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    <p>If you look at the W3C's documents, you'll find some of the history of this. Basically, URIs were chosen for the same reason reverse-domain-names are often used for project names in Java: they aren't inherently meaningful, but they do provide some hint about who created them and what they might mean, and we already have practices in place to ensure against their being unintentionally reused (which could cause malfunctions.)</p> <p>Back in the early days of Namespaces, we were using URLs as namespace names. It was generalized to URIs later (after the decision was made that the namespace name didn't necessarily point to anything), but many of the issues were specifically phrased in terms of URLs so my discussion's going to mainly reflect that subset of URI space.</p> <p>There was at one point an intent that namespace names might be more meaningful. Tim Berners-Lee wanted them to point to someplace that provided additional information about the node types bound to that namespace -- not just a schema but an entry into his "Semantic Web" concept. That seems to have been dropped; I haven't heard much about the Semantic Web in years, at least not on more than a much-simplified and far-more-local level.</p> <p>There was also an attempt to define behavior for relative URLs as namespace names. After much debate, TB-L conceded that this really didn't have workable semantics either, and the W3C declared that all namespace names should be formatted as absolute URLs or URIs and compared simply as strings. (More officially phrased: The use of relative URLs as namespace names is Deprecated until and unless someone comes up with a strong reason to use them. Individual tools may not check this, but documents using relative URLs as namespace names are Not Portable so you're very, very strongly advised to avoid them.)</p> <p>If you are indecently lucky and/or whoever developed this document type is still trying to leverage the Semantic Web concept, there <em>may</em> be additional information about the namespace available at that URI. (That is, it may actually be functioning as a Uniform Resource Locator rather than just as a Uniform Resource Identifier.) But as far as the standards go right now, there's no promise that this point in URI-space is occupied, or what can be found there if so. Basically, dereferencing a namespace name is an undefined operation.</p> <p>It really is simpler, and more correct for now, to just treat the Namespace Name as a string that must be formatted as an absolute URL or URI for uniqueness' sake. </p> <p>(I used to have a pointer to an excellent document which explained this more clearly and more officially, but it's been years since anyone has asked me this particular question and I've misplaced it. If I find it, I'll update this answer.)</p>
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    1. POFundamental Confusion about XML Namespaces
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    1. COBeg pardon? I did say URI throughout my note. I'm not sure what you're objecting to. (I was involved in some of these discussions. It's an ... interesting ... experience to win an argument against a guy whose name is almost synonymous with the Web.)
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    2. COCan you give an example of a relative URI which is not a URL?
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      1. USJohn Saunders
    3. COAs mentioned in http://en.wikipedia.org/wiki/Uniform_resource_identifier, among others, "Technical publications, especially standards produced by the IETF and by the W3C, normally reflect a view outlined in a W3C Recommendation of 2001, which acknowledges the precedence of the term URI rather than endorsing any formal subdivision into URL and URN. I was on a W3C working group; I use the terms W3C-style. "Relative" applies to URLs, but URLs are a subset of URIs, and are the main form which was being referenced when the Namespaces In XML Recommendation was being designed, debated, and approved.
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