Friday, December 13, 2013

XML Schemas

XML Schemas are much more powerful than DTDs.

XML Schemas Support Data Types

One of the greatest strength of XML Schemas is the support for data types.
With support for data types:
  • It is easier to describe allowable document content
  • It is easier to validate the correctness of data
  • It is easier to work with data from a database
  • It is easier to define data facets (restrictions on data)
  • It is easier to define data patterns (data formats)
  • It is easier to convert data between different data types

XML Schemas use XML Syntax

Another great strength about XML Schemas is that they are written in XML.
Some benefits of that XML Schemas are written in XML:
  • You don't have to learn a new language
  • You can use your XML editor to edit your Schema files
  • You can use your XML parser to parse your Schema files
  • You can manipulate your Schema with the XML DOM
  • You can transform your Schema with XSLT

XML Schemas Secure Data Communication

When sending data from a sender to a receiver, it is essential that both parts have the same "expectations" about the content.
With XML Schemas, the sender can describe the data in a way that the receiver will understand.
A date like: "03-11-2004" will, in some countries, be interpreted as 3.November and in other countries as 11.March.
However, an XML element with a data type like this:
<date type="date">2004-03-11</date>
ensures a mutual understanding of the content, because the XML data type "date" requires the format "YYYY-MM-DD".

XML Schemas are Extensible

XML Schemas are extensible, because they are written in XML.
With an extensible Schema definition you can:
  • Reuse your Schema in other Schemas
  • Create your own data types derived from the standard types
  • Reference multiple schemas in the same document

Well-Formed is not Enough

A well-formed XML document is a document that conforms to the XML syntax rules, like:
  • it must begin with the XML declaration
  • it must have one unique root element
  • start-tags must have matching end-tags
  • elements are case sensitive
  • all elements must be closed
  • all elements must be properly nested
  • all attribute values must be quoted
  • entities must be used for special characters
Even if documents are well-formed they can still contain errors, and those errors can have serious consequences.
Think of the following situation: you order 5 gross of laser printers, instead of 5 laser printers. With XML Schemas, most of these errors can be caught by your validating software.
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 "note.xml":
<?xml version="1.0"?>
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

A DTD File

The following example is a DTD file called "note.dtd" that defines the elements of the XML document above ("note.xml"):
<!ELEMENT note (to, from, heading, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
The first line defines the note element to have four child elements: "to, from, heading, body".
Line 2-5 defines the to, from, heading, body elements to be of type "#PCDATA".

An XML Schema

The following example is an XML Schema file called "note.xsd" that defines the elements of the XML document above ("note.xml"):
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.w3schools.com"
xmlns="http://www.w3schools.com"
elementFormDefault="qualified">

<xs:element name="note">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="to" type="xs:string"/>
      <xs:element name="from" type="xs:string"/>
      <xs:element name="heading" type="xs:string"/>
      <xs:element name="body" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

</xs:schema>
The note element is a complex type because it contains other elements. The other elements (to, from, heading, body) are simple types because they do not contain other elements. You will learn more about simple and complex types in the following chapters.

A Reference to a DTD

This XML document has a reference to a DTD:
<?xml version="1.0"?>

<!DOCTYPE note SYSTEM
"http://www.w3schools.com/dtd/note.dtd">

<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

A Reference to an XML Schema

This XML document has a reference to an XML Schema:
<?xml version="1.0"?>

<note
xmlns="http://www.w3schools.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3schools.com note.xsd">
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>


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