The World Wide Web Consortium (W3C) has announced that XML could finally "become usable". The not-for-profit organisation has released its XML schema as a candidate recommendation, putting it one step closer to final ratification - at this stage, the creation of a standard XML schema is all but guaranteed.
Much has been said about schemas recently, but few people really understand the difference between the one proposed for XML and its predecessor, the Document Type Definition (DTD).
XML on its own is merely a language used to write other languages, generally for application or market-specific tasks. A DTD is a document containing information about a particular XML-based language - call it a dictionary for a particular electronic dialect.
According to Michael Sperberg-McQueen, co-chairman of the XML Schema Working Group, there are three main enhancements that a schema brings over a DTD.
Firstly, DTDs are written using non-XML notation, meaning that a separate tool is needed to parse them. An XML schema is already XML-based, so it can be read using an XML editor.
Secondly, the XML schema supports name spaces much more effectively than DTDs. This means there will be less chance for different XML schema documents to use names that clash with those used by other documents.
XML Schema documents will support inheritance much more effectively. Inheritance is an object-oriented concept, in which different objects can inherit the characteristics of other objects. Under the XML schema specification, different schema documents can inherit each other’s characteristics.
Thirdly, XML schema documents contain data types, meaning that, for the first time, a piece of data can be defined within a document as a date, for example, rather than simply a string. This makes data easier to validate.
This is a key business benefit, according to Sperberg-McQueen. “You can push your data validation rules out onto the client,” he said. “It is foreseeable now that, when you send data to the client for processing, you can send the validation rules with it and the client will return cleaner data.” Microsoft is strongly supporting the XML schema specification, in spite of the fact that it pre-empted the W3C with the XML Data Reduced (DR) proposal, which is an inherent part of the yet-to-be released Biztalk Server product.
According to David Turner, technology evangelist at Microsoft, the company will release Biztalk Server with XML DR support but will implement XML schema in a later release.
Because XML DR is a subset of XML schema, Turner said he does not anticipate any problems, but added that the company will ship tools for converting XML DR documents to the XML schema format.
The W3C ratification process
- A briefing package is sent to the members describing the work to be done on a particular topic.
- A working group is formed. The XML Schema Working Group was formed in September 1998 and the first meeting was in November the same year.
- The working group develops a requirements document.
- The working group begins issuing working drafts that come continually closer to meeting those requirements.
- A last-call working draft is released, this is a last chance to read the specification while it is still relatively fluid. The XML Schema Working Group made its last call draft in April.
- A candidate is released as a signal to implementers that it is stable for implementation.
- Feedback is received from implementers and schema authors.
- The working group publishes the specification as a proposed recommendation.
- The specification is approved as a recommendation, which is as close as the W3C gets to an official standard.