The Role of Schema

The word schema traditionally refers to the order or structure of data, such as the tables and columns in a relational database. But it actually applies to any organized collection of data, including XML. In fact, XML schemas are central to building and deploying XML-based applications.

Schema: Rules for XML Data

XML data fields, like HTML markup, consist of ranges of text (called elements) set off by angle-bracketed tags. An XML schema is a document that defines what elements are legal within a given set of data. Therefore, a schema must describe information such as the following:

• Names and types of elements
• Which elements are required and which are optional
• Which elements can be repeated and which may appear only once.

For example, a schema for an XML expense report might specify that it contain elements for the employee’s name, destination, and travel dates; an optional note element for the employee’s use; and a repeating series of elements for specific expense items.

XML schemas are written in a language called XML Schema that is defined by a World Wide Web Consortium (W3C) standard and often referred to as XSD (XML Schema Definition). An XSD file is itself an XML file, which simplifies development because it eliminates the need for a separate, specialized schema parser.