About XML

XML

Extensible Markup Language [XML] is a extensible, portable, and structured text format.

XML is plays an important role in the exchange of various types of data on the Web and between different applications.

It provides a standard and common data structure for sharing data between disparate systems. Additionally, XML has built-in data validation, which guarantees that the structure of the data that is received is valid.

Related XML Standards

XML has a number of related standards:

XSL—Extensible Stylesheet language - used to transform XML data into a customizable presentation.

XLink and XQuery - provide a way to provide flexible query facilities to extract data from real and virtual XML documents on the Web.

XPath and XPointer - languages for addressing parts of an XML document.

XML Schema

XML Schema is one of the key components of XML.

Schemas provide a means of defining the structure, content, and semantics of XML documents that can be shared between different types of computers and documents.

XML Schemas provides a means of creating a set of rules that can be used to identify document rules governing the validity of the XML documents that you create.

The XML Schema specification plays an important role in the design and implementation of Web Services. WSDL files are also built using XML Schema as the underlying syntax.

Using XML

Using XML requires creating XML documents and consuming XML documents.

• Creating an XML document is done using editors and tools.
• Consuming XML documents involves parsing the XML documents and extracting the useful data.

Creating XML Documents

Creating XML documents is a two step process:

1. Defining the grammar and restrictions over data for the XML document.
2. Creating the XML document itself and validating the grammar DTD and Schema.

DTD and schema are used to specify the structure of instance documents and the datatype of each element/attribute.

XML Schema is approved by W3C Recommendation and is now being widely used for structuring XML documents for e-commerce and Web Services applications.

The design of the XML Schema standard focused on:

• Expressing Object Oriented design principles found in common OO programming languages into the specification.
• Providing rich datatyping support similar to the datatyping functionality available in most relational database systems.

XML Tags

XML uses the "<> </>" tags used in HTML. The two primary building blocks of XML are elements and attributes.

Elements

Elements are tags that have values, and are structured as a tree. Thus elements organized in a hierarchical fashion with a 'parent' element and 'child' elements. For example the parent tag <employee> may have a child tag <phone> , which in turn may also have child tages <office> and <mobile>.

The characteristics of Elements are:

Can contain data - such as <number>

Can contain attributes

Can contain both data and attributes.

Elements must be constructed in accordance to set rules, such as valid names, proper nesting etc.

Attributes

Attributes give more meaning to an element and describe it more clearly.

For example <shift> element has an attribute "id" with values "counter" and "help_desk". These attributes define that an employee can be working at a counter or help desk. The core purpose of attributes is to provide more information about the element and should not be used to contain the data itself. Just as with elements, attributes have many rules associated with them.

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