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10.7. Schemas for XML, XML Base and XLink

XML Base, XLink, and XML 1.0 all define vocabularies that are composed only of attributes. Attributes such as xml:base, xlink:href, or xml:lang are designed to work with elements from any vocabulary, imparting their meaning to those elements and sometimes the descendants of those elements.

10.7.1. XML Attributes

In this section, we will discuss how to allow the use of a XML attribute in our document. To illustrate this, let's use a xml:lang attribute to qualify the language of several descriptions in several languages for our book:

<?xml version="1.0"?>
<library xmlns="http://dyomedea.com/ns/library">
  <book id="b0836217462">
    <title>
      Being a Dog Is a Full-Time Job
    </title>
    <description xml:lang="en">
      Its title says it all !
    </description>
    <description xml:lang="fr">
      Son titre le résume parfaitement !
    </description>
  </book>
</library>

Following Namespaces in XML 1.0, W3C XML Schema considers xml:lang, xml:space (from the XML 1.0 recommendation) and xml:base (from the XML Base specification) to belong to the XML 1.0 namespace identified by the URI reference "http://dyomedea.com/ns/people".

TIP: The XML 1.0 namespace is an exception in the namespaces world, since we don't need to declare it in the instance documents. Its prefix "xml" is reserved for this usage and has a special meaning, even for the parsers that do not support the namespaces. In theory, any other prefix can be assigned to the XML 1.0 namespace, but this practice is confusing and shouldn't be encouraged.

To use attributes from this namespace, we will declare the XML namespace, import a schema, and reference both of them, as we've done previously for the elements from our "people" namespace:

<?xml version="1.0"?> 
<xs:schema targetNamespace="http://dyomedea.com/ns/library"
  elementFormDefault="qualified" attributeFormDefault="unqualified"
  xmlns:lib="http://dyomedea.com/ns/library"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"> 
  <xs:import namespace="http://www.w3.org/XML/1998/namespace"
    schemaLocation="xml.xsd"/>
  <xs:element name="description">
    <xs:complexType>
      <xs:simpleContent>
        <xs:restriction base="xs:string">
          <xs:attribute ref="xml:lang"/>
        </xs:restriction>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>
  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="lib:book"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="book">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="title" type="xs:string"/>
        <xs:element ref="lib:description" maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:attribute name="id" type="xs:ID" use="required"/>
    </xs:complexType>
  </xs:element>
</xs:schema>

In this example, we import a local schema (xml.xsd) for the XML namespace. The W3C has defined its own schema for the xml:lang, xml:space, and xml:base attributes. This is available at http://www.w3.org/2001/xml.xsd:

<?xml version='1.0'?>
<!DOCTYPE xs:schema PUBLIC "-//W3C//DTD XMLSCHEMA 200102//EN" "XMLSchema.dtd" >
<xs:schema targetNamespace="http://www.w3.org/XML/1998/namespace" xmlns:xs="http://
www.w3.org/2001/XMLSchema" xml:lang="en">

 <xs:annotation>
  <xs:documentation>
   See http://www.w3.org/XML/1998/namespace.html and
   http://www.w3.org/TR/REC-xml for information about this namespace.
  </xs:documentation>
 </xs:annotation>

 <xs:annotation>
  <xs:documentation>This schema defines attributes and an attribute group
        suitable for use by
        schemas wishing to allow xml:base, xml:lang or xml:space attributes
        on elements they define.

        To enable this, such a schema must import this schema
        for the XML namespace, e.g. as follows:
        &lt;schema . . .>
         . . .
         &lt;import namespace="http://www.w3.org/XML/1998/namespace"
                    schemaLocation="http://www.w3.org/2001/03/xml.xsd"/>

        Subsequently, qualified reference to any of the attributes
        or the group defined below will have the desired effect, e.g.

        &lt;type . . .>
         . . .
         &lt;attributeGroup ref="xml:specialAttrs"/>
 
         will define a type which will schema-validate an instance
         element with any of those attributes</xs:documentation>
 </xs:annotation>

 <xs:annotation>
  <xs:documentation>In keeping with the XML Schema WG's standard versioning
   policy, this schema document will persist at
   http://www.w3.org/2001/03/xml.xsd.
   At the date of issue it can also be found at
   http://www.w3.org/2001/xml.xsd.
   The schema document at that URI may however change in the future,
   in order to remain compatible with the latest version of XML Schema
   itself.  In other words, if the XML Schema namespace changes, the version
   of this document at
   http://www.w3.org/2001/xml.xsd will change
   accordingly; the version at
   http://www.w3.org/2001/03/xml.xsd will not change.
  </xs:documentation>
 </xs:annotation>

 <xs:attribute name="lang" type="xs:language">
  <xs:annotation>
   <xs:documentation>In due course, we should install the relevant ISO 2- and 3-letter
         codes as the enumerated possible values . . .</xs:documentation>
  </xs:annotation>
 </xs:attribute>

 <xs:attribute name="space" default="preserve">
  <xs:simpleType>
   <xs:restriction base="xs:NCName">
    <xs:enumeration value="default"/>
    <xs:enumeration value="preserve"/>
   </xs:restriction>
  </xs:simpleType>
 </xs:attribute>

 <xs:attribute name="base" type="xs:anyURI">
  <xs:annotation>
   <xs:documentation>See http://www.w3.org/TR/xmlbase/ for
                     information about this attribute.</xs:documentation>
  </xs:annotation>
 </xs:attribute>

 <xs:attributeGroup name="specialAttrs">
  <xs:attribute ref="xml:base"/>
  <xs:attribute ref="xml:lang"/>
  <xs:attribute ref="xml:space"/>
 </xs:attributeGroup>

</xs:schema>
TIP: Among the many controversial issues surrounding the usage of URIs, the decision to use a schema at its original location (on the W3C web site) versus using a local copy is not easy. Using a schema at its original location provides a guarantee of always accessing the most recent version, but this can be a problem if the schema becomes out of sync with the applications using it. It can also impact the ability to process documents when the hosting site is unreachable and may open a possibility of malicious substitutions of schemas.

10.7.2. XLink Attributes

To illustrate the usage of XLink attributes, we can replace the definition of the author that is currently within the description of a book under the authors element, with a reference to an author described as a "person" in another XML document:

<?xml version="1.0"?> 
<library xmlns="http://dyomedea.com/ns/library"
  xmlns:xlink="http://www.w3.org/1999/xlink">
  <book id="b0836217462">
    <title>
      Being a Dog Is a Full-Time Job
    </title>
    <authors>
      <person xlink:href="authors.xml#CMS"/>
    </authors>
  </book>
</library>

Again, we need to define the namespace for XLink and to import a schema before referencing the XLink attributes:

<?xml version="1.0"?> 
<xs:schema targetNamespace="http://dyomedea.com/ns/library"
  elementFormDefault="qualified" attributeFormDefault="unqualified"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:lib="http://dyomedea.com/ns/library"> 
  <xs:import namespace="http://www.w3.org/1999/xlink"
    schemaLocation="xlink.xsd"/>
  <xs:element name="person">
    <xs:complexType>
      <xs:attribute ref="xlink:href" use="required"/>
      <xs:attribute ref="xlink:type" fixed="simple"/>
      <xs:attribute ref="xlink:show" fixed="embed"/>
      <xs:attribute ref="xlink:actuate" fixed="onLoad"/>
    </xs:complexType>
  </xs:element>
  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="lib:book"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="book">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="title" type="xs:string"/>
        <xs:element ref="lib:authors"/>
      </xs:sequence>
      <xs:attribute name="id" type="xs:ID" use="required"/>
    </xs:complexType>
  </xs:element>
  <xs:element name="authors">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="lib:person"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>
TIP: Following the suggestion of the XLink specification, we have defined additional XLink attributes (type, show, and actuate) as fixed values. While this is legitimate, one should note that these values are only available to the applications that use a W3C XML Schema post-schema validation infoset (PSVI).

Although the XLink specification doesn't provide a schema, creating the parts we need is very straightforward:

<?xml version="1.0"?> 
<xs:schema targetNamespace="http://www.w3.org/1999/xlink"
  elementFormDefault="qualified" attributeFormDefault="qualified"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:xlink="http://www.w3.org/1999/xlink">
  <xs:attribute name="type">
    <xs:simpleType>
      <xs:restriction base="xs:NMTOKEN">
        <xs:enumeration value="simple"/>
        <xs:enumeration value="extended"/>
        <xs:enumeration value="locator"/>
        <xs:enumeration value="arc"/>
        <xs:enumeration value="resource"/>
        <xs:enumeration value="title"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
  <xs:attribute name="href" type="xs:anyURI"/>
  <xs:attribute name="role" type="xs:anyURI"/>
  <xs:attribute name="arcrole" type="xs:anyURI"/>
  <xs:attribute name="title" type="xs:string"/>
  <xs:attribute name="show">
    <xs:simpleType>
      <xs:restriction base="xs:NMTOKEN">
        <xs:enumeration value="new"/>
        <xs:enumeration value="replace"/>
        <xs:enumeration value="embed"/>
        <xs:enumeration value="other"/>
        <xs:enumeration value="none"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
  <xs:attribute name="label" type="xs:NMTOKEN"/>
  <xs:attribute name="actuate">
    <xs:simpleType>
      <xs:restriction base="xs:NMTOKEN">
        <xs:enumeration value="onLoad"/>
        <xs:enumeration value="onRequest"/>
        <xs:enumeration value="other"/>
        <xs:enumeration value="none"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
  <xs:attribute name="from" type="xs:NMTOKEN"/>
  <xs:attribute name="to" type="xs:NMTOKEN"/>
</xs:schema>


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