Local and Global Definitions

Outline (Local and Global Definitions)

Local and Global Definitions [6]
1. Elements [2]
2. Attributes [3]
Names and Namespaces [3]
Identity Constraints [6]
Conclusions [1]

Local and Global Definitions E. Wilde: XSD – Part III

(4) Named and Anonymous Types

Types can be named or anonymous
- named types can be reused (for elements, attributes, or type derivation)
- anonymous types can only be used where they are defined
DTD types are always anonymous (they cannot be reused)

<!ELEMENT person (name, address) >
<!ATTLIST person id ID #REQUIRED >

DTDs have everything hardcoded
- complex types are always locally defined
- elements are always globally defined
- attributes are always locally defined

Elements

Outline (Elements)

Local and Global Definitions [6]
1. Elements [2]
2. Attributes [3]
Names and Namespaces [3]
Identity Constraints [6]
Conclusions [1]

Elements E. Wilde: XSD – Part III

(6) Local vs. Global Elements

Elements can be defined in a type or in the schema
- local elements can only be used where they are defined
- global elements can be reused, they can serve as building blocks
Elements and complex types depend on each other
- an element is defined by a type, often this will be a complex type
- a complex type is defined by its contents, which are elements and/or attributes

 <xs:complexType name="mixedType" mixed="true">
  <xs:choice maxOccurs="unbounded" minOccurs="0">
   <xs:element ref="b"/>
   <xs:element name="i" type="xs:string"/>
   <xs:element name="u" type="xs:string"/>
  </xs:choice>
  <xs:attribute ref="class"/>
 </xs:complexType>
 <xs:element name="b" type="xs:string"/>

global-local.xsd (line 4-12)

Elements E. Wilde: XSD – Part III

(7) Reusable Elements

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="billingAddress" type="addressType"/>
 <xs:element name="shippingAddress" type="addressType"/>
 <xs:complexType name="addressType">
  <xs:sequence>
   <xs:element name="name" type="xs:string"/>
   <xs:element name="street" type="xs:string"/>
   <xs:element name="city" type="xs:string"/>
   <xs:element name="state" type="xs:string" minOccurs="0"/>
   <xs:element name="zip" type="xs:decimal"/>
  </xs:sequence>
  <xs:attribute name="country" type="xs:NMTOKEN"/>
 </xs:complexType>
</xs:schema>

complex-example.xsd

Attributes

Outline (Attributes)

Local and Global Definitions [6]
1. Elements [2]
2. Attributes [3]
Names and Namespaces [3]
Identity Constraints [6]
Conclusions [1]

Attributes E. Wilde: XSD – Part III

(9) Attribute Definitions

DTDs treat attributes as something entirely different from element content
- they are defined in an ATTLIST, not in the ELEMENT definition
```
<!ELEMENT person (name, address) >
<!ATTLIST person id ID #REQUIRED >
```
- they have a special range of Attribute Types [Document Type Definition (DTD); Attribute Types (1)] as opposed to elements
```
<!ATTLIST person id ID #REQUIRED >
```
XSD overcomes these restrictions only partially
- Simple Types [XSD – Part I; Simple Types (1)] are used to define attribute (or element) contents
- attributes are still described as something entirely different from an element's content model
Attributes could be better integrated into the model
- RELAX NG [Alternative Schema Languages – Schematron; RELAX NG (1)] treats attributes as part of an element's content model
- this makes it trivial to have choices of element content and attributes

Attributes E. Wilde: XSD – Part III

(10) Reusing Attributes

DTDs treat attributes as something local to an element
- attributes are defined in an element's ATTLIST
- reusing attributes for more than on element requires Parameter Entities [Document Type Definition (DTD); Parameter Entities (1)]
XSD better supports reuse of schema components
- types can be defined locally (anonymous) or globally (named)
- elements and attributes can be defined globally or locally
Globally defined attributes can be reused
- the attribute definition does not tie it to any occurrence
- the attribute can then be referenced from an complex type definition

Attributes E. Wilde: XSD – Part III

(11) Reusing Attributes (Example)

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="p" type="mixedType"/>
 <xs:complexType name="mixedType" mixed="true">
  <xs:choice maxOccurs="unbounded" minOccurs="0">
   <xs:element ref="b"/>
   <xs:element name="i" type="xs:string"/>
   <xs:element name="u" type="xs:string"/>
  </xs:choice>
  <xs:attribute ref="class"/>
 </xs:complexType>
 <xs:element name="b" type="xs:string"/>
 <xs:attribute name="class">
  <xs:simpleType>
   <xs:restriction base="xs:string">
    <xs:enumeration value="comment"/>
    <xs:enumeration value="warning"/>
   </xs:restriction>
  </xs:simpleType>
 </xs:attribute>
</xs:schema>

global-local.xsd

Names and Namespaces

Outline (Names and Namespaces)

Local and Global Definitions [6]
1. Elements [2]
2. Attributes [3]
Names and Namespaces [3]
Identity Constraints [6]
Conclusions [1]

Names and Namespaces E. Wilde: XSD – Part III

(13) Definitions

Many XSDs define a vocabulary for a namespace
- DTDs do not have any support for namespaces
- XSD heavily builds on XML Namespaces [XML Namespaces]

XSD provides support for declaring a vocabulary's namespace

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.example.com/">

Schema-validation can check for proper namespace usage
- the targetNamespace has to be used in the instance
- if the namespace does not match, validation cannot succeed

Names and Namespaces E. Wilde: XSD – Part III

(14) Instances

The schema defines the targetNamespace of the vocabulary
- all globally defined elements, attributes, and types are in that namespace
- the instances must declare and use the namespace to be schema-valid

<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
  <title>Multicolumn Layout in HTML</title>
  <style type="text/css">

multicol.html (line 2-5)

A prefixed name is not the same as a qualified name
- if there is a default namespace, unprefixed elements are still qualified
Nasty details about XML Namespaces and attributes
- the default namespace does not apply to attributes
- attributes must therefore always be prefixed if they need to be qualified

Names and Namespaces E. Wilde: XSD – Part III

(15) Name Qualification

Global elements and attributes have to be used as qualified names
- this means that they must be referred to by their namespace-qualified name
- if a default namespace is used, elements are qualified without carrying a prefix
- since the default namespace does not apply to attributes, they always must be explicitly prefixed
Local elements and attributes may be used qualified or unqualified
- this control only applies to locally define elements or attributes
- the default defined by XSD is not a good choice
- because of how XML Namespaces work, a non-default choice is recommended
XSD allows control over how local names have to be used

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.example.com/" elementFormDefault="qualified" attributeFormDefault="unqualified">

Identity Constraints

Outline (Identity Constraints)

Local and Global Definitions [6]
1. Elements [2]
2. Attributes [3]
Names and Namespaces [3]
Identity Constraints [6]
Conclusions [1]