PB138 — XML Schema

(C) 2019 Masaryk University -- Tomáš Pitner, Luděk Bártek, Adam Rambousek

Basic sources of information

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

<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>

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>

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:

 <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>

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.

 <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"/>

  • 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.

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">