I've always been curious about the semantics (in terms of Web Architecture) of XML Catalog resolution of URIs. If an XML Catalog takes in one URI A and outputs a second URI B, what is the relationship between resource A and resource B? <A> owl:sameAs <B>? Or is the relationship at the discretion of the author of the XML Catalog under consideration?

The XML Catalogs specification does not take a position on what an alternate URI means, nor do I think it should.

While it is very often the case that the relationship is owl:sameAs, this is demonstrably not always the case. For example, on my system, all references to the DocBook 4.x DTDs are redirected to the current development version of V4. This is convenient for me (usually), but it could not reasonably be asserted that DocBook V4.1 was owl:sameAs DocBook V4.5b2.

When I've looked at XML Catalog resolution of URIs, it feels somewhat like RDF. When following an arc on an RDF graph (with no literals), you start at one URI and end at another. You can think of each step in XML Catalog resolution as one step away from a resource in the traversal of an RDF graph. Naturally, the next question is then, "why not use RDF?" Here are the features that I like about XML Catalogs for this functionality, as opposed to RDF. With XML Catalogs you get a natural functional behavior; with a graph, you would have to impose additional constraints on top of the graph model. Also, RDF syntax is unwieldy for this type of functionality. XML Catalog syntax is nice and straightforward for the behavior it provides.

If we are going to use XML Catalogs for general functional mapping of URIs, then I believe we need to correctly treat both inputs and outputs to XML catalogs as URIs (when performing URI resolution). As such, I want to take this opportunity to point out a comment that I made earlier this week, asking why the uri element does not specify that both the source (attribute name) and destination (attribute uri) are not both of type uri-reference. (See also the section of the XML Catalog specification discussing the uri element.) To me, this makes particular sense when you allow for resolution relationships besides owl:sameAs.