PB138 — XML Schema
Basic sources of information
• XML Schema Specification
• Using W3C XML Schema Tutorial brief
• XML Schema Tutorial at W3Schools
• More comprehensive complete tutorial at xfront.com
• How to add XML Schema support to Netbeans IDE available at Geertjan’s Blog
• Try this XML Schema online validator or
• Similar but more general Validome validator (not only XML schema) alternativelly available in
web archive.
Motivation
Stronger tool for XML data model specification than DTD, it allows:
• Separate type (e.g. element type) from its occurrence (i.e. element with particular name)
• More primitive data types.
• Allows to use namespaces.
• Allows to specify content model (elements) more accurate way.
• Allows new type inheritance.
• Allows modular schema design and schema reuse.
• XML Schema has an XML syntax.
XML Schema Definition Header
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  .../...
</xs:schema>
Assignment of type to element with the
given name
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<xs:element name="element_name">
 <!-- here comes the type definition --
 placed either right here (so called "local")
 or as a referenced one (so called "global") -->
</xs:element>
short type reference:
<xs:element name="element_name" type="element_type"/>
Simple Type Definition
• Does not contain any child elements. Can be used like either element or attribute type.
• Possible to define using an existing type restriction
<xs:simpleType name="TypeName">
  <xs:restriction base="BaseTypeName"> ... </xs:restriction>
</xs:simpleType>
Simple type definition (Example 1)
Content length restriction
<xs:simpleType name="nameType">
  <xs:restriction base="xs:string"> <xs:maxLength value="32"/>
</xs:restriction> </xs:simpleType>
Simple type definition (Example 2)
Content restriction using a regular expression
<xs:simpleType name="isbnType">
  <xs:restriction base="xs:string"> <xs:pattern value="[0-9]{10}"/> </xs:restriction>
</xs:simpleType>
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Simple types — union
• Approximately correspond to C "union" concept.
• Result is a simple type.
• Base type and values enumeration can be merged.
Simple types — union
 <xs:simpleType name="isbnType">
  <xs:union>
  <xs:simpleType>
  <xs:restriction base="xs:string">
  <xs:pattern value="[0-9]{10}"/>
  </xs:restriction>
  </xs:simpleType>
  <xs:simpleType>
  <xs:restriction base="xs:NMTOKEN">
  <xs:enumeration value="TBD"/>
  <xs:enumeration value="NA"/>
  </xs:restriction>
  </xs:simpleType>
  </xs:union>
 </xs:simpleType>
Simple types - values enumeration
• Type can be defined as a values list separated by white-spaces.
• The number of elements list limitation can used as a next derivation type.
Simple types - values enumeration
<xs:simpleType name="isbnTypes">
  <xs:list itemType="isbnType"/>
</xs:simpleType>
<xs:simpleType name="isbnTypes10">
  <xs:restriction base="isbnTypes">
  <xs:minLength value="1"/>
  <xs:maxLength value="10"/>
  </xs:restriction>
</xs:simpleType>
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Complex type definition
<xs:complexType name="TypeName">
  <xs:sequence>
  <xs:element ...> ...
  <xs:attribute ...>
  </xs:element>
  </xs:sequence>
</xs:complexType>
• <xs:choice> and <xs:all> can be used instead of sequence.
Complex type definition — element groups
• The group element can be used to define complex type.
• Group of elements:
<xs:group name="GroupName">
  <xs:sequence>
  <xs:element ... /> ...
  </xs:sequence>
</xs:group>
• <xs:choice> and <xs:all> can be used instead of sequence.
Complex type definition — attribute groups
• Attribute group:
 <xs:attributeGroup name="AttributesGroupName">
  <xs:attribute ... use="required"/>
  ...
 </xs:attributeGroup>
• The mandatory occurrence may be specified (use="required").
Groups usage
• Example of elements/attributes groups use:
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 <xs:complexType name="bookType">
  <xs:sequence>
  <xs:group ref="mainBookElements"/>
  <xs:element name="character" type="characterType" minOccurs="0" maxOccurs=
"unbounded"/>
  </xs:sequence>
  <xs:attributeGroup ref="bookAttributes"/>
 </xs:complexType>
Compositor sequence
• Defines occurrence of elements in the predefined order.
 <xs:element name="element_name">
  <xs:complexType>
  <xs:sequence>
  .../...
  </xs:sequence>
  .../...
  </xs:complexType>
 </xs:element>
Compositor sequence
• sequence is a content model that allows occurrence of the defined sequence of child elements.
• xs prefix is (as usually) bound to the NS with URL http://www.w3.org/2001/XMLSchema
• Either <xs:choice> or <xs:all> can be used instead of <xs:sequence>.
Compositor choice
• Defines the occurrence of only one of the specified child elements or groups of elements.
 <xs:element name="element_name">
  <xs:complexType>
  <xs:choice>
  .../...
  </xs:choice>
  .../...
  </xs:complexType>
 </xs:element>
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Compositor all
• Defines occurrence of child elements without definition of their order.
• May appear on the definition top level only.
• The cardinality of child elements can be one at most.
Compositor all
 <xs:complexType name="bookType">
  <xs:all>
  <xs:element name="title" type="xs:string"/>
  <xs:element name="author" type="xs:string"/>
  <xs:element name="character"type="characterType" minOccurs="0" maxOccurs="1"/>
  </xs:all>
  <xs:attribute name="isbn" type="isbnType" use="required"/>
 </xs:complexType>
Element simple content
 <xs:element name="book">
  <xs:complexType>
  <xs:simpleContent>
  <xs:extension base="xs:string">
  <xs:attribute name="isbn" type="isbnType"/>
  </xs:extension>
  </xs:simpleContent>
  </xs:complexType>
 </xs:element>
Mixed element content
• The text content (textual child nodes) can not be validated.
• The child elements can be validated.
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 <xs:element name="book">
  <xs:complexType mixed="true">
  <xs:all>
  <xs:element name="title" type="xs:string"/>
  <xs:element name="author" type="xs:string"/>
  </xs:all>
  <xs:attribute name="isbn" type="xs:string"/>
  </xs:complexType>
 </xs:element>
Further options
• Possibility to specify integrity limitations:
◦ value is unique — xs:unique
◦ value is a key — xs:key
◦ value is a key reference — xs:keyref
Schema annotation
• Annotation is a human-readable note (comment) of a schema or its part.
• It may contain the processing information, see example xs:appinfo as well.
• Next content is not specified (limited), see example (bind, class, …)
Schema annotation
 <xs:annotation>
  <xs:documentation xml:lang="en">Top level element.</xs:documentation>
  <xs:documentation xml:lang="fr">Element racine.</xs:documentation>
  <xs:appinfo source="http://example.com/foo/">
  <bind xmlns="http://example.com/bar/">
  <class name="Book"/>
  </bind>
  </xs:appinfo>
 </xs:annotation>
Schema definition reuse
• Direct:
 <xs:include schemaLocation="character.xsd"/>
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• With redefinition:
<xs:redefine schemaLocation="character12.xsd">
  <xs:simpleType name="nameType">
  <xs:restriction base="xs:string">
  <xs:maxLength value="40"/>
  </xs:restriction>
  </xs:simpleType>
</xs:redefine>
Abstract and final types
abstract
Type can not be instantiated.
• Can be used for inheritance derivation only.
final
Type can not be extended/derived by inheritance.
Namespaces in XML Schema
<xs:schema targetNamespace="http://example.org/ns/books/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:bk="http://example.org/ns/books/" elementFormDefault="qualified"
  attributeFormDefault="unqualified">
  .../...
</xs:schema>
Unspecified elements and attributes
• XML Schema allows to use some elements that are not known prior to its use.
<xs:complexType name="descType" mixed="true">
  <xs:sequence>
  <xs:any namespace="http://www.w3.org/1999/xhtml"
  processContents="skip" minOccurs="0" maxOccurs="unbounded"/>
  </xs:sequence>
</xs:complexType>
Use xs:anyAttribute for attributes.
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Schema definition reference
<book isbn="0836217462"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="file:library.xsd">
<book isbn="0836217462" xmlns="http://example.org/ns/books/"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="file:library.xsd">
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