As you navigate the site you will find that there are not only textual information resources but also practical ways to get involved with XML initiatives and specific activities. These are aimed at developing your basic understanding of XML and exploring the relationships between XML and legacy technology, like Electronic Data Interchange (EDI). This site will also introduce some of the more significant XML standards activities that are taking place around the globe.

XML is the Extensible Markup Language. It is designed to improve the functionality of the Web by providing more flexible and adaptable information identification.

It is called extensible because it is not a fixed format like HTML (a single, predefined markup language). Instead, XML is actually a `metalanguage' -- a language for describing other languages -- which lets you design your own customized markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard metalanguage for text markup systems.

XML is intended `to make it easy and straightforward to use SGML on the Web: easy to define document types, easy to author and manage SGML-defined documents, and easy to transmit and share them across the Web.'

It defines `an extremely simple dialect of SGML which is completely described in order to pass it between different computing systems which would otherwise be unable the XML Specification. The goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML.'

`For this reason, XML has been designed for ease of implementation, and for interoperability with both SGML and HTML'

XML is not just for Web pages: it can be used to store any kind of structured information, and to enclose or encapsulate information in to communicate.

XMLValidate is an enterprise-grade solution for validating streaming XML documents or messages against an XML Schema or DTD. This SAX-based implementation for run-time validation provides organizations with the core component in developing high bandwidth, XML-based processing. Its event-driven API makes XMLValidate an ideal candidate for industry-grade processing in a run-time environment, such as the financial community, which must reliably process between 10-20 million transactions a day.

Example

The XML shown below is both Well Formed and Valid according to the following DTD.
< ?xml version="1.0"?>
< !DOCTYPE HelloWorldDTD [
< !ELEMENT HelloWorld (To,From,Content)>
< !ELEMENT To (#PCDATA)>
< !ELEMENT From (#PCDATA)>
< !ELEMENT Content (#PCDATA)>
]>
< HelloWorld>
< To>Fred</To>
< From>Jim</From>
< Content>Hello</Content>
< /HelloWorld>

In this example the XML conforms to the syntactical rules of XML as well as conforming to the internal DTD. From the example above the <!DOCTYPE HelloWorldDTD[….]> effectively says this is a DTD and it is called "HelloWorldDTD". The next line <!ELEMENT HelloWorld (To,From,Content)> lists the elements which will appear in the basic XML, in this case they are "To", "From" and "Content". The DTD then details each element and what data will be expressed within that element. So the following specifies that this element called "To" will contain Parser Character Data (PCData) only <!ELEMENT To ( #PCDATA)>.
For more information on DTDs of Schema please see the appropriate pages of this website.

A DTD is a formal description in XML Declaration Syntax of a particular type of document. It sets out what names are to be used for the different types of element, where they may occur, and how they all fit together. For example, if you want a document type to be able to describe Lists which contain Items, the relevant part of your DTD might contain something like this:

<!ELEMENT List (Item)+>
< !ELEMENT Item (#PCDATA)>

This defines a list as an element type containing one or more items (that's the plus sign); and it defines items as element types containing just plain text (Parsed Character Data or PCDATA). Validating parsers read the DTD before they read your document so that they can identify where every element type ought to come and how each relates to the other, so that applications which need to know this in advance (most editors, search engines, navigators, databases) can set themselves up correctly. The example above lets you create lists like:

<List><Item>Chocolate</Item><Item>Music</Item><Item>Surfing</Item></List>

How the list appears in print or on the screen depends on your stylesheet: you do not normally put anything in the XML to control formatting like you had to do with HTML before stylesheets. This way you can change style easily without ever having to edit the document itself.

A DTD provides applications with advance notice of what names and structures can be used in a particular document type. Using a DTD when editing files means you can be certain that all documents which belong to a particular type will be constructed and named in a consistent and conformant manner. DTDs are less important for processing documents already known to be well-formed, but they are still needed if you want to take advantage of XML's special attribute types like the built-in ID/IDREF cross-reference mechanism.

An XML Schema really acts like a data map to explain the basic XML documents with which it is associated. The purpose of a Schema is to define the allowable building blocks in exactly the same manner as a DTD, however Schema development has been informec from the application of DTDs in the B2B arena and as a result are more advanced. They provide a means of specifying element content in terms of data types such a provision can be made for validating the content of elements as well as the mark-up itself.

Schemas, unlike DTDs are written in XML avoiding the need for processing software to be able to read DTD syntax, called Declaration Syntax. This makes schemas easier to learn than DTDs and in turn reduces development time while simultaneously reducing the burden on software which otherwise needs to interpret two distinct syntaxes.

Schemas are also easier to extent than DTDs, which makes them more re-usable, and therefore a good foundation for standardising business documents. The concept of re-usability is an important one as it will allow the basic building blocks of business exchanges to be standardised facilitating greater interoperability than exists today.

Schemas also support namespaces which are an important aspect of the XML syntax. They allow the developer/user to identify their unique extensions to a particular XML document. This is important when we consider the flexibility of XML to extensions, as namespaces will be one mechanism by which extensions can be managed. Allowing communities to trace who and why a document was extended.

For example within a particular business community it may be necessary, because of a particular business process to add an additional address field to the description of Party. This is easily achieved by the flexibility of XML schemas and indeed XML, but by using a namespace this extension can be uniquely attributed to a single user.