RDF Primer

2.1 Basic Concepts

Imagine trying to state that someone named John Smith created a particular Web page. A straightforward way to state this in a natural language such as English would be in the form of a simple statement such as:

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

Parts of this statement are emphasized to illustrate that, in order to describe the properties of something, there need to be ways to name, or identify, a number of things:

• the thing the statement describes (the Web page, in this case)
• a specific property (creator, in this case) of the thing the statement describes
• the thing the statement says is the value of this property (who the creator is), for the thing the statement describes

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

Other properties of this Web page could be described 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, the date the page was created, and the language in which the page is written, could be described using 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 being described 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 phrase "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, two things are needed:

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

Fortunately, the existing Web architecture provides both these necessary facilities.

As illustrated earlier, the Web already provides one form of identifier, the Uniform Resource Locator (URL). A URL was used in the 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, it is also important to be able to record information about many things that, unlike Web pages, do not have network locations or URLs.

The Web provides a more general form of identifier for these purposes, called the Uniform Resource Identifier (URI). URLs 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, a URI can be created to refer to anything that needs to be referred to in a statement, including

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

Because of this generality, RDF uses URIs as the basis of its mechanism for identifying the subjects, predicates, and objects in statements. To be more precise, RDF uses URI references [URIS]. 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 URIrefs can contain Unicode [UNICODE] characters (see [RDF-CONCEPTS]), allowing many languages to be reflected in URIrefs. RDF defines a resource as anything that is identifiable by a URI reference, so using URIrefs allows RDF to describe practically anything, and to state relationships between such things as well. URIrefs and fragment identifiers are discussed further in Appendix A, and in [RDF-CONCEPTS].

To represent RDF statements in a machine-processable way, RDF uses the Extensible Markup Language [XML]. XML was designed to allow anyone to design their own document format and then write a document in that format. RDF defines a specific XML markup language, referred to as RDF/XML, for use in representing RDF information, and for exchanging it between machines. An example of RDF/XML was given in Section 1. That example (Example 1) used tags such as <contact:fullName> and <contact:personalTitle> to delimit the text content Eric Miller and Dr., respectively. Such tags allow programs written with an understanding of what the tags mean to properly interpret that content. Both XML content and (with certain exceptions) tags can contain Unicode [UNICODE] characters, allowing information from many languages to be directly represented. Appendix B provides further background on XML in general. The specific RDF/XML syntax used for RDF is described in more detail in Section 3, and is normatively defined in [RDF-SYNTAX]

2.2 The RDF Model

Section 2.1 has introduced RDF's basic statement concepts, the idea of using URI references to identify the things referred to in RDF statements, and RDF/XML as a machine-processable way to represent RDF statements. With that background, this section describes how RDF uses URIs to make statements about resources. The introduction said that RDF was based on the idea of expressing simple statements about resources, where each statement consists of a subject, a predicate, and an object. In RDF, the English statement:

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

could be represented 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 URIrefs are used 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 (some of the effects of using URIrefs in this way will be discussed later in this section).

RDF models statements as nodes and arcs in a graph. RDF's graph model is defined in [RDF-CONCEPTS]. In this notation, a statement is represented by:

• a node for the subject
• a node for the object
• an arc for the predicate, directed from the subject node to the object node.

So the RDF statement above would be represented by the graph shown in Figure 2:

Figure 2: A Simple RDF Statement

Groups of statements are represented by corresponding groups of nodes and arcs. So, to reflect the additional English 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

in the RDF graph, the graph shown in Figure 3 could be used (using suitable URIrefs to name the properties "creation-date" and "language"):

Figure 3: Several Statements About the Same Resource

Figure 3 illustrates that objects in RDF statements may be either URIrefs, or constant values (called literals) represented by character strings, in order to represent certain kinds of property values. (In the case of the predicate http://purl.org/dc/elements/1.1/language the literal is an international standard two-letter code for English.) Literals may not be used as subjects or predicates in RDF statements. In drawing RDF graphs, nodes that are URIrefs are shown as ellipses, while nodes that are literals are shown as boxes. (The simple character string literals used in these examples are called plain literals, to distinguish them from the typed literals to be introduced in Section 2.4. The various kinds of literals that can be used in RDF statements are defined in [RDF-CONCEPTS]. Both plain and typed literals can contain Unicode [UNICODE] characters, allowing information from many languages to be directly represented.)

Sometimes it is not convenient to draw graphs when discussing them, so an alternative way of writing down the statements, called triples, is also used. In the triples notation, each statement in the graph is written as a simple triple of subject, predicate, and object, in that order. For example, the three statements shown in Figure 3 would be written in the triples notation as:

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

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

<http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/language> "en" .

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

The full triples notation requires that URI references be written out completely, in angle brackets, which, as the example above illustrates, can result in very long lines on a page. For convenience, the Primer uses a shorthand way of writing triples (the same shorthand is also used in other RDF specifications). This shorthand substitutes an XML qualified name (or QName) without angle brackets as an abbreviation for a full URI reference (QNames are discussed further in Appendix B). A QName contains a prefix that has been assigned to a namespace URI, followed by a colon, and then a local name. The full URIref is formed from the QName by appending the local name to the namespace URI assigned to the prefix. So, for example, if the QName prefix foo is assigned to the namespace URI http://example.org/somewhere/, then the QName foo:bar is shorthand for the URIref http://example.org/somewhere/bar. Primer examples will also use several "well-known" QName prefixes (without explicitly specifying them each time), defined as follows:

prefix rdf:, namespace URI: http://www.w3.org/1999/02/22-rdf-syntax-ns#
prefix rdfs:, namespace URI: http://www.w3.org/2000/01/rdf-schema#
prefix dc:, namespace URI: http://purl.org/dc/elements/1.1/
prefix owl:, namespace URI: http://www.w3.org/2002/07/owl#
prefix ex:, namespace URI: http://www.example.org/ (or http://www.example.com/)
prefix xsd:, namespace URI: http://www.w3.org/2001/XMLSchema#

Obvious variations on the "example" prefix ex: will also be used as needed in the examples, for instance,

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

Using this new shorthand, the previous set of triples can be written as:

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

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

ex:index.html  dc:language            "en" .

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

ex:index.html  dc:creator  exstaff:27354 .

ex:index.html  dc:creator  exstaff:00816 .

These examples of RDF statements begin to illustrate some of the advantages of using URIrefs as RDF's basic way of identifying things. For instance, in the first statement, instead of identifying the creator of the Web page by the character string "John Smith", he has been assigned 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 the identification of the statement's subject can be more precise. That is, the creator of the page is not the character string "John Smith", or any one of the thousands of people named John Smith, but the particular John Smith associated with that URIref (whoever created the URIref defines the association). Moreover, since there is a URIref to refer to John Smith, he is a full-fledged resource, and additional information can be recorded about him, simply by adding additional RDF statements with John's URIref as the subject. For example, Figure 4 shows some additional statements giving John's name and age.

Figure 4: More Information About John Smith

These examples also illustrate that RDF uses URIrefs as predicates in RDF statements. That is, rather than using character strings (or words) such as "creator" or "name" to identify properties, RDF uses URIrefs. Using URIrefs to identify properties is important for a number of reasons. First, it distinguishes the properties one person may use from different properties someone else may use that would otherwise be identified by the same character string. For instance, in the example in Figure 4, example.org uses "name" to mean someone's full name written out as a character string literal (e.g., "John Smith"), but someone else may intend "name" to mean something different (e.g., the name of a variable in a piece of program text). A program encountering "name" as a property identifier on the Web (or merging data from multiple sources) would not necessarily be able to distinguish these uses. However, if example.org writes http://www.example.org/terms/name for its "name" property, and the other person writes http://www.domain2.example.org/genealogy/terms/name for hers, it is clear that there are distinct properties involved (even if a program cannot automatically determine the distinct meanings). Also, using URIrefs to identify properties enables the properties to be treated as resources themselves. Since properties are resources, additional information can be recorded 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 supports the development and use of shared vocabularies on the Web, since people can discover and begin using vocabularies already used by others to describe things, reflecting a shared understanding of those concepts. For example, in the triple

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

the predicate dc:creator, when fully expanded as a URIref, is an unambiguous reference to the "creator" attribute in the Dublin Core metadata attribute set (discussed further in Section 6.1), a widely-used set of attributes (properties) for describing information of all kinds. The writer of this triple is effectively saying that the relationship between the Web page (identified by http://www.example.org/index.html ) and the creator of the page (a distinct person, identified by http://www.example.org/staffid/85740 ) is exactly the concept identified by http://purl.org/dc/elements/1.1/creator. Another person familiar with the Dublin Core vocabulary, or who finds out what dc:creator means (say by looking up its definition on the Web) will know what is meant by this relationship. In addition, based on this understanding, people can write programs to behave in accordance with that meaning when processing triples containing the predicate dc:creator.

Of course, this depends on increasing the general use of URIrefs to refer to things instead of using literals; e.g., using URIrefs like exstaff:85740 and dc:creator instead of character string literals like John Smith and creator. Even then, RDF's use of URIrefs does not solve all identification problems because, for example, people can still use different URIrefs to refer to the same thing. For this reason, it is a good idea to try to use terms from existing vocabularies (such as the Dublin Core) where possible, rather than making up new terms that might overlap with those of some other vocabulary. Appropriate vocabularies for use in specific application areas are being developed all the time, as illustrated by the applications described in Section 6. However, even when synonymns are created, the fact that these different URIrefs are used in the commonly-accessible "Web space" provides the opportunity both to identify equivalences among these different references, and to migrate toward the use of common references.

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

fy:joefy.iunm  ed:dsfbups  fytubgg:85740 .

The result of all this is that RDF provides a way to make statements that applications can more easily process. An application cannot actually "understand" such statements, as noted already, any more than a database system "understands" terms like "employee" or "salary" in processing a query like SELECT NAME FROM EMPLOYEE WHERE SALARY > 35000. However, if an application is appropriately written, it can deal with RDF statements in a way that makes it seem like it does understand them, just as a database system and its applications can do useful work in processing employee and payroll information without understanding "employee" and "payroll". For example, a user could search the Web for all book reviews and create an average rating for each book. Then, the user could put that information back on the Web. Another Web site could take that list of book rating averages and create a "Top Ten Highest Rated Books" page. Here, the availability and use of a shared vocabulary about ratings, and a shared group of URIrefs identifying the books they apply to, allows individuals to build a mutually-understood and increasingly-powerful (as additional contributions are made) "information base" about books on the Web. The same principle applies to the vast amounts of information that people create about thousands of subjects every day on the Web.

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

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

and information in these formats can be treated as RDF statements, allowing RDF to be used to integrate data from many sources.

2.3 Structured Property Values and Blank Nodes

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

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

However, suppose John's address needed to be recorded as a structure consisting of separate street, city, state, and postal code values? How would this be done in RDF?

Structured information like this is represented in RDF by considering the aggregate thing to be described (like John Smith's address) as a resource, and then making statements about that new resource. So, in the RDF graph, in order to break up John Smith's address into its component parts, a new node is created to represent the concept of John Smith's address, with a new URIref to identify it, say http://www.example.org/addressid/85740 (abbreviated as exaddressid:85740). RDF statements (additional arcs and nodes) can then be written with that node as the subject, to represent the additional information, producing the graph shown in Figure 5:

Figure 5: Breaking Up John's Address

or the triples:

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

This way of representing structured information in RDF can involve generating numerous "intermediate" URIrefs such as exaddressid:85740 to represent aggregate concepts such as John's address. Such concepts may never need to be referred to directly from outside a particular graph, and hence may not require "universal" identifiers. In addition, in the drawing of the graph representing the group of statements shown in Figure 5, the URIref assigned to identify "John Smith's address" is not really needed, since the graph could just as easily have been drawn as in Figure 6:

Figure 6: Using a Blank Node

Figure 6, which is a perfectly good RDF graph, uses a node without a URIref to stand for the concept of "John Smith's address". This blank node serves its purpose in the drawing without needing a URIref, since the node itself provides the necessary connectivity between the various other parts of the graph. (Blank nodes were called anonymous resources in [RDF-MS].) However, some form of explicit identifier for that node is needed in order to represent this graph as triples. To see this, trying to write the triples corresponding to what is shown in Figure 6 would produce something like:

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

where ??? stands for something that indicates the presence of the blank node. Since a complex graph might contain more than one blank node, there also needs to be a way to differentiate between these different blank nodes in a triples representation of the graph. As a result, triples use blank node identifiers, having the form _:name, to indicate the presence of blank nodes. For instance, in this example a blank node identifier _:johnaddress might be used 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:postalCode      "01730" .

In a triples representation of a graph, each distinct blank node in the graph is given a different blank node identifier. Unlike URIrefs and literals, blank 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 6 and noting that the blank node has no blank node identifier). Blank node identifiers are just a way of representing the blank nodes in a graph (and distinguishing one blank node from another) when the graph is written in triple form. Blank node identifiers also have significance only within the triples representing a single graph (two different graphs with the same number of blank nodes might independently use the same blank node identifiers to distinguish them, and it would be incorrect to assume that blank nodes from different graphs having the same blank node identifiers are the same). 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. Finally, because blank node identifiers represent (blank) nodes, rather than arcs, in the triple form of an RDF graph, blank node identifiers may only appear as subjects or objects in triples; blank node identifiers may not be used as predicates in triples.

The beginning of this section noted that aggregate structures, like John Smith's address, can be represented by considering the aggregate thing to be described as a separate resource, and then making statements about that new resource. This example illustrates an important aspect of RDF: RDF directly represents only binary relationships, e.g. the relationship between John Smith and the literal representing his address. Representing the relationship between John and the group of separate components of this address involves dealing with an n-ary (n-way) relationship (in this case, n=5) between John and the street, city, state, and postal code components. In order to represent such structures directly in RDF (e.g., considering the address as a group of street, city, state, and postal code components), this n-way relationship must be broken up into a group of separate binary relationships. Blank nodes provide one way to do this. For each n-ary relationship, one of the participants is chosen as the subject of the relationship (John in this case), and a blank node is created to represent the rest of the relationship (John's address in this case). The remaining participants in the relationship (such as the city in this example) are then represented as separate properties of the new resource represented by the blank node.

Blank nodes also provide a way to more accurately make statements about resources that may not have URIs, but that are described in terms of relationships with other resources that do have URIs. For example, when making statements about a person, say Jane Smith, it may seem natural to use a URI based on that person's email address as her URI, e.g., mailto:jane@example.org. However, this approach can cause problems. For example, it may be necessary to record information both about Jane's mailbox (e.g., the server it is on) as well as about Jane herself (e.g., her current physical address), and using a URIref for Jane based on her email address makes it difficult to know whether it is Jane or her mailbox that is being described. The same problem exists when a company's Web page URL, say http://www.example.com/, is used as the URI of the company itself. Once again, it may be necessary to record information about the Web page itself (e.g., who created it and when) as well as about the company, and using http://www.example.com/ as an identifier for both makes it difficult to know which of these is the actual subject.

The fundamental problem is that using Jane's mailbox as a stand-in for Jane is not really accurate: Jane and her mailbox are not the same thing, and hence they should be identified differently. When Jane herself does not have a URI, a blank node provides a more accurate way of modeling this situation. Jane can be represented by a blank node, and that blank node used as the subject of a statement with exterms:mailbox as the property and the URIref mailto:jane@example.org as its value. The blank node could also be described with an rdf:type property having a value of exterms:Person (types are discussed in more detail in the following sections), an exterms:name property having a value of "Jane Smith", and any other descriptive information that might be useful, as shown in the following triples:

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

(Note that mailto:jane@example.org is written within angle brackets in the first triple. This is because mailto:jane@example.org is a full URIref in the mailto URI scheme, rather than a QName abbreviation, and full URIrefs must be enclosed in angle brackets in the triples notation.)

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

In practice, using blank nodes instead of URIrefs in these cases does not change the way this kind of information is handled very much. For example, if it is known that an email address uniquely identifies someone at example.org (particularly if the address is unlikely to be reused), that fact can still be used to associate information about that person from multiple sources, even though the email address is not the person's URI. In this case, if some RDF is found on the Web that describes a book, and gives the author's contact information as mailto:jane@example.org, it might be reasonable, combining this new information with the previous set of triples, to 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 typically a shorthand for "the author of the book is someone whose mailbox is mailto:jane@example.org". Using a blank node to represent this "someone" is just a more accurate way to represent the real world situation. (Incidentally, some RDF-based schema languages allow specifying that certain properties are unique identifiers of the resources they describe. This is discussed further in Section 5.5.)

ex2terms:book78354   rdf:type          ex2terms:Book .
ex2terms:book78354   ex2terms:author   "Jane Smith" .

ex2terms:book78354   rdf:type          ex2terms:Book .
ex2terms:book78354   ex2terms:author   _:author78354 .
_:author78354        rdf:type          ex2terms:Person .
_:author78354        ex2terms:name     "Jane Smith" .

2.4 Typed Literals

The last section described how to handle situations in which property values represented by plain literals had to be broken up into structured values to represent the individual parts of those literals. Using this approach, instead of, say, recording the date a Web page was created as a single exterms:creation-date property, with a single plain literal as its value, the value would be represented as a structure consisting of the month, day, and year as separate pieces of information, using separate plain literals to represent the corresponding values. However, so far, all constant values that serve as objects in RDF statements have been represented by these plain (untyped) literals, even when the intent is probably for the value of the property to be a number (e.g., the value of a year or age property) or some other kind of more specialized value.

For example, Figure 4 illustrated an RDF graph recording information about John Smith. That graph recorded the value of John Smith's exterms:age property as the plain literal "27", as shown in Figure 7:

Figure 7: Representing John Smith's Age

In this case, the hypothetical organization example.org probably intends for "27" to be interpreted as a number, rather than as the string consisting of the character "2" followed by the character "7" (since the literal represents the value of an "age" property). However, there is no information in Figure 7's graph that explicitly indicates that "27" should be interpreted as a number. Similarly, example.org also probably intends for "27" to be interpreted as a decimal number, i.e., the value twenty seven, rather than, say, as an octal number, i.e., the value twenty three. However, once again there is no information in Figure 7's graph that explicitly indicates this. Specific applications might be written with the understanding that they should interpret values of the exterms:age property as decimal numbers, but this would mean that proper interpretation of this RDF would depend on information not explicitly provided in the RDF graph, and hence on information that would not necessarily be available to other applications that might need to interpret this RDF.

The common practice in programming languages or database systems is to provide this additional information about how to interpret a literal by associating a datatype with the literal, in this case, a datatype like decimal or integer. An application that understands the datatype then knows, for example, whether the literal "10" is intended to represent the number ten, the number two, or the string consisting of the character "1" followed by the character "0", depending on whether the specified datatype is integer, binary, or string. (More specialized datatypes could also be used to include the units information mentioned at the end of Section 2.3, e.g., a datatype integerYears, although the Primer will not elaborate on this idea.) In RDF, typed literals are used to provide this kind of information.

An RDF typed literal is formed by pairing a string with a URIref that identifies a particular datatype. This results in a single literal node in the RDF graph with the pair as the literal. The value represented by the typed literal is the value that the specified datatype associates with the specified string. For example, using a typed literal, John Smith's age could be described as being the integer number 27 using the triple:

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

or, using the QName simplification for writing long URIs:

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

or as shown in Figure 8:

Figure 8: A Typed Literal for John Smith's Age

Similarly, in the graph shown in Figure 3 describing information about a Web page, the value of the page's exterms:creation-date property was written as the plain literal "August 16, 1999". However, using a typed literal, the creation date of the Web page could be explicitly described as being the date August 16, 1999, using the triple:

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

or as shown in Figure 9:

Figure 9: A Typed Literal for a Web Page's Creation Date

Unlike typical programming languages and database systems, RDF has no built-in set of datatypes of its own, such as datatypes for integers, reals, strings, or dates. Instead, RDF typed literals simply provide a way to explicitly indicate, for a given literal, what datatype should be used to interpret it. The datatypes used in typed literals are defined externally to RDF, and identified by their datatype URIs. (There is one exception: RDF defines a built-in datatype with the URIref rdf:XMLLiteral to represent XML content as a literal value. This datatype is defined in [RDF-CONCEPTS], and its use is described in Section 4.5.) For instance, the examples in Figure 8 and Figure 9 use the datatypes integer and date from the XML Schema datatypes defined in XML Schema Part 2: Datatypes [XML-SCHEMA2]. An advantage of this approach is that it gives RDF the flexibility to directly represent information coming from different sources without the need to perform type conversions between these sources and a native set of RDF datatypes. (Type conversions would still be required when moving information between systems having different sets of datatypes, but RDF would impose no extra conversions into and out of a native set of RDF datatypes.)

Since the value that a given typed literal denotes is defined by the typed literal's datatype, and, with the exception of rdf:XMLLiteral, RDF does not define any datatypes, the actual interpretation of a typed literal appearing in an RDF graph (e.g., determining the value it denotes) must be performed by software that is written to correctly process not only RDF, but the typed literal's datatype as well. Effectively, this software must be written to process an extended language that includes not only RDF, but also the datatype, as part of its built-in vocabulary. This raises the issue of which datatypes will be generally available in RDF software. Generally, the XML Schema datatypes that are listed as suitable for use in RDF in [RDF-SEMANTICS] have a "first among equals" status in RDF. As noted already, the examples in Figure 8 and Figure 9 used some of these XML Schema datatypes, and the Primer will be using these datatypes in most of its other examples of typed literals as well (for one thing, XML Schema datatypes already have assigned URIrefs that can be used to refer to them, specified in [XML-SCHEMA2]). These XML Schema datatypes are treated no differently than any other datatype, but they are expected to be the most widely used, and therefore the most likely to be interoperable among different software. As a result, it is expected that much RDF software will also be written to process these datatypes. However, RDF software could be written to process other sets of datatypes as well, assuming they were determined to be suitable for use with RDF, as described already.

In general, RDF software may be called on to process RDF data that contains references to datatypes that the software has not been written to process, in which case there are some things the software will not be able to do. For one thing, with the exception of rdf:XMLLiteral, RDF itself does not define the URIrefs that identify datatypes. As a result, RDF software, unless it has been written to recognize specific URIrefs, will not be able to determine whether or not a URIref written in a typed literal actually identifies a datatype. Moreover, even when a URIref does identify a datatype, RDF itself does not define the validity of pairing that datatype with a particular literal. This validity can only be determined by software written to correctly process that particular datatype.

For example, the typed literal in the triple:

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

or the graph shown in Figure 10:

Figure 10: An Invalid Typed Literal for John Smith's Age

is valid RDF, but obviously an error as far as the xsd:integer datatype is concerned, since "pumpkin" is not defined as being in the lexical space of xsd:integer. RDF software not written to process the xsd:integer datatype would not be able to recognize this error.

However, proper use of RDF typed literals provides more information about the intended interpretation of literal values, and hence makes RDF statements a better means of information exchange among applications.

2.5 Concepts Summary

Taken as a whole, RDF is basically simple: nodes-and-arcs diagrams interpreted as statements about things identified by URIrefs. This section has presented an introduction to these concepts. As noted earlier, the normative (i.e., definitive) RDF specification describing these concepts is RDF Concepts and Abstract Syntax [RDF-CONCEPTS], which should be consulted for further information. The formal semantics (meaning) of these concepts is defined in the (normative) RDF Semantics [RDF-SEMANTICS] document.

However, in addition to the basic techniques for describing things using RDF statements discussed so far, it should be clear that people or organizations also need a way to describe the vocabularies (terms) they intend to use in those statements, specifically, vocabularies for:

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

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

Additional background on the basic ideas underlying RDF, and its role in providing a general language for describing Web information, can be found in [WEBDATA]. RDF draws upon ideas from knowledge representation, artificial intelligence, and data management, including Conceptual Graphs, logic-based knowledge representation, frames, and relational databases. Some possible sources of background information on these subjects include [SOWA], [CG], [KIF], [HAYES], [LUGER], and [GRAY].

3.1 Basic Principles

The basic ideas behind the RDF/XML syntax can be illustrated using some of the examples presented already. Take as an example the English statement:

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

The RDF graph for this single statement, after assigning a URIref to the creation-date property, is shown in Figure 11:

Figure 11: Describing a Web Page's Creation Date

with a triple representation of:

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

(Note that a typed literal is not used for the date value in this example. Representing typed literals in RDF/XML will be described later in this section.)

Example 2 shows the RDF/XML syntax corresponding to the graph in Figure 11:

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

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

7. </rdf:RDF>

(Line numbers are added to help in explaining the example.)

This seems like a lot of overhead. It is easier to understand what is going on by considering each part of this XML in turn (a brief introduction to XML is provided in Appendix B).

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

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

Line 3 specifies another XML namespace declaration, this time for the prefix exterms:. This is expressed as another xmlns attribute of the rdf:RDF element, and specifies that the namespace URIref http://www.example.org/terms/ is to be associated with the exterms: prefix. URIrefs beginning with the string http://www.example.org/terms/ are used for terms from the vocabulary defined by the 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 this is RDF/XML content, and to identify the namespaces being used within the RDF/XML content.

Lines 4-6 provide the RDF/XML for the specific statement shown in Figure 11. An obvious way to talk about any RDF statement is to say it is a description, and that it is about the subject of the statement (in this case, about http://www.example.org/index.html), and this is the way RDF/XML represents the statement. The rdf:Description start-tag in line 4 indicates the start of a description of a resource, and goes on to identify the resource the statement is about (the subject of the statement) using the rdf:about attribute to specify the URIref of the subject resource. Line 5 provides a property element, with the QName exterms:creation-date as its tag, to represent the predicate and object of the statement. The QName exterms:creation-date is chosen so that appending the local name creation-date to the URIref of the exterms: prefix (http://www.example.org/terms/) gives the statement's predicate URIref http://www.example.org/terms/creation-date. The content of this property element is the object of the statement, the plain literal August 19, 1999 (the value of the creation-date property of the subject resource). The property element is nested within the containing rdf:Description element, indicating that this property applies to the resource specified in the rdf:about attribute of the rdf:Description element. Line 6 indicates the end of this particular rdf:Description element.

Finally, Line 7 indicates the end of the rdf:RDF element started on line 2. Using an rdf:RDF element to enclose RDF/XML content is optional in situations where the XML can be identified as RDF/XML by context. This is discussed further in [RDF-SYNTAX]. However, it does not hurt to provide the rdf:RDF element in any case, and Primer examples will generally (but not always) provide one.

Example 2 illustrates the basic ideas used by RDF/XML to encode an RDF graph as XML elements, attributes, element content, and attribute values. The URIrefs of predicates (as well as some 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 Appendix B. The (namespace URIref, local name) pair is chosen so that concatenating them forms the URIref of the original node or predicate. The URIrefs of subject nodes are written as XML attribute values (URIrefs of object nodes may sometimes be written as attribute values as well). Literal nodes (which are always object nodes) become element text content or attribute values. (Many of these options are described later in the Primer; all of these options are described in [RDF-SYNTAX].)

An RDF graph consisting of multiple statements can be represented in RDF/XML by using RDF/XML similar to Lines 4-6 in Example 2 to separately represent each statement. For example, to write the following two statements:

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

the RDF/XML in Example 3 could be used:

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

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

8.    <rdf:Description rdf:about="http://www.example.org/index.html">
9.        <dc:language>en</dc:language>
10.   </rdf:Description>

11. </rdf:RDF>

Example 3 is the same as Example 2, with the addition of a second rdf:Description element (in lines 8-10) to represent the second statement. (An additional namespace declaration is also given in line 3 to identify the additional namespace used in this statement.) An arbitrary number of additional statements could be written in the same way, using a separate rdf:Description element for each additional statement. As Example 3 illustrates, once the overhead of writing the XML and namespace declarations is dealt with, writing each additional RDF statement in RDF/XML is both straightforward and not too complicated.

The RDF/XML syntax provides a number of abbreviations to make common uses easier to write. For example, it is typical for the same resource to be described with several properties and values at the same time, as in Example 3, where the resource ex:index.html is the subject of several statements. To handle such cases, RDF/XML allows multiple property elements representing those properties to be nested within the rdf:Description element that identifies the subject resource. For example, to represent the following group of statements about http://www.example.org/index.html:

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

whose graph (the same as Figure 3) is shown in Figure 12:

Figure 12: Several Statements About the Same Resource

the RDF/XML shown in Example 4 could be written:

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

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

10. </rdf:RDF>

Compared with the previous two examples, Example 4 adds an additional dc:creator property element (in line 8). In addition, the property elements for the three properties whose subject is http://www.example.org/index.html are nested within a single rdf:Description element identifying that subject, rather than writing a separate rdf:Description element for each statement.

Line 8 also introduces a new form of property element. The dc:language element in line 7 is similar to the exterms:creation-date element used in Example 2. Both these elements represent properties with plain literals as property values, and such elements are written by enclosing the literal within start- and end-tags corresponding to the property name. However, the dc:creator element on line 8 represents a property whose value is another resource, rather than a literal. If the URIref of this resource were written as a plain literal within start- and end-tags in the same way as the literal values of the other elements, this would say that the value of the dc:creator element was the character string http://www.example.org/staffid/85740, rather than the resource identified by that literal interpreted as a URIref. In order to indicate the difference, the dc:creator element is written using what XML calls an empty-element tag (it has no separate end-tag), and the property value is written using an rdf:resource attribute within that empty element. The rdf:resource attribute indicates that the property element's value is another resource, identified by its URIref. Because the URIref is being used as an attribute value, RDF/XML requires the URIref to be written out (as an absolute or relative URIref), rather than abbreviating it as a QName as was done in writing element and attribute names (absolute and relative URIrefs are discussed in Appendix A).

It is important to understand that the RDF/XML in Example 4 is an abbreviation. The RDF/XML in Example 5, in which each statement is written separately, describes exactly the same RDF graph (the graph of Figure 12):

 <?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:exterms="http://www.example.org/terms/">

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

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

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

 </rdf:RDF>

The following sections will describe a few additional RDF/XML abbreviations. [RDF-SYNTAX] provides a more thorough description of the abbreviations that are available.

RDF/XML can also represent graphs that include nodes that have no URIrefs, i.e., the blank nodes described in Section 2.3. For example, Figure 13 (taken from [RDF-SYNTAX]) shows a graph saying "the document 'http://www.w3.org/TR/rdf-syntax-grammar' has a title 'RDF/XML Syntax Specification (Revised)' and has an editor, the editor has a name 'Dave Beckett' and a home page 'http://purl.org/net/dajobe/' ".

Figure 13: A Graph Containing a Blank Node

This illustrates an idea discussed in Section 2.3: the use of a blank node to represent something that does not have a URIref, but can be described in terms of other information. In this case, the blank node represents a person, the editor of the document, and the person is described by his name and home page.

RDF/XML provides several ways to represent graphs containing blank nodes. These are all described in [RDF-SYNTAX]. The approach illustrated here, which is the most direct approach, is to assign a blank node identifier to each blank node. A blank node identifier serves to identify a blank node within a particular RDF/XML document but, unlike a URIref, is unknown outside the document in which it is assigned. A blank node is referred to in RDF/XML using an rdf:nodeID attribute, with a blank node identifier as its value, in places where the URIref of a resource would otherwise appear. Specifically, a statement with a blank node as its subject can be written in RDF/XML using an rdf:Description element with an rdf:nodeID attribute instead of an rdf:about attribute. Similarly, a statement with a blank node as its object can be written using a property element with an rdf:nodeID attribute instead of an rdf:resource attribute. Using rdf:nodeID, Example 6 shows the RDF/XML corresponding to Figure 13:

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

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

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

13. </rdf:RDF>

In Example 6, the blank node identifier abc is used in line 9 to identify the blank node as the subject of several statements, and is used in line 7 to indicate that the blank node is the value of a resource's exterms:editor property. The advantage of using a blank node identifier over some of the other approaches described in [RDF-SYNTAX] is that using a blank node identifier allows the same blank node to be referred to in more than one place in the same RDF/XML document.

Finally, the typed literals described in Section 2.4 may be used as property values instead of the plain literals used in the examples so far. A typed literal is represented in RDF/XML by adding an rdf:datatype attribute specifying a datatype URIref to the property element containing the literal.

For example, to change the statement in Example 2 to use a typed literal instead of a plain literal for the exterms:creation-date property, the triple representation would be:

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

with corresponding RDF/XML syntax shown in Example 7:

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

4.   <rdf:Description rdf:about="http://www.example.org/index.html">
5.     <exterms:creation-date rdf:datatype=
         "http://www.w3.org/2001/XMLSchema#date">1999-08-16
       </exterms:creation-date>
6.   </rdf:Description>

7. </rdf:RDF>

In line 5 of Example 7, a typed literal is given as the value of the exterms:creation-date property element by adding an rdf:datatype attribute to the element's start-tag to specify the datatype. The value of this attribute is the URIref of the datatype, in this case, the URIref of the XML Schema date datatype. Since this is an attribute value, the URIref must be written out, rather than using the QName abbreviation xsd:date used in the triple. A literal appropriate to this datatype is then written as the element content, in this case, the literal 1999-08-16, which is the literal representation for August 16, 1999 in the XML Schema date datatype.

<!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]> 

1. <?xml version="1.0"?>
2. <!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]>

3. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.             xmlns:exterms="http://www.example.org/terms/">

5.   <rdf:Description rdf:about="http://www.example.org/index.html">
6.     <exterms:creation-date rdf:datatype="&xsd;date">1999-08-16
       </exterms:creation-date>
7.   </rdf:Description>

8. </rdf:RDF>

Although additional abbreviated forms for writing RDF/XML are available, the facilities illustrated so far provide a simple but general way to express graphs in RDF/XML. Using these facilities, an RDF graph is written in RDF/XML as follows:

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

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

3.2 Abbreviating and Organizing RDF URIrefs

So far, the examples have assumed that the resources being described have been given URIrefs already. For instance, the initial examples provided descriptive information about example.org's Web page, whose URIref was http://www.example.org/index.html. This resource was identified in RDF/XML using an rdf:about attribute citing its full URIref. Although RDF does not specify or control how URIrefs are assigned to resources, sometimes it is desirable to achieve the effect of assigning URIrefs to resources that are part of an organized group of resources. For example, suppose a sporting goods company, example.com, wanted to provide an RDF-based catalog of its products, such as tents, hiking boots, and so on, as an RDF/XML document, identified by (and located at) http://www.example.com/2002/04/products. In that resource, each product might be given a separate RDF description. This catalog, along with one of these descriptions, the catalog entry for a model of tent called the "Overnighter", might be written in RDF/XML as shown in Example 9:

1.   <?xml version="1.0"?>
2.   <!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]>
3.   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.               xmlns:exterms="http://www.example.com/terms/">

5.     <rdf:Description rdf:ID="item10245">
6.          <exterms:model rdf:datatype="&xsd;string">Overnighter</exterms:model>
7.          <exterms:sleeps rdf:datatype="&xsd;integer">2</exterms:sleeps>
8.          <exterms:weight rdf:datatype="&xsd;decimal">2.4</exterms:weight>
9.          <exterms:packedSize rdf:datatype="&xsd;integer">784</exterms:packedSize>
10.    </rdf:Description>

  ...other product descriptions...

11.  </rdf:RDF>

Example 9 is similar to previous examples in the way it represents the properties (model, sleeping capacity, weight) of the resource (the tent) being described. (The surrounding xml, DOCTYPE, RDF, and namespace information is included in lines 1 through 4, and line 11, but this information would only need to be provided once for the whole catalog, not repeated for each entry in the catalog. Note also that although the datatypes associated with the various property values are given explicitly, the units associated with some of these property values are not, even though this information should be available to properly interpret the values. Representing units and similar information that may be associated with property values is discussed in Section 4.4. In this example, the value of exterms:sleeps is the number of persons the tent can sleep, the value of exterms:weight is given in kilograms, and the value of exterms:packedSize is given in square centimeters, the area the tent occupies on a backpack.)

An important difference from previous examples is that, in line 5, the rdf:Description element has an rdf:ID attribute instead of an rdf:about attribute. Using rdf:ID specifies a fragment identifier, given by the value of the rdf:ID attribute (item10245 in this case, which might be the catalog number assigned by example.com), as an abbreviation of the complete URIref of the resource being described. The fragment identifier item10245 will be interpreted relative to a base URI<