RDF 1.2 XML Syntax

The RDF Concepts and Abstract Syntax document [ RDF12-CONCEPTS ] defines the RDF Graph data model and the RDF Graph abstract syntax. Along with the RDF Semantics [ RDF12-SEMANTICS ] this provides an abstract syntax with a formal semantics for it. The RDF graph has nodes and labeled directed arcs that link pairs of nodes and this is represented as a set of RDF triples where each triple contains a subject node , predicate and object node . Nodes are IRIs, literals, or blank nodes. Blank nodes may be given a document-local identifier called a blank node identifier. Predicates are IRIs and can be interpreted as either a relationship between the two nodes or as defining an attribute value (object node) for some subject node.

In order to encode the graph in XML, the nodes and predicates have to be represented in XML terms — element names, attribute names, element contents and attribute values. RDF/XML uses XML QNames as defined in Namespaces in XML [ XML-NAMES ] to represent IRIs. All QNames have a namespace name which is an IRI and a short local name . In addition, QNames can either have a short prefix or be declared with the default namespace declaration and have none (but still have a namespace name)

The IRI represented by a QName is determined by appending the local name part of the QName after the namespace name (IRI) part of the QName. This is used to shorten the IRI of all predicates and some nodes. IRIs identifying subject and object nodes can also be stored as XML attribute values. RDF literals which can only be object nodes, become either XML element text content or XML attribute values.

A graph can be considered a collection of paths of the form node, predicate arc, node, predicate arc, node, predicate arc, ... node which cover the entire graph. In RDF/XML these turn into sequences of elements inside elements which alternate between elements for nodes and predicate arcs. This has been called a series of node/arc stripes. The node at the start of the sequence turns into the outermost element, the next predicate arc turns into a child element, and so on. The stripes generally start at the top of an RDF/XML document and always begin with nodes.

Several RDF/XML examples are given in the following sections building up to complete RDF/XML documents. Example 7 is the first complete RDF/XML document.

An RDF graph is given in Figure 1 where the nodes are represented as ovals and contain their IRIs where they have them, all the predicate arcs are labeled with IRIs and string literals nodes have been written in rectangles.

If we follow one node, predicate arc ... , node path through the graph shown in Figure 2 :

The left hand side of the Figure 2 graph corresponds to the node/predicate arc stripes:

1. Node with IRI http://www.w3.org/TR/rdf-syntax-grammar
2. Predicate Arc labeled with IRI http://example.org/terms/editor
3. Node with no IRI
4. Predicate Arc labeled with IRI http://example.org/terms/homePage
5. Node with IRI http://purl.org/net/dajobe/

In RDF/XML, the sequence of 5 nodes and predicate arcs on the left hand side of Figure 2 corresponds to the usage of five XML elements of two types, for the graph nodes and predicate arcs. These are conventionally called node elements and property elements respectively. In the striping shown in Example 1 , rdf:Description is the node element (used three times for the three nodes) and ex:editor and ex:homePage are the two property elements.

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

The Figure 2 graph consists of some nodes that are IRIs (and others that are not) and this can be added to the RDF/XML using the rdf:about attribute on node elements to give the result in Example 2 :

<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>

Adding the other two paths through the Figure 1 graph to the RDF/XML in Example 2 gives the result in Example 3 (this example fails to show that the blank node is shared between the two paths, see 2.10 Identifying Blank Nodes: rdf:nodeID ):

<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>
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
  <ex:editor>
    <rdf:Description>
      <ex:fullName>Dave Beckett</ex:fullName>
    </rdf:Description>
  </ex:editor>
</rdf:Description>
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
  <dc:title>RDF 1.2 XML Syntax</dc:title>
</rdf:Description>

There are several abbreviations that can be used to make common uses easier to write down. In particular, it is common that a subject node in the RDF graph has multiple outgoing predicate arcs. RDF/XML provides an abbreviation for the corresponding syntax when a node element about a resource has multiple property elements. This can be abbreviated by using multiple child property elements inside the node element describing the subject node.

Taking Example 3 , there are two node elements that can take multiple property elements. The subject node with IRI http://www.w3.org/TR/rdf-syntax-grammar has property elements ex:editor and ex:title and the node element for the blank node can take ex:homePage and ex:fullName . This abbreviation gives the result shown in Example 4 (this example does show that there is a single blank node):

<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>
      <ex:fullName>Dave Beckett</ex:fullName>
    </rdf:Description>
  </ex:editor>
  <dc:title>RDF 1.2 XML Syntax</dc:title>
</rdf:Description>

When a predicate arc in an RDF graph points to an object node which has no further predicate arcs, which appears in RDF/XML as an empty node element <rdf:Description rdf:about="..."> </rdf:Description> (or <rdf:Description rdf:about="..." /> ) this form can be shortened. This is done by using the IRI of the object node as the value of an XML attribute rdf:resource on the containing property element and making the property element empty.

In this example, the property element ex:homePage contains an empty node element with the IRI http://purl.org/net/dajobe/ . This can be replaced with the empty property element form giving the result shown in Example 5 :

 
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar">
  <ex:editor>
    <rdf:Description>
      <ex:homePage rdf:resource="http://purl.org/net/dajobe/"/>
      <ex:fullName>Dave Beckett</ex:fullName>
    </rdf:Description>
  </ex:editor>
  <dc:title>RDF 1.2 XML Syntax</dc:title>
</rdf:Description>

When a property element's content is string literal, it may be possible to use it as an XML attribute on the containing node element. This can be done for multiple properties on the same node element only if the property element name is not repeated (required by XML — attribute names are unique on an XML element) and any in-scope xml:lang on the property element's string literal (if any) are the same (see 2.7 Languages: xml:lang ) This abbreviation is known as a Property Attribute and can be applied to any node element.

This abbreviation can also be used when the property element is rdf:type and it has an rdf:resource attribute the value of which is interpreted as a IRI object node.

In Example 5 :, there are two property elements with string literal content, the dc:title and ex:fullName property elements. These can be replaced with property attributes giving the result shown in Example 6 :

<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar"
           dc:title="RDF 1.2 XML Syntax">
  <ex:editor>
    <rdf:Description ex:fullName="Dave Beckett">
      <ex:homePage rdf:resource="http://purl.org/net/dajobe/"/>
    </rdf:Description>
  </ex:editor>
</rdf:Description>

To create a complete RDF/XML document, the serialization of the graph into XML is usually contained inside an rdf:RDF XML element which becomes the top-level XML document element. Conventionally the rdf:RDF element is also used to declare the XML namespaces that are used, although that is not required. When there is only one top-level node element inside rdf:RDF , the rdf:RDF can be omitted although any XML namespaces must still be declared.

The XML specification also permits an XML declaration at the top of the document with the XML version and possibly the XML content encoding. This is optional but recommended.

Completing the RDF/XML could be done for any of the correct complete graph examples from Example 4 onwards but taking the smallest Example 6 and adding the final components, gives a complete RDF/XML representation of the original Figure 1 graph in Example 7 :

It is possible to omit rdf:RDF in Example 7 above since there is only one rdf:Description inside rdf:RDF but this is not shown here.

RDF/XML permits the use of the xml:lang attribute as defined by 2.12 Language Identification of XML 1.0 [ XML10 ] to allow the identification of content language. The xml:lang attribute can be used on any node element or property element to indicate that the included content is in the given language. Typed literals which includes XML literals are not affected by this attribute. The most specific in-scope language present (if any) is applied to property element string literal content or property attribute values. The xml:lang="" form indicates the absence of a language identifier.

Some examples of marking content languages for RDF properties are shown in Example 8 :

RDF allows typed literals to be given as the object node of a predicate. Typed literals consist of a literal string and a datatype IRI. These are written in RDF/XML using the same syntax for literal string nodes in the property element form (not property attribute) but with an additional rdf:datatype=" datatypeURI " attribute on the property element. Any IRI can be used in the attribute.

An example of an RDF typed literal is given in Example 10 where there is a single RDF triple with the subject node IRI http://example.org/item01 , the predicate IRI http://example.org/stuff/1.0/size (from ex:size ) and the object node with the typed literal ("123", http://www.w3.org/2001/XMLSchema#int ) to be interpreted as an XML Schema [ XMLSCHEMA-2 ] datatype int .

Blank nodes in the RDF graph are distinct but have no IRI identifier. It is sometimes required that the same graph blank node is referred to in the RDF/XML in multiple places, such as at the subject and object of several RDF triples. In this case, a blank node identifier can be given to the blank node for identifying it in the document. Blank node identifiers in RDF/XML are scoped to the containing XML Information Set document information item . A blank node identifier is used on a node element to replace rdf:about=" IRI " or on a property element to replace rdf:resource=" IRI " with rdf:nodeID=" blank node identifier " in both cases.

Taking Example 7 and explicitly giving a blank node identifier of abc to the blank node in it gives the result shown in Example 11 . The second rdf:Description property element is about the blank node.

Blank nodes (not IRI nodes) in RDF graphs can be written in a form that allows the <rdf:Description> </rdf:Description> pair to be omitted. The omission is done by putting an rdf:parseType="Resource" attribute on the containing property element that turns the property element into a property-and-node element, which can itself have both property elements and property attributes. Property attributes and the rdf:nodeID attribute are not permitted on property-and-node elements.

Taking the earlier Example 7 , the contents of the ex:editor property element could be alternatively done in this fashion to give the form shown in Example 12 :

If all of the property elements on a blank node element have string literal values with the same in-scope xml:lang value (if present) and each of these property elements appears at most once and there is at most one rdf:type property element with a IRI object node, these can be abbreviated by moving them to be property attributes on the containing property element which is made an empty element.

Taking the earlier Example 5 , the ex:editor property element contains a blank node element with two property elements ex:fullname and ex:homePage . ex:homePage is not suitable here since it does not have a string literal value, so it is being ignored for the purposes of this example. The abbreviated form removes the ex:fullName property element and adds a new property attribute ex:fullName with the string literal value of the deleted property element to the ex:editor property element. The blank node element becomes implicit in the now empty ex:editor property element. The result is shown in Example 13 .

It is common for RDF graphs to have rdf:type predicates from subject nodes. These are conventionally called typed nodes in the graph, or typed node elements in the RDF/XML. RDF/XML allows this triple to be expressed more concisely. by replacing the rdf:Description node element name with the namespaced-element corresponding to the IRI of the value of the type relationship. There may, of course, be multiple rdf:type predicates but only one can be used in this way, the others must remain as property elements or property attributes.

The typed node elements are commonly used in RDF/XML with the built-in classes in the RDF vocabulary : rdf:Seq , rdf:Bag , rdf:Alt , rdf:Statement , rdf:Property and rdf:List .

For example, the RDF/XML in Example 14 could be written as shown in Example 15 .

RDF/XML allows further abbreviating IRIs in XML attributes in two ways. The XML Infoset provides a base URI attribute xml:base that sets the base URI for resolving relative IRIs, IRI references , otherwise the base URI is that of the document. The base URI applies to all RDF/XML attributes that deal with IRIs which are rdf:about , rdf:resource , rdf:ID and rdf:datatype .

The rdf:ID attribute on a node element (not property element, that has another meaning) can be used instead of rdf:about and gives a relative IRI reference equivalent to # concatenated with the rdf:ID attribute value. So for example if rdf:ID="name" , that would be equivalent to rdf:about="#name" . rdf:ID provides an additional check since the same name can only appear once in the scope of an xml:base value (or document, if none is given), so is useful for defining a set of distinct, related terms relative to the same IRI.

Both forms require a base URI to be known, either from an in-scope xml:base or from the URI of the RDF/XML document.

Example 16 shows abbreviating the node IRI of http://example.org/here/#snack using an xml:base of http://example.org/here/ and an rdf:ID on the rdf:Description node element. The object node of the ex:prop predicate is an absolute IRI resolved from the rdf:resource XML attribute value using the in-scope base URI to give the IRI http://example.org/here/fruit/apple .

RDF has a set of container membership properties and corresponding property elements that are mostly used with instances of the rdf:Seq , rdf:Bag and rdf:Alt classes which may be written as typed node elements. The list properties are rdf:_1 , rdf:_2 etc. and can be written as property elements or property attributes as shown in Example 17 . There is an rdf:li special property element that is equivalent to rdf:_1 , rdf:_2 in order, explained in detail in 6.4 List Expansion Rules . The mapping to the container membership properties is always done in the order that the rdf:li special property elements appear in XML — the document order is significant. The equivalent RDF/XML to Example 17 written in this form is shown in Example 18 .

RDF/XML allows an rdf:parseType="Collection" attribute on a property element to let it contain multiple node elements. These contained node elements give the set of subject nodes of the collection. This syntax form corresponds to a set of triples connecting the collection of subject nodes, the exact triples generated are described in detail in 6.2.19 Production parseTypeCollectionPropertyElt . The collection construction is always done in the order that the node elements appear in the XML document. Whether the order of the collection of nodes is significant is an application issue and not defined here.

Example 19 shows a collection of three nodes elements at the end of the ex:hasFruit property element using this form.

The rdf:ID attribute can be used on a property element to reify the triple that it generates (See 6.3 Reification Rules for the full details). The identifier for the triple should be constructed as a IRI made from the relative IRI reference # concatenated with the rdf:ID attribute value, resolved against the in-scope base URI. So for example if rdf:ID="triple" , that would be equivalent to the IRI formed from relative IRI reference #triple against the base URI. Each ( rdf:ID attribute value, base URI) pair has to be unique in an RDF/XML document, see constraint-id .

Example 20 shows a rdf:ID being used to reify a triple made from the ex:prop property element giving the reified triple the IRI http://example.org/triples/#triple1 .

Contact:

Dan Brickley

See also:

How to Register a Media Type for a W3C Specification

Internet Media Type registration, consistency of use
TAG Finding 3 June 2002 (Revised 4 September 2002)

The Internet Media Type (formerly known as MIME Type) for RDF/XML is application/rdf+xml — [ RFC3870 ].

It is recommended that RDF/XML files have the extension ".rdf" (all lowercase) on all platforms.

It is recommended that RDF/XML files stored on Macintosh HFS file systems be given a file type of "rdf " (all lowercase, with a space character as the fourth letter).

The application/rdf+xml media type has been registered at IANA as [ RFC3870 ].

The RDF namespace IRI (or namespace name) is http://www.w3.org/1999/02/22-rdf-syntax-ns# and is typically used in XML with the prefix rdf although other prefix strings may be used. The RDF Vocabulary is identified by this namespace name and consists of the following names only:

Syntax names — not concepts

RDF Description ID about parseType resource li nodeID datatype

Class names

Seq Bag Alt Statement Property XMLLiteral List

Property names

subject predicate object type value first rest _ n
where n is a decimal integer greater than zero with no leading zeros.

Resource names

nil

Any other names are not defined and SHOULD generate a warning when encountered, but should otherwise behave normally.

Within RDF/XML documents it is not permitted to use XML namespaces whose namespace name is the · RDF namespace IRI · concatenated with additional characters.

Throughout this document the terminology rdf: name will be used to indicate name is from the RDF vocabulary and it has a IRI of the concatenation of the · RDF namespace IRI · and name . For example, rdf:type has the IRI http://www.w3.org/1999/02/22-rdf-syntax-ns#type

The RDF Concepts document [ RDF12-CONCEPTS ] defines the three types of RDF data that can act as node and/or predicate:

IRI

IRIs can act as node (both subject and object) and as predicate.

IRIs can be either:

• given as XML attribute values interpreted as relative IRIs that are resolved against the in-scope base URI as described in 4.3 Resolving IRIs to give absolute IRIs
• transformed from XML namespace-qualified element and attribute names (QNames)
• transformed from rdf:ID attribute values.

Within RDF/XML, XML QNames are transformed into IRIs by appending the XML local name to the namespace name (IRI). For example, if the XML namespace prefix foo has namespace name (IRI) http://example.org/somewhere/ then the QName foo:bar would correspond to the IRI http://example.org/somewhere/bar . Note that this restricts which IRIs can be made and the same IRI can be given in multiple ways.

The rdf:ID values are transformed into IRIs by appending the attribute value to the result of appending "#" to the in-scope base URI which is defined in 4.3 Resolving IRIs

Literal

Literals can only act as object nodes.

Literals always have a datatype. Language-tagged strings get the datatype rdf:langString . When there is no language tag or datatype specified the literal is assumed to have the datatype xsd:string .

Blank Node

Blank nodes can act as subject node and as object node.

Blank nodes have distinct identity in the RDF graph. When the graph is written in a syntax such as RDF/XML, these blank nodes may need graph-local identifiers and a syntax in order to preserve this distinction. These local identifiers are called blank node identifiers and are used in RDF/XML as values of the rdf:nodeID attribute with the syntax given in Production nodeIdAttr . Blank node identifiers in RDF/XML are scoped to the XML Information Set document information item .

If no blank node identifier is given explicitly as an rdf:nodeID attribute value then one will need to be generated (using generated-blank-node-id, see 5.3.3 Grammar Action Notation ). Such generated blank node identifiers must not clash with any blank node identifiers derived from rdf:nodeID attribute values. This can be implemented by any method that preserves the distinct identity of all the blank nodes in the graph, that is, the same blank node identifier is not given for different blank nodes. One possible method would be to add a constant prefix to all the rdf:nodeID attribute values and ensure no generated blank node identifiers ever used that prefix. Another would be to map all rdf:nodeID attribute values to new generated blank node identifiers and perform that mapping on all such values in the RDF/XML document.

RDF/XML supports XML Base [ XMLBASE ] which defines a · base-uri · accessor for each · root event · and · element event · . Relative IRIs IRI references are resolved into IRIs according to the algorithm specified in [ XMLBASE ] (and RFC 2396). These specifications do not specify an algorithm for resolving a fragment identifier alone, such as #foo , or the empty string "" into an IRI. In RDF/XML, a fragment identifier is transformed into an IRI by appending the fragment identifier to the in-scope base URI. The empty string is transformed into an IRI by substituting the in-scope base URI.

An empty same document reference "" resolves against the URI part of the base URI; any fragment part is ignored. See Uniform Resource Identifiers (URI) [ RFC3986 ].

constraint-id

Each application of production idAttr matches an attribute. The pair formed by the · string-value · accessor of the matched attribute and the · base-uri · accessor of the matched attribute is unique within a single RDF/XML document.

The syntax of the names must match the rdf-id production .

Note

Test: Indicated by test014.rdf and test014.nt

There are nine types of event defined in the following subsections. Most events are constructed from an Infoset information item (except for IRI , blank node , plain literal and typed literal ). The effect of an event constructor is to create a new event with a unique identity, distinct from all other events. Events have accessor operations on them and most have the string-value accessor that may be a static value or computed.

To transform the Infoset into the sequence of events in document order , each information item is transformed as described above to generate a tree of events with accessors and values. Each element event is then replaced as described below to turn the tree of events into a sequence in document order.

1. The original element event
2. The value of the children accessor recursively transformed, a possibly empty ordered list of events.
3. An end element event

The following notation is used to describe matching the sequence of data model events as given in 5. Syntax Data Model and the actions to perform for the matches. The RDF/XML grammar is defined in terms of mapping from these matched data model events to triples, using notation of the form:

number event-type event-content

action ...

N-Triples

where the event-content is an expression matching event-types (as defined in 5.1 Events ), using notation given in the following sections. The number is used for reference purposes. The grammar action may include generating new triples to the graph, written in N-Triples [ RDF12-N-TRIPLES ] format.

The following sections describe the general notation used and that for event matching and actions.

For the given IRI event r and the statement with terms s , p and o corresponding to the N-Triples:

add the following statements to the graph:

For the given element e , create a new IRI u := concat("http://www.w3.org/1999/02/22-rdf-syntax-ns#_", e . li-counter ), increment the e . li-counter property by 1 and return u .