Re: XML Database Decision Tree?
Date: Wed, 24 Oct 2001 10:36:12 +0200 From: Nicolas LEHUEN <nicolas.lehuen@u...> This raises the following question : are there schema mutation tools in native XML databases ? I.e. you provide a new target schema and the XSLT stylesheet to apply to move one collection of document from an old schema to the new one, and the database processes all relevant documents in a safe way, e.g. by locking the collection or any other means that would prevent clients to get a mix of old and new documents. Short answer: Yes. eXcelon's "Stylus Studio" tool has a component called the "XML mapper", which provides assistance in defining a mapping from one DTD to another, and generates a stylesheet that implements the defined mapping. Caveats: Some possible mappings (even some possible DTD's) aren't expressible using the XML mapper's graphical notation. Stylus Studio allows you to edit the generated stylesheet by hand, and does its best to recover the graphical representation of the modified mapping, but that recovery isn't always possible. Considering stylesheets in isolation as representations of mappings, there is no assurance that all possible mappings are representable in XSLT, and the properties of a particular stylesheet with respect to source and target DTD's are not easily (if at all) provable. I don't know of any standard notation or theory for reasoning about DTD's and relations or mappings between DTD's, in general. Assuming that you can find such a notation for the particular case at hand, and can represent the desired mapping as a stylesheet (and somehow prove or test or otherwise convince yourself that it does in fact represent the desired mapping), then you can use the XSLT processor in XIS to generate a new file from the old. Also, the current implementation may have effective size limits on the XML document being transformed; future releases will increase or eliminate these limits. (This info is from Paul Rabin, the engineering manager in charge of eXcelon's native XML database product, officially known as Extensible Information Server or XIS.) (Also, XML documents in XIS aren't required to have prescriptive schemas. So if your position was that you didn't want a prescriptive schema in the first place, but just wanted to allow any well-formed XML, XIS can work that way too. But I guess you were referring to cases where you do want a prescriptive schema and then you want to change it.) -- Dan
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