Re: DTD versioning
John Wilson <tug@w...> asked about DTD versioning. Thre was some work done at OCLC on software to manipulate DTDs; try the Research section of www.oclc.org. Where I forsee this being important, I generally take one of two different tactics:  include a DTDrev attribute on the outermost "doctype" element, and make it #REQUIRED, <!ATTLIST document DTDrevision "2" #REQUIRED> Now every document must contain this to be valid, and I can therefore find documents that may need changing if the DTD's major revision changes.  include the DTD version in the filename, SYSTEM and/or PUBLIC identifier of the DTD. It's required in practice in a PUBLIC identifier for SGML, since resolving (fetching) the same PUBLIC identifier twice must always result in the same information. In XML, support for PUBLIC identifiers is optional, so it's best to avoid them and use only SYSTEM identifiers, but you can still include a version number there. For SGML, it's actually a fairly difficult problem to compute compatible DTD changes, especially in the face of the rules about inclusions and how they affect the interpretation of whitespace. XML does not have these complexities. None the less, a mechanical change is often the smallest part of the effort, because SGML has in the past been used mostly for representing documents rather than for database interchange. In documents, especially in the _descriptive_ or transcriptional use of SGML, well, an example: if i prevoisly had <placename> <keyword> <italic> in my DTD and i now decide that some italic words are really people, so i add <people> i must now inspect every <placement> <keyword> or <italic> to see if perhaps a person's name has been put there, and change it. If I could do that automatically, I probably would have tagged it like that in the first place :) Not making existing documents syntactically invalid is one thing; not making them semantically invalid is another. For database interchange, the semantic issues can be controlled more tightly, I think, and this is an area that could be researched more. Lee -- Liam Quin -- the barefoot typographer -- Toronto lq-text: freely available Unix text retrieval IRC: discuss XML/SGML/XSL/XLL/DSSSL Mondays irc.technonet.net in #XML email address: l i a m q u i n, at host: i n t e r l o g dot c o m xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...)
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