W3C home > Mailing lists > Public > www-rdf-interest@w3.org > July 2003

Re: URIQA questions

From: Benja Fallenstein <b.fallenstein@gmx.de>
Date: Mon, 07 Jul 2003 22:58:06 +0200
Message-ID: <3F09DEDE.2040806@gmx.de>
To: Patrick.Stickler@nokia.com
CC: www-rdf-interest@w3.org


Ok. So URIQA makes the following design decisions:

- It does not provide all authoritative information about a resource; 
instead, it provides an somewhat arbitrary, but well-defined subset that 
is judged to be reasonable in many cases.

- It doesn't provide a way to query a server for anything else than this 
subset of the authoritative information about a URI under the server's 
authority.

- In particular, it doesn't provide a way to query the server for 
additional information that may be necessary to understand the 
abovementioned subset of a URI's authoritative information.

I think I understand these design decisions. I was misled by my original 
assumptions about the goals of URIQA.

Thanks for your clarifications.
- Benja

Patrick.Stickler@nokia.com wrote:
> 
>>-----Original Message-----
>>From: ext Benja Fallenstein [mailto:b.fallenstein@gmx.de]
>>Sent: 06 July, 2003 02:30
>>To: Stickler Patrick (NMP/Tampere)
>>Cc: rdf-i
>>Subject: URIQA questions
>>
>>
>>
>>Dear Patrick,
>>
>>reading the URIQA spec at
>>
>>http://216.239.39.100/search?q=cache:HHP1uPSr1EUJ:sw.nokia.com
>>/URIQA.html+uriqa&hl=en&ie=UTF-8
>>
>>(for some reason I cannot access sw.nokia.com at the moment), 
> 
> 
> Our firewalls got upgraded last weekend and for some reason the
> routing to that particular server got munged. It's being fixed.
> Sorry.
> 
> 
>>I'm left 
>>wondering about two points. I would expect to be able to use 
>>the URIQA 
>>interface to query any information I need about a resource from its 
>>authoriative server, 
> 
> 
> Well, any authoritative information, at least. Not necessarily
> any information that is available everywhere on the SW on ever
> server.
> 
> 
>>but if I'm not missing something, this 
>>would not be 
>>possible.
>>
>>
>>First, the directionality in concise bounded resource 
>>descriptions seems 
>>an arbitrary restriction that means I cannot query many, but not all 
>>statements about a resource. 
> 
> 
> A concise bounded restriction is entirely resource-centric. It's
> not meant to replace or compete with more general query services
> or functionality, even those operating on the same knowledge
> base.
> 
> It is a lowest common denominator which can be broadly and 
> effectively implemented by all semantic web enabled servers,
> regardless of what additional functionality those servers might
> offer.
> 
> We can't expect every web server on the planet to implement an
> identical high level query API. Even if such a standard API
> exists (and there are folks working on it) we can't expect every
> web server to provide it, yet the URIQA model is both simple
> enough yet powerful enough to provide a common, standardized
> core of functionality for resource discovery that will enable
> SW agents to obtain authoritative knowledge about resources
> based solely on their URIs.
> 
> 
>>Unless the vocabulary in use has been 
>>designed to be used with URIQA, I may be missing out on something 
>>important. For example, consider this graph:
>>
>>     <http://example.org/>   homePageOf   
>><http://example.org/institute>.
>>     <http://example.org/institute>   rdf:label   "The Foo 
>>insititute" .
>>     <http://example.org/institute>   founded     "2003-03-17" .
>>
>>If I start from http://example.org/, all is fine; I can find the 
>>institute URI and then query its own description to get the name and 
>>founding date.
>>
>>But if I start at http://example.org/institute, I have no way 
>>of finding 
>>out about its home page, even though the semweb server knows about it 
>>and this can certainly be considered crucial information 
>>regarding the 
>>institute.
> 
> 
> With most SW and knowledge representation operations, ontology counts
> for alot.
> 
> 
>>There could of course be an inverse hasHomePage property, but why add 
>>this if it isn't necessary, except for URIQA?
> 
> 
> Well, this of course presumes that you know about the property homePageOf.
> 
> 
> 
>>It would seem much better to make the concise bounded description go 
>>both forward and backward-- i.e., also include statements in 
>>which the 
>>queried URI is an *object*, and so on.
> 
> 
> A concise bounded description is intended to be an optimal amount
> of knowledge for efficient interchange. If you include such a reverse
> lookup, the result graphs can quickly grow to be huge, depending
> on the resource. 
> 
> E.g., if you want a description about a URI denoting a classificatory
> term, in order to get its labels, description, etc., including all
> statements where it is the object could make the results jump from
> a dozen or so statements to millions of statements, particularly
> if inference is used (which is to be expected in many/most cases).
> 
> E.g. if subclass inference is employed, and one were to submit
> a query for rdf:Resource, one would get a statement for every
> resource that exists in the knowledge base (in addition to the
> rest of the relevant statements) since it can be infered that
> 
>   Ax ( x rdf:type rdf:Resource )
> 
> etc.
> 
> Even if inference is not employed, the resulting graphs can be
> very large (unexpectedly large) and could easily overload smaller
> agents.
> 
> 
> 
>>Second, the concise bounded description may include URIs not 
>>under the 
>>authority of the same web server, and yet the knowledge this 
>>server has 
>>about that URI may be crucial in interpreting the meaning of the 
>>statements served by the server. We can query this 
>>information using the 
>>URIQA servlet, but the reason for the HTTP extension to exist in the 
>>first place is that we may not know the location of the authoriative 
>>servlet in advance-- yet the HTTP extension doesn't seem to offer any 
>>way to discover it.
> 
> 
> Sure you do, if the URI includes a web authority component, such as
> an http: URI.
> 
> If you have http://example.com/foo and you execute a GET with
> URI-Resolution-Mode: Description (per the URIQA model) and the
> resulting description references http://anotherexample.com/bar
> then you can execute another GET with URI-Resolution-Mode:
> Description to get the description of the second resource, etc.
> ad nauseum until you have all the information you need.
> 
> Note that the real core of URIQA is not the Servlet API that is
> defined for the reference implementation, but the extensions to
> the Web architecture that allow one to query a semantic web
> enabled server based in the URI alone.
> 
> 
>>As an example, consider this graph:
>>
>>     <http://example.org>   ex:hasCreator   _:x .
>>
>>     _:x   foaf:name       "Jane Tripleson" .
>>     _:x   foaf:mailbox    <mailto:triples@example.org> .
>>
>>This is straight-forward enough, but consider this graph:
>>
>>     <http://example.org> ex:hasCreator
>>     <urn:urn-5:gK0wObL42bRyFllUsU+8cPL5cQBi> .
>>
>>     <urn:urn-5:gK0wObL42bRyFllUsU+8cPL5cQBi>
>>         foaf:name   "Jane Tripleson" .
>>
>>     <urn:urn-5:gK0wObL42bRyFllUsU+8cPL5cQBi>
>>         foaf:mailbox   <mailto:triples@example.org> .
>>
>>(A urn-5 is just a random number which is long enough to be 
>>unique, see 
>>http://www.iana.org/assignments/urn-informal/urn-5 .)
>>
>>This graph is perfectly valid-- but using URIQA, as far as I 
>>can see I 
>>can only retrieve a triple that means exactly nothing to me, 
>>without a 
>>way to find out more.
> 
> 
> Well, something like DDDS along with HTTP+URIQA would be needed
> if you are working with URNs.
> 
> Alternately, and what I and others recommend, you would use PURLs.
> 
> The combination of PURLs with URIQA make for a powerful solution
> that IMO nearly eliminates any need for the urn: naming scheme
> (with the single exception of those who need/want URIs that are
> entirely independent of any identifiable organization)
> 
> If you were to use, instead of urn:urn-5:gK0wObL42bRyFllUsU+8cPL5cQBi,
> something akin to http://purl.example.com/urn-5/gK0wObL42bRyFllUsU+8cPL5cQBi
> then you could simply do
> 
> GET http://purl.example.com/urn-5/gK0wObL42bRyFllUsU+8cPL5cQBi HTTP/1.1
> URI-Resolution-Mode: Description
> 
> to get a description of the resource, which could also include
> statements about which representations that URI would redirect to,
> etc. in addition to all other authoritative knowledge about the
> resource.
> 
> Services could be founded for the creation and maintanance of PURLs
> and their associated metadata, which have (nearly) all of the desirable
> qualities of URNs but exploit the web architecture, with semantic
> web extensions such as URIQA, to the fullest.
> 
> 
>>Ok, you could use
>>
>>     GET urn:urn-5:gK0wObL42bRyFllUsU+8cPL5cQBi HTTP/1.1
>>     Host: something
>>
>>using Host: to get this through proxies, but this would seem 
>>to violate 
>>the HTTP spec--
>>
>>     The Host field value MUST represent the naming authority
>>     of the origin server or gateway given by the original URL.
>>
>>Besides, this wouldn't help with URIs containing fragids.
> 
> 
> Don't get me started about fragids ;-)
> 
> 
>>IMHO URIQA should simply provide the URI of its servlet in its HTTP 
>>responses. (If this were also included in the response to 
>>OPTIONS, the 
>>use of URI-Resolution-Mode could even be completely avoided, using 
>>OPTIONS to find the servlet URI, then using the web service 
>>interface; 
>>it seems to me that if URIQA clients supported this, servers could be 
>>more easily configured for URIQA-- just add one header to the OPTIONS 
>>response and plug in a normal servlet or CGI.)
> 
> 
> This would, though, result in SW agents having to make multiple calls
> to the server for every single request for information.
> 
> SW agents should be able to surf the semantic web, from description
> to description, just as easily as web agents can surf the traditional
> web, from representation to representation.
> 
> This requires analogous behavior defined for semantic web enabled
> servers by the architecture. The header URI-Resolution-Mode: provides
> this in an elegant manner, by simply indicating to the server whether it
> should resolve requests in terms of descriptions or representations.
> 
> Otherwise, interaction between agents and servers is roughly identical.
> 
> SW agents should not be made second-class citizens of the web by having
> to make two system calls where other traditional agents only need make
> one.
> 
> Cheers,
> 
> Patrick
> 
> --
> Patrick Stickler
> Nokia, Finland
> patrick.stickler@nokia.com
>  
> 
> 
Received on Monday, 7 July 2003 16:59:33 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Monday, 7 December 2009 10:52:00 GMT