May 15, 2009
Linked Data and httpRange-14 Concepts and Relationships
There is some understandable confusion about the relationship between concepts in Linked Data and httpRange-14. This class diagram may help:
Note that unlike httpRange-14, a Linked Data interpretation requires additional, but relatively lightweight, assumptions on the Architecture of the World Wide Web. Here are at least some of them:
- Clients can unambiguously classify a resource as a Real World Object (RWO) either by the presence of a hash in the HTTP URI (deduced prior to the HTTP request) or an HTTP 303 See Other status in the response (deduced after the HTTP request). In comparison, an HTTP 200 OK can be used to unambiguously classify the resource as a Web Document/Information Resource.
- This implies that Real World Object vs. Web/Document/Information Resource are disjoint.
- Every Real World Object needs to have at least one associated Web Document/Information Resource that contains information about it.
- Every Web Document describes one RWO. The fact that some of these RWOs are mash-ups doesn't change this fact. Traditional understandings of the Web commonly fail to identify these Real World Objects with HTTP URIs.
- The Web Document returned by dereferencing a RWO URI is expected to contain information about the RWO. Linked Data/httpRange-14 do not split hairs between data and metadata. It is all just information and information is invariably about something in the real world.
One of the beauties of embracing the Linked Data assumptions as a matter of policy is that they benefit others who may not even be aware of their significance.
Jeff
2 comments so far
Hi Jeff,
I think I disagree with the first two parts of your point (1), your point (3) and your point (4).
It certainly isn't true that a 303 redirect allows me to conclude that the identified resource is a "Real World Object".
See point (b) in http://lists.w3.org/Archives/Public/www-tag/2005Jun/0039.html: "the resource identified by that URI could be any resource" - "any resource" at all, including a Web document. The fact that the URI owner does not serve a representation does not mean that it does not identify a Web document.
It's only in the 200 case that - from the response code alone - I'm allowed to infer anything about the nature of the identified resource i.e. that it is a Web document/information resource.
And I don't think it is true that the presence of a # in an http URI allows me to conclude that the identified resource is a "Real World Object"; again, a hash URI can identify anything, including a Web document. e.g. I could say
<http://my.example.org/readinglist#doc1> owl:sameAs <http://your.example.org/doc1/>
and you could serve HTTP 200 and a representation for that second URI. (It might seem an odd thing for me to do but I think it's OK architecturally!)
I certainly disagree with (3): the ringbinder on my desk is a Real World Object, but there is no Web Document about it.
And I could even write you an email telling you I'm assigning an http URI (under a domain I own) to my ringbinder, and I can do that without creating a Web Document about it.
And I think I disagree with (4): I can create an HTML page containing only the text "gobbledygook". It's a Web Document but it doesn't describe a Real World Object. Or I could have a Web Document that describes another Web Document, not a RWO.
Also, a Web Document may describe multiple RWOs e.g. a single Web Document may describe several cars or several dogs or several ringbinders.
It may be that each of those cars/dogs/ringbinders is identified by a hash URI with a different fragment appended to the URI of that single Web Document.
Or for the 303 case, similarly: I could issue 303 redirects from several different URIs to the URI of a single Web Document containing descriptions of those multiple resources.
Pete,
This is excellent feedback. The issues are many and subtle, so I will try to address them in blog entries rather than as a direct reply here.
Jeff