[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: URNs as SYSTEM IDs
At 10:41 AM 27/11/00 -0500, Michael Mealling wrote: >Correct me if I'm wrong here: so it is always the case that, in order >to parse some XML according to a DTD and the SYSTEM literal is a URI, >then I must have connectivity in order to download the DTD? In that >case, how are names for DTD standardized? I rarely see anyone related >to XML using a PUBLIC identifier. They all tend to use the SYSTEM id >as some sort of name instead. Well, the spec (4.2.2) says "The SystemLiteral is called the entity's system identifier. It is a URI, which may be used to retrieve the entity." I don't think this rules out strategies, ranging from caching to the use of "well-known" names, that don't require actually dereferencing the URI every time you see it. In fact, a URI can be a URN, and URNs are designed in part to *avoid* direct dereferencing. Another issue is that across the universe of XML processing, the proportion of times that the DTD or schema actually gets fetched is pretty small; for example, your average XHTML agent is not going to go chasing after DTDs in the course of displaying web pages, and your average b2b code probably doesn't do a lot of DTD munging. So I've always felt that SYSTEM identifiers were the way to go, and that this wasn't really a problem operationally. My feeling is that PUBLIC identifiers are a legacy concession to the SGML community, who had made good use of them, and a reflection of the fact that in that space, you can't do anything no matter how trivial without access to the DTD. But lots of smart people disagree with me and think that you should build your addresses around PUBLIC identifiers; history will tell. It is the case that the design of XML 1.0 is clearly biased toward the use of SYSTEM identifiers. >I can easily deprecate assigning a URN to a DTD in this spec. But >it does raise the issue of what the suggested practice from the W3C is. Deprecating URNs might be reasonable - lots of people think they're irrelevant or DOA - but nothing de facto or de jure about XML should make you feel you ought to do this. >Whether you give up or not, people will start using addresses as names >whether its possible to do so or not. The notion that there's a clean dividing line between addresses and names has been disproved over and over. But to the extent you can separate the naming and addressing functions, it seems that you win. -Tim
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