Schemas

Elliotte Rusty Harold

XMLOne London

Tuesday, March 20, 2001

elharo@metalab.unc.edu

http://www.ibiblio.org/xml/

What are Schemas?

Generically, a document that describes what a correct document may contain
Specifically, a W3C Recommendation for an XML-document syntax that describes the permissible contents of XML documents

About Schemas

Created by W3C XML Schema Working Group based on many different submissions
No known patent, trademark, or other IP restrictions
XML Schema Part 1: Structures: http://www.w3.org/TR/xmlschema-1/
XML Schema Part 2: Datatypes: http://www.w3.org/TR/xmlschema-2/

What's Wrong with DTDs?

Unusual, non-XML like syntax
No data typing, especially for element content
Limited extensibility
Only marginally compatible with namespaces
Cannot use mixed content and enforce order and number of child elements
Cannot enforce number of child elements without also enforcing order. (i.e. no & operator from SGML)

DTDs vs. Schemas

DTDs	Schemas
`<!ELEMENT>` declaration	`xsd:element` element
`<!ATTLIST>` declaration	`xsd:attribute` element
`<!NOTATION>` declaration
`<!ENTITY>` declaration
	`Data types`

Schema versions

Last call working draft from April 7, 2000
Candidate Recommendation October 24, 2000
Proposed Recommendation March 16, 2001

greeting.xml

<?xml version="1.0"?>
<GREETING>
Hello XML!
</GREETING>

greeting.xsd

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="GREETING" type="xsd:string"/>

</xsd:schema>

Attaching the schema to the document without namespaces

xsi:noNamespaceSchemaLocation attribute on root element
xsi prefix is mapped to http://www.w3.org/2001/XMLSchema-instance URI

For example,

<?xml version="1.0"?>
<GREETING xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="greeting.xsd">
Hello XML!
</GREETING>

Other means of connecting schemas to documents are allowed

Validating the document with Xerces-J 1.3.0

D:\schemas\examples>java sax.SAX2Count -v greeting2.xml
greeting2.xml: 701 ms (1 elems, 1 attrs, 0 spaces, 12 chars)

An Invalid Document

<?xml version="1.0"?>
<GREETING xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="greeting.xsd">
  <P>Hello XML!</P>
</GREETING>

Checking the Invalid Document

D:\speaking\XMLOne London\schemas\examples>java sax.SAX2Count -v greeting3.xml
[Error] greeting3.xml:4:6: Element type "P" must be declared.
[Error] greeting3.xml:5:13: Datatype error: In element 'GREETING' : Can not have
 element children within a simple type content.
greeting3.xml: 781 ms (2 elems, 1 attrs, 0 spaces, 14 chars)

A More Complex Document

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="simple_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Complex vs. Simple Types

Complex types can have child elements and attributes
Simple types cannot have children or attributes

Three main schema elements:

xsd:element declares an element and assigns it a type
xsd:attribute declares an attribute and assigns it a type
xsd:complexType defines a new type

A More Complex Schema

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0" maxOccurs="1"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="YEAR"      type="xsd:gYear"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Validating the Song Document

D:\speaking\XMLOne London\schemas\examples>java sax.SAX2Count -v hotcop.xml [Error] hotcop.xml:10:25: Datatype error: java.text.ParseException: Illegal or misplaced separator.

Here's the problem:

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

This is not in the schema time duration format! which is ISO 8601 "PnYn MnDTnH nMnS, where nY represents the number of years, nM the number of months, nD the number of days, 'T' is the date/time separator, nH the number of hours, nM the number of minutes and nS the number of seconds. The number of seconds can include decimal digits to arbitrary precision. An optional preceding minus sign ('-') is allowed, to indicate a negative duration."

Fixed Hot Cop

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="simple_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>P0YT6M20S</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

Xerces doesn't get this one right yet!

A Smaller Schema

Default value of minOccurs is 1
Default value of maxOccurs is 1

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Anonymous Types

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0" maxOccurs="1"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:string"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:element>
 
</xsd:schema>

Data Typing

Consider this document:

<foo>
    <value>45.67</value>
</foo>

What is the type of value?

Possible types

A decimal monetary type, as in COBOL
A fixed point number
An infinitely precise floating point number such as represented by the java.math.BigDecimal class
An IEEE754 double
A Java double
An IEEE 754 float
A VAX Fortran REAL
An imprecisely known decimal number with 4 significant digits that's plus or minus 1 in the last place.
An imprecisely known decimal number with 4 significant digits that's plus or minus 5 in the last place.
Build 67 of version 45 of Microsoft Word
A regular expression matching all strings that begin with the two characters '4' and '5', followed by a single character, followed by the two characters '6' and '7'.
A string of characters a monkey typed on a keyboard

Other interpretations are doubtless possible, and even make sense in particular contexts. There's no guarantee that the string 45.67 in fact represents any particular type.

The PSVI

A schema assigns an identifiable type to each element
Schema validation produces in a Post Schema Validation Infoset, PSVI for short
Schema aware applications using schema aware parsers and APIs can make use of the types of elements

Primitive Data Types for Schemas

Boolean
String
URIs
Numeric types
Time types
XML types
No money types. However, these can be derived

Numeric Data Types for Schemas

XML Schema Built-In Simple Types
Name	Type	Examples
float	IEEE 754 32-bit floating point number	-INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN
double	IEEE 754 64-bit floating point number	-INF, 1.401E-90, -1E4, -0, 0, 12.78E-2, 12, INF, NaN, 3.4E42
number	arbitrary precision, decimal numbers	-2.7E400, 5.7E-444, -3.1415292, 0, 7.8, 90200.76, 3.4E1024
integer	an arbitrarily large or small integer	-500000000000000000000000, -9223372036854775809, -126789, -1, 0, 1, 5, 23, 42, 126789, 9223372036854775808, 456734987324983264987362495809587095720978
nonPositiveInteger	an integer less than or equal to zero	0, -1, -2, -3, -4, -5, ...
negativeInteger	an integer strictly less than zero	-1, -2, -3, -4, -5, ...
long	an eight-byte two's complement integer such as Java's `long` type	-9223372036854775808, -12678967543233, -1, 9223372036854775807
int	an integer that can be represented as a four-byte, two's complement number such as Java's `int` type	-2147483648, -1, 0, 1, 5, 23, 42, 2147483647
short	an integer that can be represented as a two-byte, two's complement number such as Java's `short` type	-32768, -1, 0, 1, 5, 23, 42, 32767
byte	an integer that can be represented as a one-byte, two's complement number such as Java's `byte` type	-128, -1, 0, 1, 5, 23, 42, 127
nonNegativeInteger	an integer greater than or equal to zero	0, 1, 2, 3, 4, 5, ...
unsignedLong	an eight-byte unsigned integer	0, 1, 2, 3, 4, 5, ...18446744073709551614, 18446744073709551615
unsignedInt	a four-byte unsigned integer	0, 1, 2, 3, 4, 5, ...4294967294, 4294967295
unsignedShort	a two-byte unsigned integer	0, 1, 2, 3, 4, 5, ...65534, 65535
unsignedByte	a one-byte unsigned integer	0, 1, 2, 3, 4, 5, ...254, 255
positiveInteger	an integer strictly greater than zero	1, 2, 3, 4, 5, 6, ...

Time Data Types for Schemas

XML Schema Built-In Simple Types
Name	Type	Examples
timeInstant	a particular moment in Coordinated Universal Time; up to an arbitrarily small fraction of a second	1999-05-31T13:20:00.000-05:00
gMonth	A given month in a given year	2000-10
gYear	a given year	2000
recurringDate	a date in no particular year, or rather in every year	--10-31
recurringDay	a day in no particular month, or rather in every mnonth	----31
timeDuration	a length of time, without fixed endpoints, to an arbitrary fraction of a second	P2000Y10M31DT09H32M7.4312S
date	a specific day in history	2000-10-31
time	a specific time of day, that recurs every day	14:30:00.000, 09:30:00.000-05:00

XML Data Types for Schemas

XML Schema Built-In Simple Types
Name	Type	Examples
ID	XML 1.0 ID attribute type	any XML name that's unique among ID type attributes
IDREF	XML 1.0 IDREF attribute type	any XML name that's used as an ID type attribute elsewhere in the document
ENTITY	XML 1.0 ENTITY attribute type	any XML name that's declared as an unparsed entity in the DTD
NOTATION	XML 1.0 NOTATION attribute type	any XML name that's declared as a notation name in the DTD
language	valid values for xml:lang as defined in XML 1.0	en-GB, en-US, fr
IDREFS	XML 1.0 IDREFS attribute type	a white space separated list of IDREF names
ENTITIES	XML 1.0 ENTITIES attribute type	a white space separated list of ENTITY names
NMTOKEN	XML 1.0 NMTOKEN attribute type	12 are you ready
NMTOKENS	XML 1.0 NMTOKENS attribute type	a white space separated list of name tokens
Name	An XML 1.0 Name	set, title, rdf, math, math123, href
QName	a prefixed name	song:title
NCName	a local name without any colons	title

Assorted Data Types for Schemas

XML Schema Built-In Simple Types
Name	Type	Examples
string	Parsed Character Data; #PCDATA	Hot Cop
normalizedString	A string that does not contain any tabs, carriage returns, or linefeeds	PIC1, PIC2, PIC3, cow_movie, MonaLisa, Hello World , Warhol, red green
token	A string with no leading or trailing white space, no tabs, no linefeeds, and not more than one consecutive space	p1 p2, ss123 45 6789, _92, red, green, NT Decl, seventeenp1, p2, 123 45 6789, ^&^&_92, red green blue, NT-Decl, seventeen; Mary had a little lamb, The love of money is the root of all Evil.
boolean	C++'s bool type	true, false, 1, 0
anyURI	relative or absolute URI	http://www.w3.org/TR/2000/WD-xmlschema-2-20000407/#timeDuration, /javafaq/reports/JCE1.2.1.html
hexBinary	Arbitrary binary data encoded in hexadecimal form	A4E345EC54CC8D52198000FFEA6C
base64Binary	Arbitrary binary data encoded in Base64	6jKpNnmkkWeArsn5Oeeg2njcz+nXdk0f9kZI892ddlR8Lg1aMhPeFTYuoq3I6neFlb BjWzuktNZKiXYBfKsSTB8U09dTiJo2ir3HJuY7eW/p89osKMfixPQsp9vQMgzph6Qa lY7j4MB7y5ROJYsTr1/fFwmj/yhkHwpbpzed1LE=

Derived Types

You can derive new simple types from existing types by restricting the type to a subset of its normal values
An xsd:simpleType element defines the restricted type
The name attribute of xsd:simpleType assigns a name to the new type, by which it can be referred to in xsd:element type attributes.
An xsd:restriction child element specifies what type is being restricted via its base attribute.
Facet children of xsd:restriction specify the constraints on the type.

For example, this xsd:simpleType element defines a phonoYear as any year from 1877 (the year Edison invented the phonograph) on:

<xsd:simpleType name="phonoYear">
  <xsd:restriction base="xsd:gYear">
    <xsd:minInclusive value="1877"/>
  </xsd:restriction>
</xsd:simpleType>

Then you declare the year element like this:
<xsd:element type="phonoYear" />

Facets

Facets include:
- length
- minLength
- maxLength
- pattern
- enumeration
- whiteSpace
- maxInclusive
- maxExclusive
- minInclusive
- minExclusive
- totalDigits
- fractionDigits
- period
- duration
Not all facets apply to all types.

Facets for strings: length, minLength, maxLength

The number of characters allowed in a string
Must be a non-negative integer
Applies to string, normalizedString, token, hexBinary, base64Binary, QName, NCname, ID, IDREF, IDREFS, language, anyURI, ENTITY, ENTITIES, NOTATION, NOTATIONS, NMTOKEN and NMTOKENS type items

For example, to say that all names and titles must contain between 1 and 255 characters:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>
 
  <xsd:simpleType name="Str255">
    <xsd:restriction base="xsd:string">
       <xsd:minLength value="1"/>
       <xsd:maxLength value="255"/>
    </xsd:restriction>
  </xsd:simpleType>
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="Str255"/>
      <xsd:element name="COMPOSER"  type="Str255"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="Str255" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="Str255" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="Str255" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Facets for ordered items: minExclusive, maxExclusive, minInclusive, maxInclusive

Determines the minimum and maximum allowed values
Applies to ordered simple types including byte, unsignedByte, integer, positiveInteger, negativeInteger, nonNegativeInteger, nonPositiveInteger, int, unsignedInt, long, number, unsignedLong, short, unsignedShort, number, float, double, time, timeInstant, timePeriod, timeDuration, date, gMonth, gYear, recurringDay, and recurringDate.

For example, to say that the year must be between 1877 and 2100:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>
 
  <xsd:simpleType name="phonoYear">
    <xsd:restriction base="xsd:gYear">
      <xsd:minInclusive value="1877"/>
      <xsd:maxInclusive value="2100"/>
    </xsd:restriction>
  </xsd:simpleType> 
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="phonoYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

whiteSpace

Determines what the application should do with white space found in the content
Three possible values:
- preserve: The white space in the input document is left unchanged
- replace: Each tab, carriage return and linefeed is replaced with a single space.
- collapse: Each tab, carriage return and linefeed is replaced with a single space. Furthermopre, after this replacement is performed, all runs of multiple spaces are condensed to a single space. leading and trailing white space is deleted.
No effect on validation
Applies to string, normalizedString and token type items
Per XML 1.0, white space in attributes is normalized irregardless of the schema

For example, to say that white space should be collapsed in all names and titles:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>

  <xsd:simpleType name="NormalizedString">
    <xsd:restriction base="xsd:string">
       <xsd:whiteSpace value="collapse"/>
    </xsd:restriction>
  </xsd:simpleType>
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="NormalizedString"/>
      <xsd:element name="COMPOSER"  type="NormalizedString"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="NormalizedString" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="NormalizedString" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="NormalizedString" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Facets for decimal numbers: totalDigits and fractionDigits

The totalDigits facet specifies the maximum number of decimal digits in a number as a positive integer
The fractionDigits facet specifies the maximum number of decimal digits to the right of the decimal point as a non-negative integer
Applies to all number types except base64Binary and hexBinary including byte, unsignedByte, byte, integer, positiveInteger, negativeInteger, nonNegativeInteger, nonPositiveInteger, int, unsignedInt, long, unsignedLong, short, unsignedShort and number
You can specify at most two fractional digits or at most seven decimal digits, but not at least two fractional digits or exactly seven decimal digits

Facets for time: period and duration

The period facet defines the frequency of recurrence (after what duration it recurs) for time types. Its value is a time duration.
The duration facet defines the the length of the duration for time types. Its value is also a time duration.
Applies to time types: time, timeInstant, timePeriod, date, month, year, recurringDay, and recurringDate.
For example, you might use the period facet to define a twoWeek type with a fourteen day and a fourteen day period duration for paychecks.

Enumeration

The enumeration facet lists all allowed values
Applies to all simple types except boolean

For example, to say that the publisher must be one of the oligopoly that controls 90% of U.S. music (Warner-Elektra-Atlantic, Universal Music Group, Sony Music Entertainment, Inc., Capitol Records, Inc., BMG Music)

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="songType"/>
 
  <xsd:simpleType name="oligopolyMember">
    <xsd:restriction base="xsd:string">
      <xsd:enumeration value="Warner-Elektra-Atlantic"/>
      <xsd:enumeration value="Universal Music Group"/>
      <xsd:enumeration value="Sony Music Entertainment, Inc."/>
      <xsd:enumeration value="Capitol Records, Inc."/>
      <xsd:enumeration value="BMG Music"/>
    </xsd:restriction>
  </xsd:simpleType> 
 
  <xsd:complexType name="songType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="oligopolyMember" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Adding a Price

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="priced_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
  <PRICE>$1.35</PRICE>  
</SONG>

The pattern facet

Suppose you want a money type to specify that the PRICE element content must look like $1.35 or ¥11000
Derive this from the xsd:string type by restriction
Use the pattern facet to specify a regular expression instances must match

Regular Expressions

More or less Perl-like with some Unicode extensions
The money regular expression:
\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?

\p{Sc}

Any Unicode currency indicator; e.g. $, ¥, £, ¤, etc.

\p{Nd}
A Unicode decimal digit character

\p{Nd}+
One or more Unicode decimal digit characters

\.
The period character

(\.\p{Nd}\p{Nd})
A string with a period followed by two decimal digits like .35

(\.\p{Nd}\p{Nd})?
Zero or one strings of the form .35

The Price Schema

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:simpleType name="money">
    <xsd:restriction base="xsd:string">             
      <xsd:pattern value="\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?"/>
    <!-- 
       Regular Expression:
       \p{Sc}             Any Unicode currency indicator; e.g. $, &#xA5, &#xA3, &#A4, etc.
       \p{Nd}             A Unicode decimal digit character
       \p{Nd}+            One or more Unicode decimal digit characters
       \.                 The period character
       (\.\p{Nd}\p{Nd})
       (\.\p{Nd}\p{Nd})?  Zero or one strings of the form .35
       
       This works for any decimalized currency. 
       
    -->
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRICE" type="money"/>      
     </xsd:sequence> 
  </xsd:complexType>
  
</xsd:schema>

Complex Types

Elements that contain child elements or have attributes or both
Defined by an xsd:complexType element

A Document with Attributes

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="attribute_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Attributes

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <!-- An empty element -->
  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0""/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Element Content

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="nested_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME>
      <GIVEN>Jacques</GIVEN>
      <FAMILY>Morali</FAMILY>
    </NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>
      <GIVEN>Henri</GIVEN>
      <FAMILY>Belolo</FAMILY>
    </NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>
      <GIVEN>Victor</GIVEN>
      <FAMILY>Willis</FAMILY>
    </NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME>
      <GIVEN>Jacques</GIVEN>
      <FAMILY>Morali</FAMILY>
    </NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Complex Types

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="ComposerType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
             <xsd:element name="GIVEN"  type="xsd:string"/>
             <xsd:element name="FAMILY" type="xsd:string"/>  
          </xsd:sequence>    
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="ProducerType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/>
          </xsd:sequence>      
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="ComposerType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="ProducerType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Sharing Content Models

PRODUCER and COMPOSER are really the same type.

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
             <xsd:element name="GIVEN"  type="xsd:string"/>
             <xsd:element name="FAMILY" type="xsd:string"/>  
          </xsd:sequence>    
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Mixed Content

Schemas let you enforce order and appearance of elements in mixed content.

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="mixed_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Jacques</GIVEN> <FAMILY>Morali</FAMILY> Esq.</NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Henri</GIVEN> L. <FAMILY>Belolo</FAMILY>, M.D.</NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Victor</GIVEN> C. <FAMILY>Willis</FAMILY></NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME>Mr. <GIVEN>Jacques</GIVEN> S. <FAMILY>Morali</FAMILY></NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Mixed Content

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType mixed="true">
          <xsd:sequence>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/>
           </xsd:sequence>      
         </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

When Order Doesn't Matter

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="unordered_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME><FAMILY>Morali</FAMILY> <GIVEN>Jacques</GIVEN></NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME><GIVEN>Henri</GIVEN> <FAMILY>Belolo</FAMILY></NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME><FAMILY>Willis</FAMILY> <GIVEN>Victor</GIVEN></NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME><GIVEN>Jacques</GIVEN> <FAMILY>Morali</FAMILY></NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

The xsd:all Group

Each element in the xsd:all group must occur zero or once; that is minOccurs and maxOccurs must each be 0 or 1
The xsd:all group must be the top level element of its type
The xsd:all group may contain only individual element declarations; no choice or sequences

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:all>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/> 
          </xsd:all>     
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <!-- An empty element -->
  <xsd:complexType name="PhotoType" content="empty">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Choices

xsd:choice requires exactly one of a group of specified elements to appear
The choice can have minOccurs and maxOccurs attributes that adjust this from zero to any given number.

Sequences

xsd:sequence requires each child element it specifies to appear in the specified order
The sequence can have minOccurs and maxOccurs attributes that repeat each sequence zero to any given number of times.

Default Namespace

<?xml version="1.0"?>
<GREETING 
  xmlns="http://ibiblio.org/xml/schemas/greeting/"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://ibiblio.org/xml/schemas/greeting/
                      greeting_defaultNS.xsd">
  Hello XML!
</GREETING>

The targetNamespace attribute

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://ibiblio.org/xml/schemas/greeting/"
>
 
  <xsd:element name="GREETING" type="xsd:string"/>

</xsd:schema>

A Song with a Namespace

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://ibiblio.org/xml/namespace/song"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://ibiblio.org/xml/namespace/song namespace_song.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

A Schema for a Document that Uses the Default Namespace

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://ibiblio.org/xml/namespace/song"
  targetNamespace="http://ibiblio.org/xml/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE" type="xsd:string"/>
      <xsd:element name="PHOTO" type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>    
      <xsd:element name="YEAR" type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Multiple Namespaces, Multiple Schemas

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://ibiblio.org/xml/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://ibiblio.org/xml/namespace/song xlink_song.xsd
        http://www.w3.org/1999/xlink xlink.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO xlink:type="simple" xlink:href="hotcop.jpg"
         xlink:actuate="onLoad" xlink:show="embed"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

XLink Schema

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://www.w3.org/1999/xlink"
  targetNamespace="http://www.w3.org/1999/xlink"
  attributeFormDefault="unqualified"
>

  <xsd:attribute name="type" type="xsd:string" 
                 use="fixed" value="simple"  />
  <xsd:attribute name="href" type="xsd:anyURI"/>
  <xsd:attribute name="actuate" type="xsd:string"
                 use="fixed" value="onLoad"  />
  <xsd:attribute name="show" type="xsd:string"
                 use="fixed" value="embed"  />

</xsd:schema>

Song Schema with XLink Support

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://ibiblio.org/xml/namespace/song"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  targetNamespace="http://ibiblio.org/xml/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>

  <xsd:import namespace="http://www.w3.org/1999/xlink"
              schemaLocation="xlink.xsd"/>

  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PhotoType">
    <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"
                   use="required" />
    <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"
                   use="required" />
    <xsd:attribute name="ALT"    type="xsd:string"
                   use="required" />
    <xsd:attribute ref="xlink:type"/>
    <xsd:attribute ref="xlink:href" use="required"/>
    <xsd:attribute ref="xlink:actuate"/>
    <xsd:attribute ref="xlink:show"/>                 
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
                   maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string"
                   minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string"
                   minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:timeDuration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string"
                   maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Annotations

The top-level xsd:annotation element describes the schema
Its xsd:documentation child element describes the schema for human readers
Its xsd:appInfo child element describes the schema for computer programs; e.g. stylesheet instructions

  <xsd:annotation>
   <xsd:documentation>
    Song schema for XML and Java Example at XMLOne London
    Copyright 2001 Elliotte Rusty Harold. 
   </xsd:documentation>
  </xsd:annotation>

What Schemas don't do

Cannot declare entities
Parent models
Extra-document validation

Schema Alternatives

Rick Jelliffe's Schematron
Murato Makoto's RELAX
James Clark's TREX
Rick Jelliffe's Hook
DTDs

Schematron

According to Schematron inventor Rick Jelliffe:
The Schematron differs in basic concept from other schema languages in that it not based on grammars but on finding tree patterns in the parsed document. This approach allows many kinds of structures to be represented which are inconvenient and difficult in grammar-based schema languages.
XSLT/XPath based
W3C Schemas are conservative: everything not permitted is forbidden.
Schematron is liberal: everything not forbidden is permitted.
No data typing; validation only
Handles unordered structures very well
Handles descendant constraints very well
Almost self-documenting
http://www.ascc.net/xml/resource/schematron/schematron.html

A Schematron schema for songs

A schema contains a title and a pattern
Each pattern contains rules
Each rule contains assert and report elements and has a context attribute
Each assert and report element has a test attribute containing an XPath expression whihc returns a boolean.
The contents of each assert element is printed if the assertion test fails
The contents of each report element is printed if the report test succeeds

<?xml version="1.0"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
  <title>A Schematron Schema for Songs</title>
  <pattern>
    <rule context="SONG">
      <assert test="TITLE">
        A SONG must contain an initial TITLE element.
      </assert>
      <assert test="TITLE[position()=1]">
        The TITLE element must be the initial element of the SONG element.
      </assert>
      <assert test="COMPOSER">
        A SONG must contain at least COMPOSER element.
      </assert>
      <assert test="ARTIST">
        A SONG must contain at least one ARTIST element.
      </assert>
    </rule>
  </pattern>
</schema>

RELAX

Murato Makoto
JIS standard/Proposed ISO standard
Uses W3C Schema data types
No derived types
Mostly DTD-like structures
http://www.xml.gr.jp/relax/

A RELAX schema for songs

<?xml version="1.0?>
<module
      moduleVersion="1.2"
      relaxCoreVersion="1.0"
      targetNamespace=""
      xmlns="http://www.xml.gr.jp/xmlns/relaxCore">

  <!-- Elements allowed as document roots -->
  <interface>
    <export label="SONG"/>
  </interface>

  <elementRule role="SONG">
    <sequence>
      <ref label="TITLE"/>
      <ref label="COMPOSER" occurs="*"/>
      <ref label="PRODUCER" occurs="*"/>
      <ref label="PUBLISHER" occurs="?"/>
      <ref label="YEAR"/>
      <ref label="ARTIST" occurs="+"/>
      <ref label="PRICE"  occurs="?"/>
    </sequence>
  </elementRule>
  
  <elementRule role="TITLE"     type="string"/>
  <elementRule role="COMPOSER"  type="string"/>
  <elementRule role="PRODUCER"  type="string"/>
  <elementRule role="PUBLISHER" type="string"/>
  <elementRule role="YEAR"      type="year"/>
  <elementRule role="ARTIST"    type="string"/>
  <elementRule role="PRICE"     type="string"/>

</module>

TREX

Tree Regular Expressions for XML
Invented by James Clark of XSLT fame
Proposed OASIS standard
Uses W3C Schema data types
http://www.thaiopensource.com/trex/

Hook

Rick Jelliffe's Hook: A One-Element Language for Validation of XML Documents based on Partial Order
XSLT/XPath based
No data typing; validation only

A Hook schema for XHTML (adapted from Rick Jelliffe):

<hook:order targetNamespace="http://www.w3.org/1999/xhtml" >
  html head  [ title; meta. link. base. ]   body
  [ a br. blockquote caption; div  dl; h1; h2; h3; h4; h5; h6;  
    img. ol; p; pre; table; ul; ]  
  [ tr;  dt; dd; li; ]  td 
  [ a br. blockquote div  form img. ol; ul; li; ]  
  [ input; label; select; textarea; ]  [ option. ]
  [ abbr acronym address cite code dfn em kbd q samp span strong var object; ] 
  param 
</hook:order>

http://www.ascc.net/xml/hook/

DTDs aren't Dead!

To Learn More

XML Bible, second edition, Chapter 24
- Elliotte Rusty Harold
- Hungry Minds, 2001
- ISBN 0-7645-4760-7
This presentation: http://www.ibiblio.org/xml/slides/xmlonelondon2001/schemas
W3C Schema Primer: http://www.w3.org/TR/xmlschema-0/
XML Schema Part 1: Structures: http://www.w3.org/TR/xmlschema-1/
XML Schema Part 2: Datatypes: http://www.w3.org/TR/xmlschema-2/

Index | Cafe con Leche