[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] is xml schema bad wrt schema extensions?
hi. recently (http://lists.xml.org/archives/xml-dev/200209/msg00032.html), paul prescod brought up the interesting issue of whether xml schema extensibility is a bad thing because naively designed software might break. i think this is a very interesting and important issue. paul is right that schema's derivation methods make it possible for software to fail when processing instances of derived schemas. so one basic question is: is this the fault of xml schema of or sloppy software authors? and, regardless of that, how can this be made more safe? i think so far there has been remarkably little effort to point out how to *safely and reasonably use* xml schema, in contrast to the large amount of effort in rather emotional schema bashing that can be seen on xml-dev... one point i would be very interested in is to look at this a bit more formally. for example, what kind of xpath can i safely apply to instances of schemas and derived schemas? only child paths? anything else? and if i restrict extension capabilities by some of schema's features, restricting type restriction and/or extension or type substitution, does this change the "safe set" of xpath? i have no answer to this question, but maybe this could lead to some framework for how to design schemas for extensibility, and how to restrict the possibilities for extensions to make it easier for software to deal with schema extensions. in a next step, one could start to think about how software authors could be supported to not write naively designed software. writing software which is forward compatible never has been an easy thing to do, but maybe it could be made easier, for example by disallowing certain xpath expressions which could break with extended instances. in a very futuristic view of the world of xml, i could even imagine a very smart dom module which only allows access to instances in a robust manner, making it impossible to write code that breaks. maybe some people will find this too complicated to be of any use, but there are a lot of people out there trying to use xml schema in a reasonable and robust way, and if all xml experts can come up with is advice like "it's complicated and dangerous", then we do not provide them a lot of help. cheers, erik wilde - tel:+41-1-6325132 - fax:+41-1-6321035 mailto:net.dret@d... - http://dret.net/ computer engineering and networks laboratory swiss federal institute of technology (eth) * try not. do, or do not. there is no try. *
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