Test Drive - XML

Imagine a technology that offered you all the power of a DTD without the associated unpleasantness of those strange symbols and arcane commands. Sounds too good to be true? Say hello to XML Schema.

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By: Harish Kamath, (c) Melonfire

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October 15, 2001

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Now that you've got the basics down, let's look at a couple of composite examples just to put everything in perspective. Consider the following two document instances, and then see if you can write appropriate schema definitions for each:

<?xml version="1.0"?>
<!-- id is a required attribute -->
<recipe id="3450">
<name>Chicken 
Tikka</name>
<author>Mr. Cluck</author>
<date>1999-06-08</date>
<ingredients>
<!-- 
quantity is a required attribute, units is optional -->
<item quantity="2">Boneless 
chicken breasts</item>
<item quantity="2">Chopped onions</item>
<item 
quantity="1" units="tsp">Ginger</item>
<item quantity="1" units="tsp">Garlic</item>
<item 
quantity="1" units="tsp">Red chili powder</item>
<item quantity="1" units="tsp">Coriander 
seeds</item>
<item quantity="2" units="tbsp">Lime juice</item>
<item 
quantity="2" units="tbsp">Butter</item>
</ingredients>
<process>
<step>Cut 
chicken into cubes, wash and apply lime juice and salt</step>
<step>Add 
ginger, garlic, chili, coriander and lime juice in a separate
bowl</step>
<step>Mix 
well, and add chicken to marinate for 3-4 hours</step>
<step>Place chicken 
pieces on skewers and barbeque</step>
<step>Remove, apply butter, and 
barbeque again until meat is tender</step>
<step>Garnish with lemon and 
chopped onions</step>
</process>
</recipe>

Here's the second one:

<?xml version="1.0"?>
<weather>
<!-- id is a required attribute -->
<city 
id="52320">
<name>Boston</name>
<temperature>
<!-- units is 
a required attribute, restricted to values "celsius" and
"fahrenheit" -->
<high 
units="celsius">23</high>
<low units="celsius">5</low>
</temperature>
<!-- 
forecast may be any one of "rain", "sun", "snow" or "fog" -->
<forecast>snow</forecast>
</city>
<city 
id="9010">
<name>New York</name>
<temperature>
<high units="celsius">11</high>
<low 
units="celsius">-5</low>
</temperature>
<forecast>snow</forecast>
</city>
<city 
id="8239">
<name>London</name>
<temperature>
<high units="celsius">27</high>
<low 
units="celsius">12</low>
</temperature>
<forecast>sun</forecast>
</city>
</weather>

Here's a schema definition for the first one:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:annotation>
<xsd:documentation>This 
version uses references and anonymous
elements</xsd:documentation>
</xsd:annotation>
<xsd:element 
name="step" type="xsd:string" />
<xsd:element name="name" type="xsd:string" 
/>
<xsd:element name="author" type="xsd:string" />
<xsd:element name="date" 
type="xsd:date" />
<xsd:attribute name="id" type="xsd:integer" />
<xsd:attribute 
name="units" type="xsd:string" />
<xsd:attribute name="quantity" type="xsd:integer" 
/>
<xsd:element name="process">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
ref="step" maxOccurs="unbounded" />
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="item">
<xsd:complexType>
<xsd:simpleContent>
<xsd:restriction 
base="xsd:string">
<xsd:attribute ref="quantity" use="required" />
<xsd:attribute 
ref="units" use="optional" />
</xsd:restriction>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="ingredients">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
ref="item" maxOccurs="unbounded" />
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="recipe">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
ref="name" />
<xsd:element ref="author" />
<xsd:element ref="date" 
/>
<xsd:element ref="ingredients" />
<xsd:element ref="process" 
/>
</xsd:sequence>
<xsd:attribute ref="id" use="required" />
</xsd:complexType>
</xsd:element>
</xsd:schema>

This version first defines various elements and then references those definitions to construct a schema. In case this doesn't work for you, you can derive and use named datatypes instead of references - here's an alternative version of the schema above:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:annotation>
<xsd:documentation>This 
version uses named datatypes</xsd:documentation>
</xsd:annotation>
<xsd:complexType 
name="itemDType">
<xsd:simpleContent>
<xsd:restriction base="xsd:string">
<xsd:attribute 
name="quantity" type="xsd:integer" use="required" />
<xsd:attribute name="units" 
type="xsd:string" use="optional" />
</xsd:restriction>
</xsd:simpleContent>
</xsd:complexType>
<xsd:complexType 
name="ingredientsDType">
<xsd:sequence>
<xsd:element name="item" type="itemDType" 
maxOccurs="unbounded" />
</xsd:sequence>
</xsd:complexType>
<xsd:complexType 
name="processDType">
<xsd:sequence>
<xsd:element name="step" type="xsd:string" 
maxOccurs="unbounded" />
</xsd:sequence>
</xsd:complexType>
<xsd:complexType 
name="recipeDType">
<xsd:sequence>
<xsd:element name="name" type="xsd:string" 
/>
<xsd:element name="author" type="xsd:string" />
<xsd:element name="date" 
type="xsd:date" />
<xsd:element name="ingredients" type="ingredientsDType" 
/>
<xsd:element name="process" type="processDType" />
</xsd:sequence>
<xsd:attribute 
name="id" type="xsd:integer" use="required" />
</xsd:complexType>
<!-- 
all type definitions done, now simply create an element of type
"recipe" -->
<xsd:element 
name="recipe" type="recipeDType" />
</xsd:schema>

In a similar manner, you can create a schema for the weather forecast example, using either references

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element 
name="name" type="xsd:string"/>
<xsd:attribute name="id" type="xsd:integer"/>
<xsd:attribute 
name="units">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration 
value="fahrenheit"/>
<xsd:enumeration value="celsius"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
<xsd:element 
name="high">
<xsd:complexType>
<xsd:simpleContent>
<xsd:restriction 
base="xsd:integer">
<xsd:maxInclusive value="150"/>
<xsd:minInclusive 
value="-150"/>
<xsd:attribute ref="units" use="required"/>
</xsd:restriction>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="low">
<xsd:complexType>
<xsd:simpleContent>
<xsd:restriction 
base="xsd:integer">
<xsd:maxInclusive value="150"/>
<xsd:minInclusive 
value="-150"/>
<xsd:attribute ref="units" use="required"/>
</xsd:restriction>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="forecast">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration 
value="fog"/>
<xsd:enumeration value="rain"/>
<xsd:enumeration value="sun"/>
<xsd:enumeration 
value="snow"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element 
name="temperature">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
ref="high"/>
<xsd:element ref="low"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="city">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="name"/>
<xsd:element 
ref="temperature"/>
<xsd:element ref="forecast"/>
</xsd:sequence>
<xsd:attribute 
ref="id" use="required"/>
</xsd:complexType>
</xsd:element>
<xsd:element 
name="weather">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
ref="city" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

or derived types.

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:simpleType 
name="nameDType">
<xsd:restriction base="xsd:string"/>
</xsd:simpleType>
<xsd:simpleType 
name="forecastDType">
<xsd:restriction base="xsd:string">
<xsd:enumeration 
value="fog"/>
<xsd:enumeration value="rain"/>
<xsd:enumeration value="sun"/>
<xsd:enumeration 
value="snow"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType 
name="unitsDType">
<xsd:restriction base="xsd:string">
<xsd:enumeration 
value="celsius"/>
<xsd:enumeration value="fahrenheit"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType 
name="temperatureDType">
<xsd:simpleContent>
<xsd:restriction base="xsd:integer">
<xsd:maxInclusive 
value="150"/>
<xsd:minInclusive value="-150"/>
<xsd:attribute name="units" 
type="unitsDType" use="required"/>
</xsd:restriction>
</xsd:simpleContent>
</xsd:complexType>
<xsd:complexType 
name="temperatureWrapperDType">
<xsd:sequence>
<xsd:element name="high" 
type="temperatureDType"/>
<xsd:element name="low" type="temperatureDType"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType 
name="cityDType">
<xsd:sequence>
<xsd:element name="name" type="nameDType"/>
<xsd:element 
name="temperature" type="temperatureWrapperDType"/>
<xsd:element name="forecast" 
type="forecastDType" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute 
name="id" type="xsd:integer" use="required"/>
</xsd:complexType>
<xsd:complexType 
name="weatherDType">
<xsd:sequence>
<xsd:element name="city" type="cityDType" 
maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<!-- 
now simply declare an element of type "weather" -->
<xsd:element name="weather" 
type="weatherDType"/>
</xsd:schema>

Of these two approaches, I've always found the derived types approach to be a bit more flexible, not to mention more clearly-structured and logical - although it does take a bit of getting used to.

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