What XML Schema does
The XML Schema specification consists of two parts.
Part 1 describes a language (the XML Schema Definition language) that is used to describe the high-level structure of an XML document.
Part 2 describes the list of allowable data types that can be used by the XML Schema Definition language (and thus in XML documents themselves).
It's very important for developers to understand that XML Schema documents are actually XML metainformation: They describe the structure of XML documents and don't contain end-user data themselves.
By using the XML Schema Definition associated with an XML data stream, an XML parser can automatically verify not just the syntax of the XML data but also its structure and logical correctnessâ€"-a big step forward. XML Schema replaces the obscure and far less-powerful XML Document Type Definition standard.
For example, using XML Schema, companies can now detect if received XML files have missing data, data that's been improperly formatted (such as dates with only two-digit year values or fields with words entered where there should be numbers) or data that's obviously wrong (such as numbers that are clearly too large or too small to possibly be valid).
The range of characteristics that XML Schema defines is very comprehensive and includes a large selection of basic data types, such as integers, floating-point numbers, strings, times and dates; it also includes ways to constrain values to valid data ranges or to lists of valid values, the ability to define default values for missing data and the ability to make data elements required.
XML also defines complex types composed of groupings of simple types (such as an "address" type).
Regular expressions can be used to check for valid data, and XML Schema documents can inherit from and then partially override the behaviour of other XML Schema documents for object-oriented development.
XML Schema does not provide a way to see whether values that look correct actually are correct (by checking against values in a database, for example), but vendors such as Data Junction are now starting to provide this capability.
Because it provides a way for organisations to share high-level definitions of how XML data should be structured, business exchanges are grabbing up XML Schema like there's no tomorrow.
Centralised repositories of XML file format information, such as Microsoft's BizTalk, are now accepting submissions of industry-specific XML document definitions in XML Schema format.
