Designing XML Schema
XML documents can have a reference to a DTD or to an
XML Schema.
A Simple XML Document
Look at this simple XML document called "E-mail.xml":
<?xml version="1.0"?>
<E-mail>
<To>Rohan</To>
<From>Amit</From>
<Subject>Surprise....</Subject>
<Body>Be ready for a cruise...</Body>
</E-mail>
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XML Schema
The following example is a XML Schema file called
"E-mail.xsd" that defines the elements of the XML document above
("E-mail.xml"):
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.roseindia.net"
xmlns="http://www.roseindia.net"
elementFormDefault="qualified">
<xs:element name="E-mail">
<xs:complexType>
<xs:sequence>
<xs:element name="To" type="xs:string"/>
<xs:element name="From" type="xs:string"/>
<xs:element name="Subject" type="xs:string"/>
<xs:element name="Body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
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We will discuss the building blocks of this
schema latter in this section further.
Add a reference to the above declared XML
document
Now this XML document (E-mail.xml) has a reference to
above declared XML Schema(E-mail.xsd)
<?xml version="1.0"?>
<E-mail
xmlns="http://www.roseindia.net"
xmlns:xsi="http://www.w3.org/2001/XMLSchema"
xsi:schemaLocation="http://www.roseindia.net/Schema E-mail.xsd">
<To>Rohan</To>
<From>Amit</From>
<Subject>Surprise....</Subject>
<Body>Be ready for a cruise...</Body>
</E-mail>
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In the above xml document xmlns declares the XML
namespaces (we will discuss it in the
coming segment of current page).
Save E-mail.xml
and E-mail.xsd
in the same location. Open the file E-mail.xml in a
web-browser. You will see the following :
Let's briefly discuss the concept of
XML Namespaces
XML Namespaces provide a mechanism to
avoid element's name conflicts.
Name Conflicts: Since
element names in XML are not predefined, chances for frequency to
meet name conflict increases when two different documents use the same
element names.
We solve the Name Conflicts using a Prefix with a
element name: By using a prefix, we can create two different types
of elements. Instead of using only prefixes, we add an xmlns
attribute to the conflict causing tags to give the prefix a qualified name .
The XML Namespace (xmlns)
Attribute: The XML namespace attribute is
placed in the start tag of an element and has the following syntax:
xmlns:namespace-prefix="namespaceURI"
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Example 1(taken from E-mail.xml ) :
<E-mail
xmlns="http://www.roseindia.net"
xmlns:xsi="http://www.w3.org/2001/XMLSchema"
xsi:schemaLocation="http://www.roseindia.net/Schema"
E-mail.xsd"> |
Example 2(taken from E-mail.xsd ) :
<xs:schema
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.roseindia.net"
xmlns="http://www.roseindia.net"
elementFormDefault="qualified"> |
When a namespace is defined in the start tag of an
element, all child elements with the same prefix are associated with the same
namespace. In E-mail.xsd "xs"
is the defined namespace in the start tag. So it prefixes all
the child elements with xs
eg...
<xs:element
name="E-mail">
<xs:complexType>
<xs:sequence>
<xs:element
name="To" type="xs:string"/>
<xs:element
name="From" type="xs:string"/>
<xs:element
name="Subject" type="xs:string"/>
<xs:element
name="Body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element> |
Note that the address used to identify the
namespace is not used by the parser to look up information. The only purpose is
to give the namespace a unique name. However, very often companies use
the namespace as a pointer to a real Web page containing information about the
namespace.
Here a Uniform Resource Identifier (URI) is a string of characters which
identifies an Internet Resource.
Default Namespaces : Defining
a default namespace for an element saves us from using prefixes in all the child
elements. It has the following syntax:
We have not included prefixes in all the child element
tags( To, From, Subject,
Body) in our following example :
<?xml version="1.0"?>
<E-mail
xmlns="http://www.roseindia.net"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.roseindia.net/Schema E-mail.xsd">
<To>Rohan</To>
<From>Amit</From>
<Subject>Surprise....</Subject>
<Body>Be ready for a cruise...</Body>
</E-mail>
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Building blocks of a XML-Schema
XSD - The <schema> Element
The <schema> element is the root
element of every XML Schema:
<?xml version="1.0"?>
<xs:schema>
...
...
</xs:schema> |
The <schema> element may contain some attributes
like...
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.roseindia.net"
xmlns="http://www.roseindia.net"
elementFormDefault="qualified">
...
...
</xs:schema>
|
The following code:
xmlns:xs="http://www.w3.org/2001/XMLSchema"
|
indicates that the elements and data types used in the
schema come from the "http://www.w3.org/2001/XMLSchema" namespace. It
also specifies that the elements and data types that come from the
"http://www.w3.org/2001/XMLSchema" namespace should be prefixed with xs:
This code segment
targetNamespace="http://www.roseindia.net"
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indicates that the elements defined by this schema (E-mail,
To, From, Subject, Body.) come from the "http://www.roseindia.net"
namespace.
This fragment:
xmlns="http://www.roseindia.net"
|
indicates that the default namespace is "http://www.roseindia.net".
This fragment:
elementFormDefault="qualified"
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indicates that any elements used by the XML instance
document which were declared in this schema must be a namespace qualified.
Referencing a Schema in an XML Document
This XML document (E-mail.xml) has a reference to an
XML Schema (E-mail.xsd).
<?xml version="1.0"?>
<E-mail
xmlns="http://www.roseindia.net"
xmlns:xsi="http://www.w3.org/2001/XMLSchema"
xsi:schemaLocation="http://www.roseindia.net/Schema E-mail.xsd">
<To>Rohan</To>
<From>Amit</From>
<Subject>Surprise....</Subject>
<Body>Be ready for a cruise...</Body>
</E-mail>
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The following fragment:
xmlns="http://www.roseindia.net"
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specifies the default namespace declaration. This
declaration tells the schema-validator that all the elements used in this XML
document are declared in the "http://www.w3schools.com" namespace.
Once you have the XML Schema Instance namespace
available:
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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you can use the schemaLocation attribute. This
attribute has two values. The first value is the namespace to use. The second
value is the location of the XML schema to use for that namespace:
xsi:schemaLocation="http://www.roseindia.net note.xsd"
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