Dan Brickley wrote:
On 15/5/09 18:20, Manu Sporny wrote:
Kristof Zelechovski wrote:
Therefore, link rot is a bigger problem for CURIE
prefixes than for links.
There have been a number of people now that have gone to great lengths
to outline how awful link rot is for CURIEs and the semantic web in
general. This is a flawed conclusion, based on the assumption that there
must be a single vocabulary document in existence, for all time, at one
location. This has also lead to a false requirement that all
vocabularies should be centralized.
Here's the fear:
If a vocabulary document disappears for any reason, then the meaning of
the vocabulary is lost and all triples depending on the lost vocabulary
become useless.
That fear ignores the fact that we have a highly available document
store available to us (the Web). Not only that, but these vocabularies
will be cached (at Google, at Yahoo, at The Wayback Machine, etc.).
IF a vocabulary document disappears, which is highly unlikely for
popular vocabularies - imagine FOAF disappearing overnight, then there
are alternative mechanisms to extract meaning from the triples that will
be left on the web.
Here are just two of the possible solutions to the problem outlined:
- The vocabulary is restored at another URL using a cached copy of the
vocabulary. The site owner of the original vocabulary either re-uses the
vocabulary, or re-directs the vocabulary page to another domain
(somebody that will ensure the vocabulary continues to be provided -
somebody like the W3C).
- RDFa parsers can be given an override list of legacy vocabularies that
will be loaded from disk (from a cached copy). If a cached copy of the
vocabulary cannot be found, it can be re-created from scratch if
necessary.
The argument that link rot would cause massive damage to the semantic
web is just not true. Even if there is minor damage caused, it is fairly
easy to recover from it, as outlined above.
A few other points:
1. It's for the community of vocabulary-creators to help each other
out w.r.t. hosting/publishing these: I just nudged a friend to put
another 5 years on the DNS rental for a popular namespace. I think we
should put a bit more structure around these kinds of habit, so that
popular namespaces won't drop off the Web through accident.
2. digitally signing the schemas will become part of the story, I'm
sure. While it's a bit fiddly, there are advantages to having other
mechanisms beyond URI de-referencing for knowing where a schema came from
3. Parties worried about external dependencies when using namespaces
can always indirect through their own namespace, whose schema document
can declare subclass/subproperty relations to other URIs
cheers
Dan
The most important point to take from all of this, though, is that link
rot within the RDF world is an extremely rare and unlikely occurrence.
I've been working with RDF for close to a decade, and link rot has never
been an issue.
One of the very first uses of RDF, in RSS 1.0, for feeds, is still in
existence, still viable. You don't have to take my word, check it out
yourselves:
http://purl.org/rss/1.0/
Even if, and I want to strongly emphasize "if" link rot does occur, both
Manu and Dan have demonstrated multiple ways of ensuring that no meaning
is lost, and nothing is broken. However, I hope that people are open
enough to take away from their discussions that they are trying to
treat this concern respectfully, and trying to demonstrate that there's
more than one solution. Not that this forms a "proof" that "Oh my god,
if we use RDF, we're doomed!"
Also don't lose sight that this is really no more serious an issue than,
say, a company originating "com.sun.*" being purchased by another
company, named "com.oracle.*". And you can't say, "Well that's not the
same", because it is.
The only "safe" bet is to designate some central authority and give them
power over every possible name. Then we run the massive risk of this
system failing (and this applies to microdata's reverse DNS as well as
RDF's URI), or it being taken over by an entity that sees such a data
store as a way to make a great profit. We also defeat the very principle
on which semantic data on the web abides, and that's true whether you're
support microdata or RDF.
Shelley
