Mark Wilson I am the creator of TopXML. I am available for international and local (Australia) contracts. I am a Solution Architect/Business Analyst. I have worked in IT in several countries (NZ, Australia, South Africa, UK) building and training teams for government and very large non-governmental organizations. I am ex-Microsoft Consulting Services. I wrote the first book on Microsoft XML published in 2000 called XML Programming with VB and ASP. Most recently I have been building tools for the SEO industry. Ask me for a 37 point SEO health-checkup for your website.
First posted :
03/29/2000
Times viewed :
944
Choosing schemas
by Mark Wilson
Originally, SGML was the flavour of
the month. SGML documents were structured using DTDs. With the
advent of XML, the DTD had some good features and some bad features.
Before we discuss the various types of schemas, let's take a look at the pros and
cons of DTD and schemas:
they are mature and are used in
several thousand applications,
they have an enormous number of
trained developers already using them from the SGML heyday, and
there are many tools out there for
using DTDs.
Schemas were born out of the
limitations to DTDs. For example:
DTDs are not written in XML and
therefore they are not available via the DOM,
DTDs do not have support for
namespaces, and
DTDs are good at describing
structure, but not the data contents – their data typing is poor and limited.
There is no support for currencies and so on.
Schemas on the other hand:
are built in XML and can therefore
be used via the DOM [Document Object Model] in Visual Basic or VBScript in
ASP,
provides
data types such as float, currencies and so on,
provides better relationships between elements,
enables you to create your own user-defined data types,
some form of inheritance is
indicated, and
namespaces are supported. Sidebar:
Since Schemas are XML documents themselves, there is a DTD (or
Schema) available for evaluating if your Schema itself is
valid.!
While Schemas will definitely figure
in your future projects, a consensus view is that the final proposal of the
working group on Schemas will not look like the current implementation of
Schemas within Microsoft IE5. For example, in August 1998 the Schema syntax
changed substantially. It is expected that will happen again once the working
draft becomes a recommendation.
Schemas... lot's of
them!
So, what do the different Schemas
offered as standards by different companies look like? There are also at
least 5 Schema formats, which are also trying to take over from DTDs:
