XML Schema

• Specifies an arbitrarily complex syntax for a set of XML files (validation criteria)
• An XML alternative to DTD
• Fairly complicated; not easy to learn
• XML Schema is an XML language

• XML Schema consists of two parts

• XML Structure Schema (relationships)
• XML Datatype Schema (data formats)

XSD

• XSD (XML Schema Definition) (.xsd)
• An instance of an XML Schema

Namespaces

• XML Namespace (xmlns)
• Namespaces in the XSD control the validation of corresponding namespaces in the instance document (XML data file)

xmlns[:name]="remote-path-for-namespace"

Sample namespaces

• xmlns  : Default namespace (<element>)
• xmlns:xsd  : XML Schema Definition namespace (<xsd:element>)
• xmlns:lib  : An arbitrarily-named namespace for the XSD (<lib:bookType>) (<lib:book>)
• xmlns:xsi  : XML Schema Instance namespace

Sample XML Schemas

XMLSchema

• XML Schema in which the following are defined:

• schema, attribute, element, complexType, sequence, all, choice, ...
• string, boolean, date, ...

targetNamespace

• XML schema in which the following sample elements are defined:

• book, author, title, ...
• bookType, authorType, titleType, ...

XMLSchema-instance

• XML Schema used by the instance document, in which the following are defined:

• SchemaLocation, noNamespaceSchemaLocation, ...

Misc

• Local elements and attributes can be unqualified.
• Global elements and attributes have to be qualified.
  

elementFormDefault

• elementFormDefault="[qualified, unqualified]"
• Elements declared in the current schema must be namespace qualified / unqualified

attributeFormDefault

• attributeFormDefault="[qualified, unqualified]"
• Attributes declared in the current schema must be namespace qualified / unqualified

Sample XML Schemas

Embedded definitions

• targetNamespace doesn't have to be qualified

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"   (qualified XMLSchema: xsd)
            targetNamespace="remote-path-for-namespace"
            xmlns:cat="remote-path-for-namespace">         (qualified targetNamespace: cat)

<xsd:element name="catalog">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="cd" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="title" type="xsd:string"/>
            <xsd:element name="artist" type="xsd:string"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>
</xsd:schema>

Flat design using references

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"    (qualified XMLSchema: xsd)
            targetNamespace="remote-path-for-namespace"
            xmlns:cat="remote-path-for-namespace">          (qualified targetNamespace: cat)

<!-- definition of simple types -->
<xsd:element name="title" type="xsd:string"/>
<xsd:element name="artist" type="xsd:string"/>

<!-- definition of complex types -->
<xsd:element name="cd">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element ref="cat:title"/>
      <xsd:element ref="cat:artist"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>

<!-- define the root element -->
<xsd:element name="catalog">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element ref="cat:cd" minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>
</xsd:schema>

Flat design using named types

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"    (qualified XMLSchema: xsd)
            targetNamespace="remote-path-for-namespace"
            xmlns:cat="remote-path-for-namespace">          (qualified targetNamespace: cat)

<!-- definition of simple types -->
<xsd:simpleType name="titleType">
  <xsd:restriction base="xsd:string">
    <xsd:maxLength value="32"/>
  </xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="artistType">
  <xsd:restriction base="xsd:string">
    <xsd:maxLength value="32"/>
  </xsd:restriction>
</xsd:simpleType>

<!-- definition of complex types -->
<xsd:complexType name="cdType">
  <xsd:sequence>
    <xsd:element name="title" type="cat:titleType"/>
    <xsd:element name="artist" type="cat:artistType"/>
  </xsd:sequence>
</xsd:complexType>
<xsd:complexType name="catalogType">
  <xsd:sequence>
    <xsd:element name="cd" type="cat:cdType" minOccurs="0" maxOccurs="unbounded"/>
  </xsd:sequence>
</xsd:complexType>

<!-- define the root element -->
<xsd:element name="catalog" type="cat:catalogType"/>
</xsd:schema>

Qualified XMLSchema, qualified targetNamespace

• No default namespace
• Cluttered
• cat: can be confusing
• Works on PHP5

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            targetNamespace="remote-path-for-namespace"
            xmlns:cat="remote-path-for-namespace">

<!-- definition of simple types -->
<xsd:element name="title" type="xsd:string"/>
<xsd:element name="artist" type="xsd:string"/>

<!-- definition of complex types -->
<xsd:element name="cd">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element ref="cat:title"/>
      <xsd:element ref="cat:artist"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>

<!-- define the root element -->
<xsd:element name="catalog">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element ref="cat:cd" minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>
</xsd:schema>

Qualified XMLSchema, default targetNamespace

• Easy to understand, but cluttered
• On PHP5, only seems to work for embedded definitions

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"    (qualified XMLSchema: xsd)
            targetNamespace="remote-path-for-namespace"
            xmlns="remote-path-for-namespace"               (default targetNamespace)
            elementFormDefault="qualified">

<xsd:element name="catalog">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="cd" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="title" type="xsd:string"/>
            <xsd:element name="artist" type="xsd:string"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>
</xsd:schema>

Sample usage in XML files

• Instance documents
• After the XML declaration and before the root element

<?xml version="1.0" encoding="UTF-8"?>
<catalog xmlns="remote-path-for-namespace">           <!-- default namespace >
  <cd>
    <title>...</title>
    <artist>...</artist>
  </cd>
  ...
</catalog>

<?xml version="1.0" encoding="UTF-8"?>
<cat:catalog xmlns:cat="remote-path-for-namespace">      <!-- cat namespace >
  <cd>
    <title>...</title>
    <artist>...</artist>
  </cd>
  ...
</cat:catalog>

<book isbn="..."
      xmlns="remote-path-for-namespace"                        (default namespace)
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"    (xsi namespace)
      xsi:SchemaLocation="remote-path-for-namespace            (namespace)
                          local-path/filename.xsd">            (schema)
...
</book>

<book isbn="..."
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"       (xsi namespace)
      xsi:noNamespaceSchemaLocation="local-path/filename.xsd">    (schema)
...
</book>