Re: XML Object Serialization
** Reply to message from Kal Ahmed <kal@t...> on Tue, 29 Oct 2002 21:16:51 +0000 > How about coding to an abstract data model with greater expressive power than > a simple XML schema (small s). Like a topic map or RDF abstraction for > example ? (I'm only slightly joking here ;-) Interesting suggestion, so let me give you a real-life example. I'm a co-editor of MDDL, the Market Data Definition Language (www.mddl.org). MDDL treats most of its elements as "properties" whose values can be inherited from ancestors if they haven't been directly defined on a child element. This makes the instance files much smaller where there is repeated information (like the currency), but with >260 property elements, it makes the Schema impossible to write by hand with any semblance of quality control. So, for version 1.0 of MDDL, we (the editors) wrote a basic Schema by hand without any inheritance or other shorthand tricks, and I wrote an XSLT stylesheet to process that into the full Schema (as well as creating the nearest equivalent DTD). This worked, but it wasn't completely manageable. There are typically multiple ways to construct an XML Schema to achieve the instance document format that you want. That becomes a nightmare when you want to process the Schema automatically to enhance it, because of the need to cope for likely variations. Worse, as time goes by, there is a real risk that new editors will introduce new and unforseen kinds of variations in the Schema, and then everything would fall in a heap. What I needed was a way of imposing a particular style on the base Schema. You could do this by writing code to check the style, but for a generic language of any sort (schema, topic map, RDF, programming language, or anything else), this is not a trivial task. Instead, I went back 20 years to when people advocated the creation of small, custom programming languages for particular problem areas (using lexx/yacc or flex/bison), so that developers were forced to focus on the problem itself rather than on which of the many features of their bloated generic language they would use. My solution, then, was to create a small, tight, custom schema language in XML for the MDDL data model, one which only allows just enough functionality to do the things needed for MDDL, and nothing more. We have used this for MDDL 2.0, and it works! While the main MDDL Schema is 600K, the data model is 30K, and I have been able to use the data model to produce not only the Schema but a visual representation of MDDL http://www.mddl.org/LMS_MDDL/index.html which would have been all but impossible to produce from the final Schema. Now, this is not a criticism of XML Schema nor Topic Maps, RDF, etc. XML Schema is a good input format for Schema validation engines that help remove a ton of verification code from applications consuming XML. However, the variety of things that XML Schema must support in order to do that job means that it can be hard to use it as a design format and maintain sufficient consistency of design. Similarly, Topic Maps and RDF are good formats for information engines to work with, but when authoring, the same questions of how to maintain consistency arise. Until design tools allow that consistency of style and usage to be imposed on Schemas, Topic Maps, RDF, & whatever, there is a lot to be said for starting from a small, custom XML format, and then generating the various generic formats from that. That said, I expect that Topic Maps will be a similarly useful starting format once ISO finishes the Topic Map Constraint Language and tools start supporting it. Cheers, Tony. ==== Anthony B. Coates, Information & Software Architect mailto:abcoates@T... MDDL Editor (Market Data Definition Language) http://www.mddl.org/
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