I am attending the EMBRACE workshop in SOAP clients. A very interesting workshop I would say, and I'll wrote more about it when finished. During the workshop, it has been pointed out the usual issue of XML namespaces: the attribute value looks like a URL but it is not referring to anything in particular. An example from a SOAP envelope
<?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding"> </soap:Envelope>
Technically, the attribute value is a string that represents the namespace. The choice to use a URL stems by the fact that the DNS system guarantees uniqueness and authoritativeness, so if you define a new namespace, using your own domain guarantees (sort of) to be unique. However, there is no consensus about what this URL should resolve to. In some cases, it refers to the XML schema. In others to a DTD, or to a stylesheet, or more frequently to nothing. You can have a picture of the situation from these articles 1, 2, and 3. The first two articles, in particular, advocate the use of RDDL to solve the problem. Basically in the ambiguity of what to put, the answer with RDDL is: none of them. Instead, provide a RDDL document that says where to find each of them (if provided). Not a bad idea.
Waiting for the community to decide what to put at that address, my personal choice went toward a still standard but not deliberately confusing choice. I put a uuid URN.
which is valid according to standard (you can put any URN), it is unique and at least it does not pretend to look like it's referencing to something. You can generate one with
An objection to this approach is that a uuid is not easy to remember, and so you have to copy and paste it every time. Well, the URL approach has more or less the same issue. Namespacing does not work correctly if you don't specify the URL namespace exactly, so you end up copying and pasting it anyway.
Update: Just today, the W3C released an interesting plea to developers in order to limit the traffic at w3.org. Apparently, they get an insane amount of traffic, due to the attempts by various softwares around the net to get the documents referred by the addresses in DTD and namespace. This is another indication that you should be very careful in putting an URL, in particular if your format becomes very popular and you don't have big pipes to hold the traffic.