, 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. We will also make extensive use in these examples of several "well-known" QName prefixes (which we will use without explicitly specifying them), 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/

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:

<http://www.example.org/index.html> dc:creator exstaff:85740 .

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

<http://www.example.org/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 having "John Smith" as their name, 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:

      Information About John Smith
Figure 3: More Information about John Smith (SVG version)

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 (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.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

<http://www.example.org/index.html>  dc:creator  exstaff:85740 .

the predicate dc:creator is an unambiguous reference to the "creator" attribute in the Dublin Core metadata attribute set, a widely-used collection 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 defined 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 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 information (the list of book rating averages) and create a "Top Ten Highest Rated Books" page. The availability and use of a shared vocabulary about ratings allows individuals to build a mutually-understood and increasingly-powerful "information base" about books 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. This relationship is further explored in Section 8.

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 simple character string. However, suppose we wanted to record 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, as in the triple

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

However, suppose we wanted to use RDF to record the various pieces of information about his address as separate street, city, state, and Zip code values. How do we do this?

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 separate resource, and then making separate 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 (SVG version)

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 order to draw the graph representing the collection 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
Figure 5: Using a Blank Node (SVG version)

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. However, we do need some form of explicit identifier for that node if we are going 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 also need a way to differentiate between the various blank nodes in the corresponding triples representation of the graph. To do this, the triples notation uses a node identifier, having the form _:name, to indicate the presence of a blank node. 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 character string 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 collections 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.

@@Might want to say something at this point about the relationship of a "graph" with a "document".@@

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 separate resource, and then making separate 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 his address. When we try to deal with an address as an aggregate of (street, city, state, zip), we are effectly dealing with an n-ary (n-way) relationship (in this case, n=4) between the street, city, state, and zip components. In order to represent such structures directly in RDF (e.g., considering the address as a collection of street, city, state, and zip sub-components), we need to break this n-way relationship up into a collection of separate binary relationships.

Blank nodes give us a general way of representing such n-ary relationships using RDF triples. Each time we have an n-ary relationship (such as the address example), we can create a blank node representing an instance of the relationship, and then represent the 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 model resources that may not have URIs, but that are described in terms of relationships with other resources that do have URIs. For example, when representing a person, 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's email address may change when she changes jobs, and so it may be hard to combine information about Jane acquired 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 (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 fundamental problem here is that using Jane's email address as a stand-in for Jane is an inaccurate model: Jane's email address identifies a mailbox, and Jane and her mailbox are not the same thing. 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 emailaddress property having the URIref mailto:jane@example.org as its value. We can also assign the blank node a type property with a value of Person, a name property with a value of "Jane Smith", and any other descriptive information we might want to provide (we will discuss types in more detail in the following sections). This says, accurately, that "there is a resource of type Person whose email address is mailto:jane@example.org, whose name is Jane Smith, etc."

In practice, this doesn't change the way we actually handle this kind of information. 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 relate information from multiple sources. 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" simply makes what is actually happening more explicit. Of course, modeling the person and her mailbox as separate resources makes sense even when the person actually has a URI, since we still want to distinguish the person from her mailbox.

@@Note the example of the RDF/XML Syntax document editor in Section 3 has this form too. Might also note that more advanced Schema languages (described later) such as DAML+OIL allow specification of which properties are unique identifiers.@@

This is all there is to basic RDF: nodes-and-arcs diagrams interpreted as statements about concepts or digital resources identified by URIrefs . However, it should be clear that, in addition to the basic techniques for representing RDF statements in diagrams (or triples), we also need a way for people to define the vocabularies they intend to use in those statements, e.g., to define properties like "city" and "creator" (and even types of things like "Person"). The basis for defining such vocabularies in RDF is RDF Schema , which will be described in Section 4 . Additional discussion of the basic ideas underlying the RDF data model, and its role in providing a general language for describing Web information, can be found in [WEBDATA].

3. An XML Syntax for RDF

To summarize what we have said already, RDF models statements in terms of a graph consisting of nodes and arcs. The nodes describe resources that can be labeled with URIrefs, character string literals, or are blank. The arcs connect the nodes and are all labeled with URIrefs. This graph is more precisely called a labeled directed graph; each edge is an arc with a direction (an arrow) connecting two nodes. These edges can also be described as triples of subject node , at the blunt end of the arrow/arc, property arc and an object node at the sharp end of the arrow/arc. The property arc is interpreted as an attribute, relationship or predicate of the resource, with a value given by the object node content.

RDF defines an XML syntax for writing down and exchanging RDF graphs. This syntax is defined in the RDF/XML Syntax Specification [RDFXML]. 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 statement, after assigning a URIref to the creation-date property, is shown in Figure 6:

with a triple representation of:

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

Corresponding RDF/XML syntax for this graph would be:

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

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

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 namespace name (a URI) 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 ex:. This is expressed as another xmlns attribute of the rdf:RDF element, and specifies that the namespace URI http://www.example.org/terms/ is to be associated with the ex: 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.

Line 4 begins 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). This is exactly the way the RDF/XML represents the statement. The rdf:Description element in Line 4 indicates that we're starting a description, 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 <ex:creation-date> as its tag, to hold the value August 19, 1999 of the creation-date property of the statement. It is nested within the preceding rdf:Description element, indicating that this property applies to the resource specified in the containing rdf:Description element. An RDF processor would form the complete URIref of the creation-date property from the QName <ex:creation-date> by converting the ex: prefix to the namespace URI defined for it in Line 3, and appending creation-date to it.

Finally, Lines 6 and 7 indicate the ends of the rdf:Description and rdf:RDF elements, respectively.

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 URI, local name) pair are chosen such that concatenating them forms the original node URIref. The URIrefs labeling 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.

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. 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 collection of statements about http://www.example.org/index.html:

<http://www.example.org/index.html>  dc:creator  exstaff:85740 .
<http://www.example.org/index.html>  exterms:creation-date  "August 16, 1999" .
<http://www.example.org/index.html>  exterms:language "English" .

whose graph (the same as Figure 2) is shown in Figure 7:

      Statements About the Same Resource
Figure 7: Several Statements About the Same Resource (SVG version)

the RDF/XML syntax for this graph would be:

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:ex="http://www.example.org/terms/">

5.    <rdf:Description rdf:about="http://www.example.org/index.html">
6.         <ex:creation-date>August 16, 1999</ex:creation-date>
7.         <ex:language>English</ex: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 example, we've added an additional namespace declaration (in Line 3), and two additional property elements (in Lines 7 and 8). The ex:language element in Line 7 is similar to the ex:creation-date element we defined earlier. 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. The dc:creator element on Line 8 represents a property whose value is another resource, rather than a character string. In this case, we've represented the property by 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, it must be written out in full, rather than abbreviating it as a QName as we've done in writing element and attribute names (this is due to the need to conform to XML syntax). This element tag also uses a different namespace prefix, the new namespace prefix dc: we defined in Line 3.

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 tent) might be written:

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

4.     <rdf:Description rdf:ID="10245">
5.          <ex:product>tent</ex:product>
6.          <ex:model>Overnighter</ex:model>
7.          <ex:sleeps>2</ex:sleeps>
8.          <ex:weightInKg>2.4</ex:weightInKg>
9.          <ex:packedSize>14x56</ex:packedSize>
10.    </rdf:Description>

  ...other product descriptions...

11.  </rdf:RDF>

(We've included the surrounding xml, RDF, and namespace information in lines 1 through 3, and line 11, 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 ("10245" 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 10245 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 #10245 to it, giving the URIref http://www.example.com/2002/04/products#10245.

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 #10245 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="#10245" instead of rdf:ID="10245" (i.e., by specifying the relative URIref directly). The full URIref formed by RDF is the same in either case: http://www.example.com/2002/04/products#10245.

RDF located outside the catalog could refer to this catalog entry by using the full URIref, i.e., by concatenating the relative URIref #10245 of the catalog entry to the base URI of the catalog, forming the absolute URIref http://www.example.com/2002/04/products#10245. 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#10245">
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 the fact 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 an rdf:Description element that refers to the original resource using rdf:about.

The previous example indicated that fragment identifiers such as #10245 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 an RDF processor retrieved the catalog from the mirror site, the URIref generated for our example tent would be http://mirror.example.com/2002/04/products#10245, rather than http://www.example.com/2002/04/products#10245, 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 base URI of the document. 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:ex="http://www.example.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products">

5.     <rdf:Description rdf:ID="10245">
6.          <ex:product>tent</ex:product>
7.          <ex:model>Overnighter</ex:model>
8.          <ex:sleeps>2</ex:sleeps>
9.          <ex:weightInKg>2.4</ex:weightInKg>
10.         <ex:packedSize>14x56</ex:packedSize>
11.    </rdf:Description>

  ...other product descriptions...

12.  </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, #10245, will generate the same absolute URIref, http://www.example.com/2002/04/products#10245, no matter where the catalog is located.

The RDF/XML syntax has many other capabilities. For example, Figure 8 (from [RDF-XML]) 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/' ".

Graph for
      Another RDF/XML Example
Figure 8: Graph for Another RDF/XML Example (SVG version)

In this case, the graph contains a blank node representing the editor (who apparently has not been given a URIref). Some RDF/XML corresponding to Figure 8 is:

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:ex="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.       <ex:editor rdf:parseType="Resource">
8.         <ex:fullName>Dave Beckett</ex:fullName>
9.         <ex:homePage rdf:resource="http://purl.org/net/dajobe/" />
10.      </ex:editor>
11.    </rdf:Description>
12. </rdf:RDF>

Much of this XML is similar to what we have seen before. What is new is in lines 7-10, which specify the blank node, and its properties and their values. Line 7 starts with ex:editor, indicating that it is describing the ex:editor property of the containing rdf:Description element (in Line 5). However, it uses an attribute rdf:parseType="Resource to indicate that it is defining a new, unnamed resource as the value of the ex:editor property, corresponding to the blank node in the graph. Within the ex:editor start and end tags (on lines 7 and 10), lines 8 and 9 define the ex:fullName and ex:homePage properties of this new resource, respectively. The end tag </ex:editor> on line 10 indicates the end of the information provided about this new resource.

These examples 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 about writing RDF in XML, refer to the RDF/XML Syntax Specification [RDF-XML].

@@It is intended to add some additional material here briefly introducing additional RDF/XML abbreviations, the point being to indicate the capabilities provided to write RDF statements using XML that looks "almost like" more conventional XML. The RDF in HTML material is intended to go here too. Variants of the RDF instance definitions from the RDF Schema section will go here to introduce rdf:type, as a hook pointing to "type-like" things. RDF Schema will then be pointed to as a way to write these "type-like" things (languages like DAML+OIL being other ways)@@.

4. Other RDF Classes and Properties

RDF defines a number of additional classes and properties, providing capabilities for representing containers and RDF statements, and for deploying RDF information in the World Wide Web. These additional classes and properties are described in the following sections.

4.1. RDF Containers

There is often a need to represent collections 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 container types that can be used to do this.

A Bag (a resource having type rdf:Bag) is an unordered collection of resources or literals. A Bag is used to represent a collection that has multiple values, and there is no significance to the order in which the values are given. For example, a Bag might be used to represent a collection of part numbers in which the order of entry or processing of the part numbers does not matter. A Bag can contain duplicate (i.e., repeated) values.

A Sequence (a resource having type rdf:Seq) is an ordered collection of resources or literals. A Sequence is used to represent a collection that has multiple values, and the order of the values is significant. For example, a Sequence might be used to represent a collection that must be maintained in alphabetical order. A Sequence can contain duplicate values.

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

The distinction between a Bag and an Alternative 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 Alternative more accurately represents the relationship than the Bag (which might suggest there are two different authors).

To represent a specific instance of one of these types of collections, you create a new resource, and give it an rdf:type property whose value is one of the pre-defined resources rdf:Bag, rdf:Seq, or rdf:Alt (whichever is appropriate). This new container resource represents the collection as a whole, and may either be a blank node or be given a URIref. The members of the collection are then indicated by defining a membership property for each member that has the new container resource as its subject and the member resource as its object. These membership properties have the names rdf:_1, rdf_2, rdf_3, and so on, and are used specifically for defining the members of containers. Container resources may also have other properties that describe the container, in addition to the membership properties and the rdf:type property.

A typical use of a container is to represent the value of a property. For example, to represent the sentence "The students in course 6.001 are Amy, Tim, John, Mary, and Sue", the RDF graph might be as shown in Figure 10:

A Simple Bag Container
Figure 10: A Simple Bag Container (SVG version)

This can 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://mycollege.edu/courses/6.001">
            <rdf:li rdf:resource="http://mycollege.edu/students/Amy"/>
            <rdf:li rdf:resource="http://mycollege.edu/students/Tim"/>
            <rdf:li rdf:resource="http://mycollege.edu/students/John"/>
            <rdf:li rdf:resource="http://mycollege.edu/students/Mary"/>
            <rdf:li rdf:resource="http://mycollege.edu/students/Sue"/>

Since the value of the s:students property is expressed as a Bag, there is no significance in the order given for the URIrefs of each student.

Note that the RDF/XML uses li as a convenience element to avoid having to explicitly number each membership property. The RDF processor will generate the numbered properties rdf:_1, rdf:_2, and so on from the li elements as necessary. The element name li was chosen to be mnemonic with the term "list item" from HTML.

As an illustration of an Alternative container, the sentence "The source code for X11 may be found at ftp.x.org, ftp.example.org, or ftp.example2.org" would have the RDF graph shown in Figure 11:

A Simple Alternative Container
Figure 11: A Simple Alternative Container (SVG version)

This can 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://x.org/packages/X11">
            <rdf:li rdf:resource="ftp://ftp.x.org"/>
            <rdf:li rdf:resource="ftp://ftp.example.org"/>
            <rdf:li rdf:resource="ftp://ftp.example2.org"/>

In this case, the value of the s:DistributionSite site property is considered to be one of the members of the Alternative container, any one of which would be an acceptable value. An Alternative container is required to have at least one member. This member is identified by the property rdf:_1, and 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.

Alternative 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 Alternative container holding each of the language variants.

The examples above illustrate that the general structures of the RDF graphs for both Bags and Alternatives are the same (and they are also the same for Sequences); only the indicated rdf:type is different. RDF considers these types as essentially "hints" to a processing application on how to properly interpret the structures. This is because RDF processors are not in a position to control how an application actually uses these structures. For example, an RDF processor has no way to force an application to use the first member of an Alternative collection as a default value. Similarly, an RDF processor has no way to force an application to ignore order in processing a Bag.

RDF processors are also limited in their ability to enforce structural constraints on these collections. For example, these structures explicitly permit duplicate values. RDF does not define a Set container, which would be a Bag with no duplicates, because RDF processors are not necessarily in a position to enforce a no-duplicates constraint (for example, a duplicate might exist somewhere else on the web, unknown to the processor). Also, if you create the membership properties yourself, RDF does not insist that the property numbers be contiguous starting with rdf:_1. For example, you could create a legal Bag with just the membership properties rdf:_3, rdf:_7, rdf:_8, and rdf:_11 (although an RDF processor would not generate these property names from a collection of rdf:li properties).

Users are also free to choose their own representations for collections, rather than using the ones described here. These RDF containers are merely provided as common definitions that, if generally used, would help make data involving collections more interoperable.

A resource may have multiple statements with the same predicate (i.e. using the same property). This is not the same as having a single statement whose object is a container containing multiple members. The choice of which to use in a given situation in some cases involves a difference in meaning, while in other cases is a simple choice of representation.

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 ex:publication ex:AnthologyOfTime .
exstaff:Sue ex:publication ex:ZoologicalReasoning .
exstaff:Sue ex: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 true or false independently of the others Hence, using repeated properties is a reasonable choice.

However, the sentence:

The committee of Fred, Wilma, and Dino approved the resolution.

says that the three committee members as a whole voted in a certain manner; 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 the three separate approvedBy statements, one for each committee member, shown below:

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

since this could imply the vote of each individual member.

In this case, it would be better to model the sentence as a single approvedBy statement whose subject is the resolution and whose object is a separate resource representing the entire committee. The resource representing the committee could be a Bag containing the committee members' identities, as in the following:

ex:resolution ex:approvedBy _:z
_:z rdf:type rdf:Bag .
_:z rdf:_1 ex:Fred .
_:z rdf:_2 ex:Wilma .
_:z rdf:_3 ex:Dino .

Alternatively, the resource representing the committee could be a non-Bag resource. This resource might in turn have a members property with the Bag of members as its value, or separate memberOf properties for each member (since each person is independently a member of the committee).

4.3. Miscellaneous RDF Facilities

@@Organization of this section is in progress.@@


Often the value of a property is something that has additional contextual information that is considered "part of" that value. In other words, there is a need to qualify property values. Examples of such qualification include naming a unit of measure, a particular restricted vocabulary, or some other annotation. For some uses it is appropriate to use the property value without the qualifiers. For example, in the statement "the price of that pencil is 75 U.S. cents" it is often sufficient to say simply "the price of that pencil is 75".

In the RDF model a qualified property value is simply another instance of a structured value. The object of the original statement is this structured value and the qualifiers are further properties of this common resource. The principal value being qualified is given as the value of the rdf:value property of this common resource.

@@second part of description@@

The RDF data model intrinsically only supports binary relations; that is, a statement specifies a relation between two resources. In the following examples we show the recommended way to represent higher arity relations in RDF using just binary relations. The recommended technique is to use an intermediate resource with additional properties of this resource giving the remaining relations. As an example, consider the subject of one of John Smith's recent articles -- library science. We could use the Dewey Decimal Code for library science to categorize that article. Dewey Decimal codes are far from the only subject categorization scheme, so to hold the classification system relation we identify an additional resource that is used as the value of the subject property and annotate this resource with an additional property that identifies the categorization scheme that was used. As specified in Section 2.3., the RDF core includes a rdf:value property to denote the principal value of the main relation. The resulting graph might look like:

@@All these examples, if they stay, need further checking.@@

<http://www.example.org/Jan97> dc:subject         _:category .
_:category                     rdf:value          "020 - Library Science" .
_:category                     ex:classification  "Dewey Decimal Code" .

which could be exchanged as:

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

   <rdf:Description rdf:about="http://www.example.org/Jan97.html">
          rdf:value="020 - Library Science"
          ex:classification="Dewey Decimal Code"/>

Note that two namespace declarations exist for the same namespace in this example. This is frequently needed when default namespaces are declared so that attributes that do not come from the namespace of the element may be specified, as is the case with the rdf:value attribute in the dc:subject element above.

@@It's not clear the above is illustrated properly@@

A common use of this higher-arity capability is when dealing with units of measure. A person's weight is not just a number such as "200", it also includes the unit of measure used. In this case we might be using either pounds or kilograms. We could use a relationship with an additional arc to record the fact that John Smith is a rather strapping gentleman:

exstaff:85740  exterms:weight  _:johnweight .
_:johnweight   rdf:value  "200" .
_:johnweight   exterms:units   exunits:Pounds .

which can be exchanged as:

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

   <rdf:Description rdf:about="http://www.example.org/staffid/85740">
      <ex:weight rdf:parseType="Resource">
        <ex:units rdf:resource="http://www.example.org/units/Pounds/" />

provided the resource "Pounds" is defined in a example.org schema with the URIref http://www.example.org/units/Pounds/.

Boolean valued properties

RDF does not define the values TRUE and FALSE for use as property values.

Such definitions are not necessary: a similar effect can be achieved by using data types to indicate Boolean attributes of a resource. This is entirely consistent with the way that the formal semantics treats RDF classes.


@@This material is here only temporarily. It is intended to place it in Section 3, where it can be used, among other things, to illustrate the use of one of the RDF/XML abbreviation formats.@@

The RDF/XML serialization syntax provides different forms of expression that are equivalent for the purposes of RDF, but which may be handled differently when the RDF is embedded in another document format. Consider the following piece of RDF:

    <rdf:Description rdf:about="http://www.w3.org">
      <ex:publisher>World Wide Web Consortium</ex:publisher>
      <ex:title>W3C Home Page</ex:title>      

which is equivalent for RDF purposes to (using attributes instead of elements):

    <rdf:Description rdf:about="http://www.w3.org"
           ex:publisher="World Wide Web Consortium"
           ex:title="W3C Home Page"      

While these are equivalent RDF expressions, they may be treated differently by other processing engines. In particular, if these two expressions were embedded into an HTML document then the default behavior of a non-RDF-aware browser would be to display the values of the properties in the first case while in the second case there should be no text displayed (or at most a whitespace character).

5. Defining RDF Vocabularies: RDF Schema

RDF defines a data model for expressing simple statements about resources, using named properties and values. However, RDF user communities also need the ability to indicate that they are describing certain types or classes of resources, and to define specific properties to be used in describing those resources. For example, the company example.com from our examples in Section 3 would want to define classes such as ex:Tent, and properties such as ex:model, ex:weightInKg, and ex: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 describe them using properties such as ex2:author, ex2:title, and ex2:subject. Other applications might require defining classes such as ex3:Person and ex3:Company, and properties such as ex3:age, ex3:jobTitle, ex3:stockSymbol, and ex3:numberOfEmployees. The RDF data model itself provides no vocabulary for specifying these things. Instead, such classes and properties are defined in an RDF schema . The facilities for defining RDF schemas are specified in RDF Vocabulary Description Language 1.0: RDF Schema [RDFSCHEMA].

RDF Schema does not provide a specific vocabulary of application-oriented classes like ex:Tent, ex2:Book, or ex3:Person, and properties like ex:weightInKg, ex2:author or ex3:JobTitle. Instead, it provides the mechanisms needed to define such classes and properties, 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 4.3.

RDF Schema uses the RDF data model itself to define 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. These resources become part of the RDF model of any description that uses them, and extend the meaning of that description for any processor that understands the extended vocabulary. 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 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 collection of resources that belong to the class, called its instances. In this case, we intend the class xyz:MotorVehicle to represent the collection of resources that we intend to represent motor vehicles. As we noted in earlier sections, all things described in RDF are called resources, and all resources are considered to be instances of the RDFS-defined class rdfs:Resource. As a result, rdfs:Resource is the most basic class in the RDF Schema type system.

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 motor vehicle, we would define the resource xyz:companyCar with an rdf:type property whose value is xyz:MotorVehicle. This is an RDF statement that xyz:MotorVehicle is a class, and xyz:companyCar is an instance of that class.

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. 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 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, if a resource xyz:Van has an rdfs:subClassOf property whose value is another resource xyz:MotorVehicle, this is an RDF statement that both xyz:Van and xyz:MotorVehicle are classes, and that xyz:Van is a subclass of xyz:MotorVehicle. The meaning of this 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 9 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 9: A Simple Class Hierarchy (SVG version)

Some corresponding RDF/XML syntax for this schema, defining the new classes using the techniques for creating 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 are then 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 ex: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 resource ex:author an rdfs:range property whose value is the resource (class) ex:Person. This is an RDF statement that (a) ex:author is a property, (b) ex:Person is a class, and (c) the value of any ex:author property is an instance of class ex:Person.

A given property, say ex:hasMother, may 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 exactly one range property, say one specifying ex:Person as the range, this is a statement that the value of ex:hasMother is an instance of ex:Person. If ex:hasMother has more than one range property, say one specifying ex:Person as the range, and another one specifying ex:Female as the range, these statements mean that the value of ex:hasMother is an instance of all of the classes specified as the ranges, i.e., that any value of property ex:hasMother is both an instance of class ex:Female and an instance of class 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 resource ex:author an rdfs:domain property whose value is the resource (class) ex:Book. This is an RDF statement that (a) ex:author is a property, (b) ex:Book is a class, and (c) any resource that has an ex:author property is an instance of class ex:Book.

A given property, say ex:weight, may have zero, one, or more than one domain property. If ex:weight has no domain property, then we are saying nothing about the resources ex:weight properties may be used with (any resource could have a ex:weight property). If ex:weight has exactly one domain property, say one specifying ex:Book as the domain, this is a statement that any resource that has a ex:weight property is an instance of class ex:Book. If ex:weight has more than one domain property, say one specifying ex:Book as the domain and another one specifying ex:MotorVehicle as the domain, these statements mean that any resource that has a ex:weight property is an instance of all of the classes specified as the domains, i.e., that any resource that has a ex:weight property is an instance of class ex:Book and an instance of class 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. 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 a xyz:Number, which is the number of centimeters of rear seat legroom (we assume that the classes ex:Person and ex:Number are defined elsewhere). 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.example.org/classes#Number"/>


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 to relate the two properties. For example, if a resource ex:biologicalFather has an rdfs:subPropertyOf property whose value is another resource ex:biologicalParent, this is an RDF statement that both ex:biologicalFather and ex:biologicalParent are properties, and that ex:biologicalFather is a subproperty of 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.

  <?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"/>

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, John's car from the example above could also be defined 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 separate 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 ex: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 collection 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 collection 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 the programming language did. 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 processor 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 a processor 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 processor 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 the RDF Schema Specification [RDF-SCHEMA].

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. For example, there is currently no standard vocabulary in RDF to indicate that the value of a Person's "age" property must be an integer (number of years); all such literal values in RDF are currently character strings (although an application is free to interpret the character string "25" given as the value of an "age" property as being a number, RDF itself does not define this as the proper interpretation, or enforce this as a constraint when entering the value). A specification for RDF Datatyping is currently under development, and may become part of the RDF specifications at some future time.

Other richer schema capabilities have also been identified as useful. For example:

  • cardinality constraints on properties, e.g., that a Person has exactly one biological father.
  • that a given property (such as hasAncestor) is transitive, e.g., that if A hasAncestor B, and B hasAncestor C, then A hasAncestor C.
  • that two different classes, defined in separate schemas, actually represent the same concept.
  • 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 the language currently being developed by the W3C's Web-Ontology Working Group. Both these languages are based on RDF and RDF Schema (and DAML+OIL currently provides 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 several 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

@@section intro TBD@@

6.1 Dublin Core Metadata Initiative

The Dublin Core is a set of "elements" (properties) for describing documents. 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.


@@probably need to introduce the idea of "metadata" a little bit better; possibly under the DC section@@

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 (described further in section [reference]), 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 be consumers, or it may be internal users such as researchers, designers, photo editors, licensing agents, etc. To assist discovery, PRISM provides elements for the topics, formats, genre, origin, and contexts of a resource. It also provides 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 defined in the PRISM specification allows one to build descriptions 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 elements from the Dublin Core 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 elements are more specific versions of elements from the Dublin Core. For example, dc:date is extended by elements like prism:publicationTime, prism:releaseTime, prism:expirationTime, etc.

pcv: This prefix refers to the PRISM Controlled Vocuabulary 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 elements 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 elements 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 we 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 we face 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 elements whose meanings are similar, or subsets of other elements. For example, the geographic subject of a resource could be given with




Any of those elements 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.

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 collections 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 collections, 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 for describing XML-based resources. Although XPackage is an application of RDF, the package description document is defined by an XML Schema. This allows XPackage to be implemented as a general XML application without an RDF processor, while still maintaining RDF compliance of conforming documents. 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 Intelligent Routing

@@This section has gotten some minor editing, but could use more. Possibly an actual example could be included.@@

The world is full of information. Behind the millions of pages on the Internet's most visible part, the Web, there are many times as many documents flowing in and out of organizations via emails, cross-company networks and constant always-on information "feeds".

Every document that passes along the wires has to be inspected, processed or re-routed. A document written by one human being has to be read by another before anybody knows its worth or, possibly, where it should be redirected. This is fine for direct person-to-person email but, for information intended for a broad circulation, this manual inspection can be expensive, often reducing the value of the information by raising its handling cost or simply making it late.

For example, when an individual subscribes to a given source, the understanding is usually that everything from that source will be delivered without question. For the distributor to sort out the interesting items for you manually would be time-consuming, expensive and boring; so instead we accept dozens of emails and delete most of them every morning. And of course it is time-consuming, expensive and boring. As a result, subscription to certain sources is a step to be taken very seriously.

When a company subscribes to a news feed, it may be risking a deluge of unwanted data. If it intends to circulate the information within the company or to a broad range of clients, it often takes on the responsibility itself of checking every document by eye or investing in extra software technology. Without such protection, the company will waste network bandwidth or its clients will consider themselves willfully "spammed" and seek other business partners.

Selection of information from such feeds is therefore a matter of utmost commercial sensitivity in a context of huge and increasing volumes and complexity of data. The technology concerned is "routing" and, in the most modern cases, relies on RDF.

The need, traditionally, for human inspection of incoming documents comes from the fact that, on its own, text has no value. It only has value when you know what it is about, the authority of its source, and who it is intended for. For a software agent to recognize a document's worth, it must have access to an evaluation that is consistently readable, whatever the format of the document, and is reliable in its description.

For those two objectives, we need an internationally standardized language, and a globally recognized set of values. These are provided by RDF, together with RDF Schemas, such as those defined by Dublin Core and PRISM. The longed-for independent evaluation takes the form of an associated RDF document.

Not that every document from every information source comes with its associated RDF description... yet. It is the case however that almost every serious source supplies some value-based annotation in the form of metadata, the significant content of RDF. For example, news feeds generally come in one of a selection of annotated formats, mostly based on XML, such as NewsML or XMLNews. Most standards-oriented companies are adding freely-accessible metadata to their document formats. Adobe, for example, recently announced XMP whereby metadata can be inserted into (and more importantly extracted from) PDF documents. The message from such companies is that, even if you cannot understand or even have no right to read the contents, you are entitled to know enough to make an evaluation for your own use or for clients who can use the information. For freely available information, standards are the key.

This basic process (source embeds standard annotations; annotations are used to route and sort documents) is certainly not new. Email (SMTP) and news (NNTP) protocols use standard keyword-value-pair headers which are fundamental to their operation. Such documents are marked up according to known and publicized standards. What is new is to normalize all these local formats to a general one and thereby be able to appeal to a globally consistent set of values in making judgments.

For a universal router to do its job, it needs to cancel out these variations in format. Until the world adopts one standard, this will be a matter of tact and ingenuity but the existence of a core standard is important here. When multiple formats and value-systems need to be compared it is safer to have one standard to convert to first and then to compare rather than do it piecemeal - and that standard must be broader than all the others. Again, RDF (and RDF Schema) standards are the natural choice.

An Information Router collects metadata and stores these descriptions in RDF (rather like an enormous RDF document describing perhaps millions of resources at once). The descriptions can then be exported or imported in a standard form without loss or confusion. World-wide, repositories of metadata may be synchronized and refreshed by exchanging RDF. While humans are exchanging images, videos and news items, metadata servers are exchanging compact RDF evaluations of this same information.

The actual documents described by the RDF, orders of magnitude larger than the metadata, can be stored elsewhere or just left where they are (located by URI, of course). The metadata is compact and loaded with value. Judgments about distributing material can be made in a context of universally accepted and agreed-on values (e.g., systems like Dublin Core) and a vast number of alternatives, all without moving the actual documents around or indeed even looking at them, by computer or by human eye.

Judgments can be made by applying RDF "queries" which test the value of a document to the reader: whether the subject is interesting, the content is suitable, the author respected, the source reliable, the document accessible, the cost reasonable, the language intelligible, the conclusion desirable, the format tractable, the medium handleable, etc., etc. The actual form of such a query varies from product to product. (In any case, consumers would be given a human-friendly way to express their wishes.)

In one case, a query might take the form of a modified RDF description which, in effect, asks to be proved or disproved by a body of metadata. So an RDF description that stated that a document exists with the title "Financial history of Belize" could be viewed as a request to find such a document.

The news distributor's server could run, in addition to the usual server software, one of these Information Router packages, which applies queries on behalf of its clients and delivers just those documents that pass the evaluation.

If a complex multi-layered query describing just what it takes to please you is associated with your name as a subscriber, you can, using software available today, guarantee that what you are sent is exactly and only what you need. It's an end to spam, thanks to RDF.

7. Other Parts of the RDF Specification

@@section intro TBD@@

7.1 Model Theory

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 Model Theory [RDF-MODEL] 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.

7.2 Test Cases

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 the N-triples notation referred to in earlier sections of 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 some 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. RDF As a Data Model

@@This is currently a placeholder (the title might change too) for a brief discussion of several related topics: (a) how RDF relates to XML (why you might want to use RDF, rather than using other XML structures); (b) where RDF fits in the general world of data models, particularly its relationship to the relational data model, and related work on binary relational and other "semantic" data models. As part of this latter material, there will be pointers to some of the literature on database (schema) design (functional dependencies are highly relevant to RDF design), since analysis and design is going to be needed to develop robust RDF applications, and a lot of prior work exists on this subject that can be drawn from; (c) "identifier design": that deciding how to assign URIrefs to things is a design issue too, and some of the issues involved (e.g., options when different people assign different URIrefs to the same thing). The idea is mainly to point out the issues, and cite some sources for further reading. Depending on how the material turns out, it might be distributed in other sections instead of being placed here.@@

9. References

9.1 Normative References

RDF Model Theory, P. Hayes, Editor. Work in progress. World Wide Web Consortium, 14 February 2002. This version of the RDF Model Theory is http://www.w3.org/TR/2002/WD-rdf-mt-20020214. The latest version of the RDF Model Theory is at http://www.w3.org/TR/rdf-mt/.
RDF/XML Syntax Specification (Revised), D. Beckett, Editor. Work in progress. World Wide Web Consortium, 25 March 2002. This version of the RDF/XML Syntax Specification is http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20020325/. The latest version of the RDF Test Cases is at http://www.w3.org/TR/rdf-syntax-grammar.
RDF Test Cases, A. Barstow and D. Beckett, Editors. Work in progress. World Wide Web Consortium, 15 November 2001. This version of the RDF Test Cases is http://www.w3.org/TR/2001/WD-rdf-testcases-20011115/. The latest version of the RDF Test Cases is at http://www.w3.org/TR/rdf-testcases.
RDF Vocabulary Description Language 1.0: RDF Schema , D. Brickley and R. V. Guha, Editors. Work in progress, April 2002. World Wide Web Consortium, 30 April 2002. This version of RDF Schema is http://www.w3.org/TR/2002/WD-rdf-schema-20020430/. The latest version of RDF Schema is at http://www.w3.org/TR/rdf-schema/.
Extensible Markup Language (XML) 1.0, Second Edition, T. Bray, J. Paoli, C.M. Sperberg-McQueen and E. Maler, Editors. World Wide Web Consortium. 6 October 2000. This version is http://www.w3.org/TR/2000/REC-xml-20001006. latest version of XML is available at http://www.w3.org/TR/REC-xml.
Namespaces in XML, T. Bray, D. Hollander and A. Layman, Editors. World Wide Web Consortium. 14 January 1999. This version is http://www.w3.org/TR/1999/REC-xml-names-19990114. The latest version of Namespaces in XML is available at http://www.w3.org/TR/REC-xml-names.
XML Base, J. Marsh, Editor, W3C Recommendation. World Wide Web Consortium, 27 June 2001. This version of XML Base is http://www.w3.org/TR/2001/REC-xmlbase-20010627/. The latest version of XML Base is at http://www.w3.org/TR/xmlbase/.
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax, T. Berners-Lee, R. Fielding and L. Masinter, IETF, August 1998. This document is http://www.isi.edu/in-notes/rfc2396.txt.

9.2 Informational References

Addressing Schemes, D. Connolly, 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, M. J. Bates, 1996. This document is http://is.gseis.ucla.edu/research/mjbates.html.
What the Semantic Web can represent, T. Berners-Lee, 1998. This document is http://www.w3.org/DesignIssues/RDFnot.html.
DAML+OIL (March 2001) Reference Description, D. Connolly, F. van Harmelen, I. Horrocks, D. L. McGuinness, P. F. Patel-Schneider, L. A. Stein, 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/.
Naming and Addressing: URIs, URLs, ..., D. Connolly, 2002. This document is http://www.w3.org/Addressing/.
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, B. McBride, 2002. This document is http://www.w3.org/2000/03/rdf-tracking/.
Web Architecture: Describing and Exchanging Data, T. Berners-Lee, D. Connolly, and R. Swick, W3C Note, 7 June 1999. This document is http://www.w3.org/1999/04/WebData.
XML Linking Language (XLink) Version 1.0, S. DeRose, E. Maler, and D. Orchard, Editors. World Wide Web Consortium. 27 June 2001. This version is http://www.w3.org/TR/xlink/.
XML Package (XPackage) 1.0 , G. Wilson, Open eBook Forum Editor's Working Draft, 26 March 2002. This document is http://www.xpackage.org/specification/.

10. Acknowledgments

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

Appendix A: Changes

Changes since the 26 April 2002 Working Draft:

  • Added Appendix A.
  • Miscellaneous formatting corrections.
  • Introduced an abbreviated notation for triples and corrected 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, and added material on xml:base.
  • Corrected material on RDF capabilities in the Abstract and Section 1.
  • Introduced a new section to hold containers, reification, and miscellaneous capabilities, and added material on rdf:value, Boolean values, and (temporarily) RDF in HTML to that section.
  • 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 illustrating the use of blank nodes to model things that don't naturally have URIs.