Imposing Structure on XML Documents Using a Schema

Imposing Structure on XML Documents Using a Schema
An Introduction to Schemas

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

An Observation:
- DTDs have a very different syntax from XML
Problems that Arise as a Result:
- Creators of XML documents and DTDs must learn two syntaxes
- Two different parsers are required to process XML documents

Resolving these Problems

The Idea:
- Use XML to specify structure
The Result:
- Use schemas to specify structure
The History:
- Microsoft and the University of Edinburgh wer one of the first to use this approach (called XDR)
- In 2001, the W3C published a different recommendation (which is now called both XSD and WXS); version 1.1 became a recommendation in 2012

Schemas

Associating a Schema with an XML Document:
- Include attributes in the root element of the XML document
Syntax:
- < root_tag xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type=" schema ">
- where type can be either schemaLocation or noNamespaceSchemaLocation depending on whether or not the schema has a target namespace
- schema denotes the name of the schema

The Schema Element

Role:
- The same role as the DOCTYPE in a DTD
Attributes:
- The xmlns - specifies the default namespace for the elements and attributes
- The targetNamespace - An optional namespace for the schema
Contains:
- simpleType or complexType
- element
- attribute

The Schema Element (cont.)

An Example

xmlexamples/timetable/schema/timetable.xsd

(Fragment: schema)

<xs:schema 
    xmlns:xs="http://www.w3.org/2001/XMLSchema" 
    elementFormDefault="qualified">

Different Approaches to Specifying Schemas

Local Types:
- Define types, as needed, within elements and attributes
General Types:
- Define types outside of the elements and attributes (the approach that we will use for consistency and simplicity)

The attribute Element

Role:
- An attribute in an XML document
Attributes:
- name
- type
- fixed or default - an optional constant or default value
- use - either optional, prohibited, or required
Contains:
- simpleType

The element Element

Role:
- An element in a document
Attributes:
- name
- type
- minOccurs - optional minimum number of occurrences
- maxOccurs - optional maximum number of occurrences
Contains:
- Nothing (since we are not using local types)

The simpleType Element

Role:
- Similar to ELEMENT in a DTD (i.e., defines a type)
Attributes:
- name
Contains:
- restriction or list or union

The simpleType Element (cont.)

An Example

xmlexamples/timetable/schema/timetable.xsd

(Fragment: simpleType)

    <xs:simpleType name="timeoremptytype">
        <xs:union memberTypes="xs:time emptystring"/>
    </xs:simpleType>

The complexType Element

Role:
- Similar to ELEMENT in a DTD (i.e., defines a type)
Attributes:
- name
Contains:
- simpleContent or complexContent
- group or all or choice or sequence
- attribute

The complexType Element (cont.)

An Example

xmlexamples/timetable/schema/timetable.xsd

(Fragment: complexType)

    <xs:complexType name="timetabletype">
        <xs:sequence>
            <xs:element name="train" type="traintype" 
                        minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
        
        <xs:attribute name="title" type="xs:string"/>
        <xs:attribute name="subtitle" type="xs:string"/>
    </xs:complexType>

The restriction Element

Role:
- Restrictions on the definition of an element
Contains:
- Diffrent elements depending on the thing being defined

The restriction Element (cont.)

An Example

xmlexamples/timetable/schema/timetable.xsd

(Fragment: restriction)

    <xs:simpleType name="normaldaysenum">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Daily"/>
            <xs:enumeration value="Mo-Fr"/>
            <xs:enumeration value="Sa"/>
            <xs:enumeration value="Su"/>
        </xs:restriction>
    </xs:simpleType>

The extension Element

Role:
- Extend an existing simple or complex type
Attributes:
- base
Contains:
- group or all or choice or sequence
- attribute

The extension Element (cont.)

An Example

xmlexamples/transcript/schema/transcript.xsd

(Fragment: extension)

    <xs:complexType name="gradetype">
        <xs:attribute name="course" type="xs:string" 
            use="required"/>
        <xs:attribute name="passfail"/>
    </xs:complexType>
    
    
    <xs:complexType name="gradetypepf">
        <xs:complexContent>
            <xs:extension base="gradetype">
                <xs:attribute name="letter" type="gradeenumpf" 
                    use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    
    
    <xs:complexType name="gradetypeabcdf">
        <xs:complexContent>
            <xs:extension base="gradetype">
                <xs:attribute name="letter" type="gradeenumabcdf" 
                    use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>

The simpleContent Element

Role:
- Extensions or restrictions on text-only complext types or simple types
Contains:
- restriction or extension

The simpleContent Element (cont.)

An Example

xmlexamples/timetable/schema/timetable.xsd

(Fragment: simpleContent)

    <xs:complexType name="stoptimetype">
        <xs:simpleContent>
            <xs:extension base="timeoremptytype">
                <xs:attribute name="status" type="statusenum"/>
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

The complexContent Element

Role:
- Extensions or restrictions on complext types that contain elements
Contains:
- restriction or extension

The alternative Element

Role:
- Provides for type alternatives
Attributes:
- test - the test to perform
- type - the type to use when the test passes

The alternative Element (cont.)

An Example

xmlexamples/transcript/schema/transcript.xsd

(Fragment: alternative)

        <xs:sequence>
            <xs:element name="grade" type="gradetype"  
                        minOccurs="0" maxOccurs="unbounded">
                <xs:alternative test="@passfail eq 'true'" 
                                type="gradetypepf" />
                <xs:alternative                            
                                type="gradetypeabcdf" />
            </xs:element>
        </xs:sequence>

The assert Element

Role:
- Constrain the existence and values of related elements and attributes
Attributes:
- test - the test to perform

The aseert Element (cont.)

An Example

xmlexamples/transcript/schema/transcript.xsd

(Fragment: assert)

        <!-- Ensure that at most 1 course is Pass/Fail in any term -->
        <xs:assert test="count(grade[@passfail]) le 1"/>

The Complete Timetable Example

xmlexamples/timetable/schema/timetable.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
    xmlns:xs="http://www.w3.org/2001/XMLSchema" 
    elementFormDefault="qualified">
    
    <!-- Types -->    
    <xs:simpleType name="emptystring">
        <xs:restriction base="xs:string">
            <xs:enumeration value=""/>
        </xs:restriction>
    </xs:simpleType>


    <xs:simpleType name="normaldaysenum">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Daily"/>
            <xs:enumeration value="Mo-Fr"/>
            <xs:enumeration value="Sa"/>
            <xs:enumeration value="Su"/>
        </xs:restriction>
    </xs:simpleType>
    
    
    <xs:simpleType name="statusenum">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Ar"/>
            <xs:enumeration value="Dp"/>
        </xs:restriction>
    </xs:simpleType>


    <xs:complexType name="stoptimetype">
        <xs:simpleContent>
            <xs:extension base="timeoremptytype">
                <xs:attribute name="status" type="statusenum"/>
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>


    <xs:complexType name="stoptype">
        <xs:sequence>
            <xs:element name="station" type="xs:string"/>
            <xs:element name="time" type="stoptimetype" 
                        minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
    
    
    <xs:simpleType name="timeoremptytype">
        <xs:union memberTypes="xs:time emptystring"/>
    </xs:simpleType>
    
    
    <xs:complexType name="timetabletype">
        <xs:sequence>
            <xs:element name="train" type="traintype" 
                        minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
        
        <xs:attribute name="title" type="xs:string"/>
        <xs:attribute name="subtitle" type="xs:string"/>
    </xs:complexType>
    
    
    <xs:complexType name="traintype">
        <xs:sequence> 
            <xs:element name="stop" type="stoptype" 
                        minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
        
        <xs:attribute name="businessclass" type="yesnoenum"/>
        <xs:attribute name="normaldays" type="normaldaysenum"/>
        <xs:attribute name="number" type="xs:positiveInteger"/>
        <xs:attribute name="reservations" type="yesnoenum"/>
    </xs:complexType>
    
    
    <xs:simpleType name="yesnoenum">
        <xs:restriction base="xs:string">
            <xs:enumeration value="YES"/>
            <xs:enumeration value="NO"/>
        </xs:restriction>
    </xs:simpleType>

    <!-- Elements -->

    <xs:element name="timetable" type="timetabletype" />
    
</xs:schema>

The Complete Transcript Example

xmlexamples/transcript/schema/transcript.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
    elementFormDefault="qualified"
    vc:minVersion="1.1"> 

    <!-- Types -->
    <xs:complexType name="transcripttype">
        <xs:sequence>
            <xs:element name="term" type="termtype" 
                        minOccurs="1" maxOccurs="unbounded" />
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="termtype">
        <xs:sequence>
            <xs:element name="grade" type="gradetype"  
                        minOccurs="0" maxOccurs="unbounded">
                <xs:alternative test="@passfail eq 'true'" 
                                type="gradetypepf" />
                <xs:alternative                            
                                type="gradetypeabcdf" />
            </xs:element>
        </xs:sequence>
        
        <xs:attribute name="year" type="xs:integer" />
        <xs:attribute name="semester" type="semesterenum" />
        
        <!-- Ensure that at most 1 course is Pass/Fail in any term -->
        <xs:assert test="count(grade[@passfail]) le 1"/>
    </xs:complexType>

    <xs:complexType name="gradetype">
        <xs:attribute name="course" type="xs:string" 
            use="required"/>
        <xs:attribute name="passfail"/>
    </xs:complexType>
    
    
    <xs:complexType name="gradetypepf">
        <xs:complexContent>
            <xs:extension base="gradetype">
                <xs:attribute name="letter" type="gradeenumpf" 
                    use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    
    
    <xs:complexType name="gradetypeabcdf">
        <xs:complexContent>
            <xs:extension base="gradetype">
                <xs:attribute name="letter" type="gradeenumabcdf" 
                    use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    

    <xs:simpleType name="gradeenum">
        <xs:restriction base="xs:string" />
    </xs:simpleType>
    
    
    <xs:simpleType name="gradeenumpf">
        <xs:restriction base="gradeenum">
            <xs:enumeration value="P" />
            <xs:enumeration value="F" />
        </xs:restriction>
    </xs:simpleType>
    
    
    <xs:simpleType name="gradeenumabcdf">
        <xs:restriction base="gradeenum">
            <xs:enumeration value="A" />
            <xs:enumeration value="B" />
            <xs:enumeration value="C" />
            <xs:enumeration value="D" />
            <xs:enumeration value="F" />
        </xs:restriction>
    </xs:simpleType>


    <xs:simpleType name="semesterenum">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Fall" />
            <xs:enumeration value="Spring" />
        </xs:restriction>
    </xs:simpleType>
    
    
    <!-- Elements -->
    <xs:element name="transcript" type="transcripttype" />
</xs:schema>