XML.HTMLTeacher.Com XML Schema Overview

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Overview

This schema dialect, XSDL (XML Schema Definition Language,) is an approved recommendation as of May 2001 by the W3.org and is closest to a schema standard at this time.

A schema is an XML document that defines the content and structure of one or more XML documents.

A schema is always created as a separate file and is referenced by the XML document.

Schemas are usually stored using the .xsd extension (XML Schema Definition.) However, the extension may be any extension such as .txt and .xs.

Schemas offer some very important functions:

Content Model Validity - This ensures that the element hierarchy and document structure are correct. It checks to make sure that elements are ordered and nested correctly. (Much like DTDs.) Or may give other ordering options.

Datatype Validity - This ensures that element and attribute content adheres to the defined datatype. A datatype can define a scope for legal values as well as define a base type such as integer, decimal or string.

Extensibility - Schemas allow for greater generalities in terms of describing the structure of the document, which in turn, allows for greater control in the creation of the XML document and in reusability of the schema markup to be utilized in other areas.

Namespaces - Namespaces are the mechanism designed to help define a unique identifier for markup tags. For example, "mustang," the car, is the same as "mustang," the horse. Therefore, both of these <mustang> elements are identical. Through the use of namespaces, a schema can clearly identify each of these elements as having different meanings (semantics) or uses.

Schema versus DTD

DTDs are not likely to be totally abandoned if already in use. They are supported in many tools, are widely understood and are currently in use in many applications. They continue to be useful as a lightweight alternative to schemas.

If a DTD is currently being utilized and is meeting your requirement base, then it is not likely you will need to translate the DTD into a schema. If just beginning to convert to XML, it is likely that a company may choose to develop a schema rather than a DTD, simply because more function, such as extensibility, datatyping and reusability is offered with a schema.

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