W3C

RDF Primer

W3C Editor's Draft 12 March 2002

This version:
rdf-primer-20020312
Latest version:
rdf-primer
Previous version:
rdf-primer-20020308
Editors:
Frank Manola, The MITRE Corporation, fmanola@mitre.org
Eric Miller, W3C, em@w3.org

Copyright ©2001,2002 W3C ® ( MIT , INRIA , Keio ), All Rights Reserved. W3C liability , trademark , document use and software licensing rules apply .


Abstract

The Resource Description Framework (RDF) is a general-purpose language for representing information 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, the copyright and syndication information about a Web document, the availability schedule for some shared resource, or the description of a Web user's preferences for information delivery. RDF provides a common framework for expressing this information in such a way that 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. Exchanging 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 the basic fundamentals required to effectively use RDF in their particular applications.

Status of this document

This is a draft document, is undergoing active editorial work, 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 it as other than "work in progress".


Table of contents

1. Introduction
2. Making Statements About Resources

2.1 Identifiers: Uniform Resource Identifier (URI)
2.2 Documents: Extensible Markup Language (XML)
2.3 The RDF Model
2.4 Structured Property Values

3. An XML Syntax for RDF
4. Defining RDF Vocabularies: RDF Schema

4.1 Core Classes
4.2 Core Properties
4.3 Constraints

5. RDF In The Field: Some RDF Application

5.1 Dublin Core Metadata Initiative
5.2 PRISM
5.3 Intelligent Routing
5.4 RSS: RDF Site Summary 1.0

6. Other Parts of RDF
7. References
8. Acknowledgements


1. Introduction

The Resource Description Framework (RDF) is a general-purpose language for representing information 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, the copyright and syndication 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 be used to represent information about anything that can be identified on the Web, such as information about items available from online shopping facilities (e.g., information about prices, publishers, and availability of books or recordings).

RDF provides a common framework for expressing this information in such a way that 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. Exchanging 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#"
xmlns="http://www.w3.org/2000/10/swap/pim/contact#">
<Person rdf:about="http://www.w3.org/People/EM/contact#me">
<mailbox rdf:resource="mailto:em@w3.org"/>
<fullName>Eric Miller</fullName>
<personalTitle>Semantic Web Activity Lead</personalTitle>
</Person>
</rdf:RDF>

This example roughly translates as a collection of statements "there is someone called Eric Miller, with the email address em@w3.org, and who is the 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 the values "em@w3.org", and "Eric Miller".

Like HTML, this form of information is machine processable, and links pieces of data acorss the Web. However, unlike conventional hypertext, RDF links can also be to any identifiable things, 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 links 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:

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.:

The creator of http://www.example.org/index.html 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:

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, e.g.:

The creation-date of http://www.example.org/index.html is August 16, 1999

The language of http://www.example.org/index.html is English

(note the use of "August 16, 1999" to identify a date).

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 this case) is called the predicate, and the part that identifies the value of that property is called the object . So, taking the English statement

The creator of http://www.example.org/index.html is John Smith

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

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:

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 Identifiers: Uniform Resource Identifier (URI)

If we want to discuss something, we must first identify it. How else will you know what one is referring to? In everyday communication, identity is assigned in many ways: "Bob", "The Moon", "373 Whitaker Ave.", "California", "VIN 2745534", "todays weather", etc., and ambiguities are generally resolved due to a shared semantic context between the sender and the receiver. To identify "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 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 URLs. For example, I don't have a 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), shots 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 an official 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). URIs are similar to URLs, in that different persons or organizations can independently create them, and use them to identify things. However, unlike URLs, 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

URIs essentially constitute an infinite stock of names that can be used to identify things. 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, and you can create URIs for things you don't own (just as you can use whatever name you like for things you don't own in ordinary language). 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 its mechanism for identifying the subjects, objects, and predicates in statements. In fact, RDF defines a resource as anything that is identifiable by a URI, and hence using URIs allows RDF to describe practically anything, and to state relationships between such things as well. We'll see how this works just a bit further on. But before we do that, we need to introduce a way for RDF statements to be physically represented and exchanged.
@@May want a better seque to the next section.@@
@@Also need to resolve where the fragment identifiers text goes. It logically goes here with the URI discussion, but the explanatory text uses N-triples, which are not introduced until later. NB: That explanatory text also needs to be reformulated to use absolute URIs.@@

2.2 Documents: Extensible Markup Language (XML)

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, andmarkup in the form of tags. This markup allows a processing program to interpret the various pieces of content (elements). In HTML, the set of permissable 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 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 for example.

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

To a machine, this is the exact same structure as the previous example. However, it is no longer clear what is being said. Moreover, others may have used the same words 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 . 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 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.

<my:sentence my:xmlns="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>.
</my:sentence>

Since everyone's tags have their own URIs, we don't have to worry about tag names conflicting. The elements mean the same if they have the same URIs.

RDF defines a specific XML markup language for use in writing down RDF information, and for exchanging it between machines. An example of this language was given in Section 1 , and the language is described in Section 3 .

@@Needs some brief additional explanation of the namespace mechanism, and how it's used.@@

2.3 The RDF Model

Now that we've introduced URIs 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, using subjects, predicates, and objects. In RDF, we could represent our original English statement:

The creator of http://www.example.org/index.html is John Smith

by an RDF statement having:

RDF models statements as nodes and arcs in a graph. In this notation, a statement is represented by a node for the subject, a node for the object, and a labeled arc between them for the predicate. So the RDF statement above would be represented by the graph

A Simple RDF Statement

Figure 1: A Simple RDF Statement

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

The creation-date of http://www.example.org/index.html is August 16, 1999

The language of http://www.example.org/index.html is English

we could, introducing suitable URIs to name the properties "creation-date" and "language", use the following graph:

Several Statements About the Same Resource

Figure 2: Several Statements About the Same Resource

This graph illustrates that RDF permits the objects in statements to be simple strings, if necessary to represent property values, as well as URIs. In drawing RDF graphs, nodes that represent URIs are shown as ellipses, while nodes that represent strings are shown as boxes. 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, 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, the N-triple 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-triple representation of the graph, but only once in the drawn graph.

These examples begin to illustrate some of the advantages of using URIs 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 string "John Smith", we've assigned him a URI, in this case (using a URI based on his employee number) http://www.example.org/staffid/85740 . An advantage of using a URI in this case is that we can be more precise in our identification. That is, the creator of the page isn't the 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 URI (whoever created the URI defines the association). Moreover, since we have a URI 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

More Information About John Smith

Figure 3: More Information about John Smith

The examples also illustrate that RDF uses URIs as predicates in RDF statements. That is, rather than using strings such as "creator" or "name" to identify properties, RDF uses URIs. Using URIs 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 text string. For instance, in our example, example.org uses "name" to mean someone's full name written out as a 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://example.geneology.org/terms/name for hers, we can keep straight the fact that there are distinct properties involved (even if a program can't automatically determine the distinct meanings). Another reason why it is important to use URIs 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 URI as the subject.

Using URIs as subjects, objects, and predicates 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 N-triple

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

the predicate http://purl.org/dc/elements/1.1/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.

As a result, 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 website could take that information (the list of book rating averages) and create a "Top Ten Highest Rated Books" page. @@This discussion of machine-processability could use some further qualification and amplification.@@

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

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

2.4 Structured Property Values

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 string value. 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 string, as in the N-triple

<http://www.example.org/staffid/85740> <http://www.example.org/terms/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 URI to identify it, say http://www.example.org/addressid/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 below:

Breaking Up John&apos;s Address

Figure 4: Breaking Up John's Address

or the N-triples:

<http://www.example.org/staffid/85740>
<http://www.example.org/terms/address>
<http://www.example.org/addressid/85740> .

<http://www.example.org/addressid/85740>
<http://www.example.org/terms/street>
"1501 Grant Avenue" .

<http://www.example.org/addressid/85740>
<http://www.example.org/terms/city>
"Bedford" .

<http://www.example.org/addressid/85740>
<http://www.example.org/terms/state>
"Massachusetts" .

<http://www.example.org/addressid/85740>
<http://www.example.org/terms/Zip>
"01730" .

In the drawing of the graph above, the new URI we assigned to identify "John Smith's address" really serves no purpose, since we could just as easily have drawn the graph:

Using a bNode

Figure 5: Using a bNode

In this drawing, 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 bNode (for blank node) functions perfectly well in the drawing without needing a URI. However, we do need some form of explicit identifier for that node in order to represent this graph as N-triples. To see this, we can try to write the N-triples corresponding to what is shown in the drawn graph. What we would get would be something like:

<http://www.example.org/staffid/85740>
<http://www.example.org/terms/address>
??? .

???
<http://www.example.org/terms/street>
"1501 Grant Avenue" .

???
<http://www.example.org/terms/city>
"Bedford" .

???
<http://www.example.org/terms/state>
"Massachusetts" .

???
<http://www.example.org/terms/Zip>
"01730" .

where ??? stands for something that indicates the presence of the bNode. Since in a complex graph there might be more than one such bNode, we also need a way to differentiate between the various bNodes in the corresponding N-triples representation. To do this, the N-triples notation uses a concept of node identifiers (or nodeIDs) to identify bNodes. These are temporary identifiers distinct from URIs (and having their own syntax in N-triples) that are used to indicate the presence of bNodes in the N-triples representation. In this example, we might generate the node identifier _:johnaddress to refer to the bNode, in which case the resulting triples might be:

<http://www.example.org/staffid/85740>
<http://www.example.org/terms/address>
_:johnaddress .

_:johnaddress
<http://www.example.org/terms/street>
"1501 Grant Avenue" .

_:johnaddress
<http://www.example.org/terms/city>
"Bedford" .

_:johnaddress
<http://www.example.org/terms/state>
"Massachusetts" .

_:johnaddress
<http://www.example.org/terms/Zip>
"01730" .

This is all there is to basic RDF: nodes-and-arcs diagrams interpreted as statements about concepts or digital resources represented by URIs . However, the need for standardized vocabularies for things like "city" and the predicate "creator" is evident. The basis for 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've said already, RDF models statements in terms of a graph consisting of nodes and arcs. The nodes describe resources that can be labelled with URIs, string literals or are blank. The arcs connect the nodes and are all labelled with URIs. This graph is more precisely called a directed edge-labelled 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 also defines an XML syntax for writing down and exchanging RDF graphs. This syntax is defined in the RDF/XML Syntax Specification . In order to encode the graph in XML, the nodes and arcs are turned into XML elements, attributes, element content and attribute values. The URI labels for properties and object nodes are written in XML using XML Namespaces ( [XML-NS] ) which gives a namespace URI for a short prefix along with namespace-qualified elements and attributes names called local names. The (namespace URI, local name) pair are chosen such that concatenating them forms the original node URI. The URIs labelling subject nodes are stored in XML attribute values. The nodes labelled by string literals (which are always object nodes) become element text content or attribute values.

This transformation turns paths in the graph of the form Node, Arc, Node, Arc, Node, Arc, ... into sequences of nested elements (elements inside elements). This results in a striping when the elements are written down; alternating between node elements and property elements . The Node at the start of the sequence is always a subject node and turns into a containing element called an rdf:Description that is written at the top level of RDF/XML, after the XML document element (in this case rdf:RDF). So the chains of stripes start at the top of an RDF/XML document and always begin with nodes.

For example, the figure below shows a graph saying "there exists a document (identified by its URL) with a title, 'RDF/XML Syntax Specification (Revised)' and that document has an editor, the editor has a name 'Dave Beckett' and a home page 'http://purl.org/net/dajobe/' ".

Graph for RDF/XML Example

Figure 6: Graph for RDF/XML Example

If we take the path through the graph shown below:

One Path Through the Graph

Figure 7: One Path Through the Graph

this corresponds to the node/arc stripes:

  1. Node: http://www.w3.org/TR/rdf-syntax-grammar
  2. Arc: http://example.org/stuff/1.0/editor
  3. Node: [a bNode]
  4. Arc: http://example.org/stuff/1.0/homepage
  5. Node: http://purl.org/net/dajobe/

In RDF/XML this sequence of 5 nodes and arcs correponds to 5 XML elements (using some namespace abbreviations)):

<rdf:Description>
<ex:editor>
<rdf:Description>
<ex:homePage>
<rdf:Description>
</rdf:Description>
</ex:homePage>
</rdf:Description>
</ex:editor>
</rdf:Description>

Which consists of some nodes with known URIs that can be filled in and others that remain blank:

<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
<ex:editor>
<rdf:Description>
<ex:homePage>
<rdf:Description rdf:about="http://purl.org/net/dajobe/">
</rdf:Description>
</ex:homePage>
</rdf:Description>
</ex:editor>
</rdf:Description>

There are several abbreviations that can be used to make very common uses more easy to write down. For example, it is typical for the same resource to be described with multiple properties and values at the same time, so multiple child elements can be put inside rdf:Description , all of which are properties of that node.

Similarly, when the property value is a string it can be encoded more simply as an XML attribute and value, as an attribute of the node element. This is known as a property attribute .

Another very common use is when a node is an instance of a class with rdf:type relationship, usually called a typed node. This shorthand is done by replacing the rdf:Description element name with the namespaced-element corresponding to the URI of the value of the type relationship.

The above forms the basis of the RDF/XML syntax. although there are some other abbreviated forms, such as for generating RDF list properties and for skipping having to write down a blank element node. This latter abbreviation breaks the striping, but is useful for, among other things, encoding properties with multiple values.

The full example above, filled out and completed, and using some of these additional abbreviations, gives:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:ex="http://example.org/stuff/1.0/">
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
<dc:title>RDF/XML Syntax Specification (Revised)</dc:title>
<ex:editor rdf:parseType="Resource">
<ex:fullName>Dave Beckett</ex:fullName>
<ex:homePage rdf:resource="http://purl.org/net/dajobe/" />
</ex:editor>
</rdf:Description>
</rdf:RDF>

For a longer introduction to the RDF/XML striped syntax with a historical perspective, see RDF: Understanding the Striped RDF/XML Syntax ( [STRIPEDRDF] ).

Two kinds of tools are useful when learning RDF/XML: parsers and visualisers. The first such tool was Janne Sareela's SiRPAC; there are now a large number of RDF parsers available, in a variety of programming languages. An RDF parser is a tool that takes an XML encoding ("serialization") of an RDF graph, and returns a textual or programmatic representation of the graph. Playing with an RDF parser such as ARP, the parser used by W3C's RDF Validation Service makes it easy to experiment with RDF/XML files and see the associated node-edge-node triples that constitute the corresponding graph structure.

Another tool that can help an RDF developer get to grips with the syntax is GraphViz, or one of the GraphViz-based RDF visualization tools such as RDFViz. GraphViz is a graph visualisation toolkit. It can take descriptions of (various kinds of) graph and generate reasonably pretty pictures in various image formats. There are now a variety of filters that take the output from an RDF/XML parser and generate .dot input files for GraphViz. This can be incredibly useful when learning the RDF/XML syntax, or debugging RDF content. A GraphViz-based RDF visualizer is now also part of W3C's RDF Validator service.

@@Discussion of these examples may need amplification@@
@@The discussion of tools is a bit brief. Could alternatively have a separate section on tools that includes this material, plus a brief discussion of other tools and tool types@@

4. Defining RDF Vocabularies: RDF Schema

RDF defines a simple data model for describing the properties of resources, and interrelationships among resources, in terms of named properties and values. However, the RDF data model itself provides no mechanisms for declaring specific types or classes of resources, or for declaring which properties might apply to which classes of resources. However, RDF user communities require the ability to say certain things about certain kinds of resources. For describing bibliographic resources, for example, descriptive attributes including "author", "title", and "subject" are common. For digital certification, attributes such as "checksum" and "authorization" are often required. The declaration of these properties (attributes) are defined for RDF in an RDF schema . A schema defines not only the properties of a resource (e.g., title, author, subject, size, color, etc.) but may also define the kinds of resources being described (books, Web pages, people, companies, etc.).

The RDF Schema specification does not specify a specific vocabulary of descriptive elements such as "author". Instead, it specifies the mechanisms needed to define such elements, to define the classes of resources they may be used with, and to restrict possible combinations of classes and relationships. In other words, the RDF Schema mechanism provides a basic type system for use in RDF models. The RDF Schema type system is similar to the type systems of object-oriented programming languages such as Java. For example, the type system 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 Dog might be defined as a subclass of Mammal which is a subclass of Animal , meaning that any resource which is of rdf:type Dog is also considered to be of rdf:type Animal . However, RDF differs from many programming language type systems in 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 apply. This is done using domain and range constraints on the properties. For example, we could define the author property to have a domain of Book and a range of Literal , whereas a classical object-oriented system might typically define a class Book with an attribute called author of type Literal . One benefit of the RDF property-based approach is that it is very easy for anyone to say anything they want about existing resources, which is one of the architectural principles of the Web [ BERNERS-LEE98 ].

4.1. Core Classes

This RDF type system is specified in terms of the basic RDF data model, as a set of pre-defined RDF resources and properties. As a result, the resources that make up the type system become part of the RDF model of any description that uses them. The following resources are the core classes that are defined as part of the RDF Schema vocabulary. Every RDF model that draws upon the RDF Schema namespace (implicitly) includes these.

rdfs:Class :

This corresponds to the generic concept of a Type or Category, similar to the notion of a Class in object-oriented programming languages such as Java. When a schema defines a new class, the resource representing that class must have an rdf:type property (one of the core properties defined below) whose value is the resource rdfs:Class. RDF classes can be defined to represent almost anything, such as Web pages, people, document types, databases or abstract concepts.

rdfs: Resource :

All things being described by RDF expressions are called resources, and are considered to be instances of the class rdfs :Resource.

rdf: Property:

rdf:Property represents the subset of RDF resources that are properties.

4.2. Core Properties

Every RDF model which uses the schema mechanism also (implicitly) includes the following core properties. These are instances of the rdf:Property class, and provide a mechanism for expressing relationships between classes and their instances or superclasses .

rdf: type:

This property indicates that a resource is a member of a class, and thus has all the characteristics that are to be expected of a member of that class. When a resource has an rdf :type property whose value is some specific class, we say that the resource is an instance of the specified class. The value of an rdf:type property for some resource is another resource which must be an instance of rdfs:Class . The resource known as rdfs:Class is itself a resource whose rdf:type isrdfs:Class. Individual classes (for example, 'Dog') will always have an rdf:type property whose value is rdfs:Class (or some subclass of rdfs:Class, as described below). A resource may be an instance of more than one class.

rdfs: subClassOf:

This property specifies a subset/superset relation between classes. The rdfs:subClassOf property is transitive. If class A is a subclass of some broader class B, and B is a subclass of C, then A is also implicitly a subclass of C. Consequently, resources that are instances of class A will also be instances of C, since A is a sub-set of both B and C. Only instances of rdfs :Class can have the rdfs:subClassOf property and the property value is always of rdf:type rdfs:Class . A class may be a subclass of more than one class.

The following example expresses a simple class hierarchy. We first define a class MotorVehicle. We then define three subclasses of MotorVehicle, namely PassengerVehicle, Truck and Van . We then define a class Minivan which is a subclass of both Van and PassengerVehicle .

A Simple Class Hierarchy

Figure 8: A Simple Class Hierarchy

Some corresponding RDF/XML serialization syntax is shown below:

<rdf:RDF xml:lang="en"  
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">

<!-- Note: this RDF schema would typically be used in RDF instance data by referencing it with an XML namespace declaration, for example xmlns:xyz="http://www.w3.org/2000/03/example/vehicles#". This allows us to use abbreviations such as xyz:MotorVehicle to refer unambiguously to the RDF class 'MotorVehicle'. -->
<rdf:Description ID="MotorVehicle"> <rdf:type 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:Description ID="PassengerVehicle"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description>
<rdf:Description ID="Truck"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description>
<rdf:Description ID="Van"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description>
<rdf:Description ID="MiniVan"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#Van"/> <rdfs:subClassOf rdf:resource="#PassengerVehicle"/> </rdf:Description>
</rdf:RDF>

rdfs: subPropertyOf:

The property rdfs:subPropertyOf is an instance of rdf:Property that is used to specify that one property is a specialization of another. A property may be a specialization of zero, one or more properties. If some property P2 is a subPropertyOf another more general property P1, and if a resource A has a P2 property with a value B, this implies that the resource A also has a P1 property with value B.

As an example, if the property biologicalFather is a subproperty of the broader property biologicalParent, and if Fred is the biologicalFather of John, then it is implied that Fred is also the biologicalParent of John.

<rdf:RDF xml:lang="en"  
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">

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

4.3. Constraints

RDF Schema also provides a mechanism for specifying simple constraints on the use of properties and classes in RDF data. The basic constraints are those that describe limitations on the types of values that are valid for some property, or on the classes to which it makes sense to assign such properties. Specifically:

Domain and range constraints are specified using the following predefined RDF properties:

rdfs: range:

A property that is used to indicate the class( es) that the values of a property must be members of. The value of a range property is always a Class . Range constraints are only applied to properties.

A property may have zero, one, or more than one range property. If there is no range property, the class of the property value is unconstrained. If there is exactly one range property, the property value must be an instance of the specified class (which is the value of the range property). If there is more than one range property, the property value must be an instance of all of the classes (that are values of those range properties). For example, if we assert that property xyz:hasMother has both a rdfs:range of Female and an rdfs:range of Person, this means that any value of property xyz:hasMother must be both an instance of class Female and an instance of class Person.

rdfs: domain:

A property that is used to indicate the class( es) on whose members a property can be used.

A property may have zero, one, or more than one domain property. If there is no domain property, the property may be used with any resource. If there is exactly one domain property, the property may only be used on resources that are instances of that class (which is the value of the domain property). If there is more than one domain property, the property can only be used on resources that are instances of all of the classes (that are values of those domain properties).

We can illustrate the use of these constraint properties by continuing with our earlier example of MotorVehicle. In this example, we define two properties : registeredTo and rearSeatLegRoom. The registeredTo property is applicable to any MotorVehicle and its value is a Person (defined in the examples below). For the sake of this example, rearSeatLegRoom only applies to PassengerVehicle s. The value is a Number , which is the number of centimeters of rear seat legroom. These definitions are shown in the RDF/XML below:

<rdf:RDF xml:lang="en"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">

<rdf:Description ID="registeredTo"> <rdf:type resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/> <rdfs:domain rdf:resource="#MotorVehicle"/> <rdfs:range rdf:resource="#Person"/> </rdf:Description>
<rdf:Description ID="rearSeatLegRoom"> <rdf:type resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/> <rdfs:domain rdf:resource="#PassengerVehicle"/>  <rdfs:range rdf:resource="http://www.w3.org/2000/03/example/classes#Number"/> </rdf:Description>
</rdf:RDF>

Although RDF Schema provides a mechanism for describing constraints, it does not say whether or how an application must process the constraint information. For example, while an RDF schema can assert that an author property is used to indicate resources that are members of the class Person , it does not say whether or how an application should act in processing that class information. Different applications might use these constraints in different ways - e.g., a validator will look for errors, an interactive editor might suggest legal values, and a reasoning application might infer the class and then announce any inconsistencies.

@@This is currently more-or-less copied from the current RDF Schema draft, and needs rewriting@@

5. Some RDF Applications: RDF in the Field

@@section intro TBD@@

5.1 Dublin Core Metadata Initiative

@@ TBD@@

5.2 PRISM

@@TBD@@


5.3 Intelligent Routing

The world is full of information. Behind the millions of pages on the Internet's publicly 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 simply written by one human being has to be read by another before anybody knows its worth or where it should be redirected. This is fine for a person-to-person email but, for information destined to a broad circulation, this 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 source of news, it's usually on the understanding that everything in that feed is of interest and so everything will be delivered without question. For the distributor to sort out the interesting ones 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. Subscription to some less self-critical 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 charges itself with checking every document by eye or investing in extra software technology. Without such protection, the company's networks will soon collapse under the load or its clients will consider themselves willfully "spammed" and withdraw their custom.

The redirection of 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's about, what authority is its source and who it's intended for. Everything else is, as we know, just material for spamming. 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 RDF, together with RDF Schemas such as 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 can't 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.

Now this basic process (source embeds standard annotations: annotations are used to divert 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 You are recommended to use CSS to specify the font and properties such as its size and color. This will reduce the size of HTML files and make them easier maintain compared with using elements. 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 a slew of 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 Schemas) standards are the natural choice.

An Information Router collects metadata and stores it (rather like an enormous RDF document describing maybe millions of resources at once). This metadata store holds the descriptions in exactly the terms of RDF. They can therefore be exported or imported as industry-standard RDF 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 them (the images, videos and news items, not the humans).

The actual documents described, 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 values (the standard predicate 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 are made by applying RDF "queries" which are testing 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 a query varies from product to product. (In any case, the consumer would be given a graphical way to express his wishes.)

In one case, a query takes the form of a modified RDF description which, if you like, 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 Beliz" can be viewed as a request to find such a document.

The news distributor's server runs, in addition to the usual server software, one of these Information Router packages, applies queries on behalf of its clients and delivers just those documents that survive 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 get sent is exactly and only what you need. It's an end to spam, thanks to RDF.

5.4 RSS: RDF Site Summary 1.0

@@ TBD@@

XMP

@@ possible additional section@@

Site Maps / Topic Maps

@@possible additional section@@

6. Other Parts of RDF

@@section intro TBD@@

6.1 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 document provides this precise definition, through what is technically called 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 simply 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 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 our case, 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.

@@Revised and/or additional discussion of the model theory to be added as time permits@@

6.2 Test Cases

The RDF Test Cases document supplements 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 used in earlier sections of the 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.

6.3 Reification

@@TBD@@

@@other parts may also be identified; also TBD@@

7. References

[BERNERS-LEE98] What the Semantic Web can represent , Tim Berners-Lee, 1998 http://www.w3.org/DesignIssues/RDFnot.html

[DC] Dublin Core Metadata Initiative , http://purl.org/dc/

[RDFMT] RDF Model Theory , W3C Working Draft, 14 February 2002 http://www.w3.org/TR/rdf-mt/

[RDFXML] RDF/XML Syntax Specification (Revised) , W3C Working Draft, 18 December 2001 http://www.w3.org/TR/2001/WD-rdf-syntax-grammar-20011218

[RDFTEST] RDF Test Cases , W3C Working Draft, 12 September 2001 (contains N-triples ) http://www.w3.org/TR/2001/WD-rdf-testcases-20010912/

[RDFSCHEMA] RDF Schema Specification 1.0 , (editor's working draft), September 2001 http://www.w3.org/2001/sw/RDFCore/Schema/20010913 /

[RDFISSUE] RDF Issue Tracking , http://www.w3.org/2000/03/rdf-tracking/

[RFC 2396] RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax , August 1998 http://www.isi.edu/in-notes/rfc2396.txt

[WEBDATA] Web Architecture: Describing and Exchanging Data , W3C Note, 7 June 1999
http://www.w3.org/1999/04/WebData

[XML] Extensible Markup Language (XML) 1.0 , W3C Recommendation, 10 February 1988, http://www.w3.org/TR/1998/REC-xml-19980210.html

[XML-NS] Namespaces in XML , W3C Recommendation, 14 January 1999, http://www.w3.org/TR/1999/REC-xml-names-19990114

@@Formatting of references needs review; e.g., cite editors or not?@@

8. Acknowledgments

The following people provided valuable contributions to this document:

@@Other editorial notes: Validation has not been done, and all sorts of cleanup of the HTML is required.@@