Yes, you can use DOI’s or LSID’s, but then all the programs that process them have to know how to dereference them. Web crawlers and other software will need to able to recognize that the identifier is a DOI and then use DOI specific code to dereference it. There is a similar problem with LSID’s.

Are those who advocate DOI’s and LSIDs promising to write and maintain this code for all the semantic web libraries in use?

I think that there is confusion about URI’s because of their similarity to URL’s. They are the same as DOI’s and LSID’s in that the are globally unique strings of characters. They are different and preferable because they are relatively well supported and understood.

There are other existing ways to deal with the issues of server persistence, location independence.
The simplest way is to make you data (or part of it) available as an RDF dump.
My URI’s work just fine on the Linked Open Data cloud even if my server is down.

There are plenty of examples where this is useful

ISBNs
DOIs
Life Science Identifiers (LSIDS)

In many cases they can be magically transformed from URNs to HTTP URIs if required.

e.g.

amazon.co.uk/exec/obidos/ASIN/1844138518

http://dx.doi.org/10.1016/S1535-6108(02)00133-2

So URNs needn’t be HTTP URIs, but the two can happily co-exist without breaking any of the webs magical goodness…

For sure if you are using a different platforms then HTTP URIs won’t be appropriate – as you say ISBN is an appropriate URN for printed books.

But If the resource is available on the web i.e. over HTTP then I see no good reason to use any other URN other than HTTP URIs.

DOIs can’t easily be dereferenced even if they identify a web resource – why add another level of indirection? So goes for XRI.

If I’ve understood you right, you’re confusing Identifiers (URIs)> and Names (URNs ), which sometimes are the same thing, other times not.

If you say everybody should just “Use HTTP URIs so that people can look up those names” then you force people to tightly couple the names they give things with the identifiers they use for them.

I reckon sometimes, this is fine but other times its a bad idea. Consider ISBN’s which are URNs which can be used to identify books, completely independently of HTTP.

urn:isbn:1844138518 is a name which can obviously have many different URIs. Despite this, it is a perfectly good name that uniquely identifies a book by Chris Anderson.

The same is true for DOIs, urn:doi:10.1016/S1535-6108(02)00133-2 is a unique name for an interesting paper in biology.

So if you insist that all URIs should be HTTP URIs, (rather than sometimes just humble URNs), you are forcing people to tightly couple their naming and identity schemes to HTTP, even when it might not be appropriate for them to do so.

Just my €0.02!