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Re: Practical issues arising from the "null relative URIs"-hack

From: <henry.story@bblfish.net>
Date: Mon, 31 Mar 2014 14:53:58 +0200
Cc: public-ldp@w3.org
Message-Id: <FD54BEE5-32B4-4A8A-A4C0-886A88BAC6A6@bblfish.net>
To: "Kingsley (Uyi) Idehen" <kidehen@openlinksw.com>

On 31 Mar 2014, at 13:49, Kingsley Idehen <kidehen@openlinksw.com> wrote:

> On 3/31/14 5:51 AM, Reto Gmür wrote:
>> 
>> 
>> 
>> On Fri, Mar 28, 2014 at 2:42 PM, Kingsley Idehen <kidehen@openlinksw.com> wrote:
>> On 3/28/14 7:02 AM, Reto Gmür wrote:
>>> On Fri, Mar 28, 2014 at 12:04 AM, Kingsley Idehen <kidehen@openlinksw.com> wrote:
>>> On 3/27/14 4:42 PM, Reto Gmür wrote:
>>>>> 
>>>> 
>>>> If you consider RFC5995 ( Using POST to Add Members to WebDAV ) http://tools.ietf.org/html/rfc5995
>>>> you need only consider that it does not say anything about relative URIs to understand
>>>> that because it says nothing it does exactly what we are proposing. If you were to use 
>>>> a RFC5995 compliant server to POST some Turtle with relative URIs in it, then you'd 
>>>> get exactly the LDP intended result. A turtle document that was posted with a <> URI would refer
>>>> to the document created.  
>>>> Granted. The same happens if you send an email with text/turtle content-type. Still, a bit far fetched to see this use as the intended design or even as to see  an established design pattern in that, imho.
>>> 
>>> This is an established design pattern, that's poorly understood. Relative URIs are really a major route to taking a lot of confusion and tedium out of Linked Data exploitation. 
>>> 
>>> I doubt about the this being established (given that it violates RFC3986). But maybe you're right and relative URIs would be elegant and powerful. But then they should be in the RDF data model rather than having LDP breaking the abstraction. (I'm a bit afraid relative URIs in RDF might also add more confusion by people expecting the URIs so be relative to the position of the resource in the graph).
>>> 
>>> What is need to create the body of a POST request using RDF toolkits with the current design:
>>> 
>>>   var i = new NamedResource("http://some.temporary.uri/that/must/not/be/used/elsewere/in/representation")
>>>   graph.addTriple(i, RDFS.descrition, "This is the resource that will be created")
>>>   ..add more triples
>>>   var turtleToPost = graph.serializeAsTurtleWithBaseUri("http://some.temporary.uri/that/must/not/be/used/elsewere/in/representation") 
>>> 
>>> In my opinion, using this throwaway URI and hoping it doesn't escape from the context of our code is a dirty hack.
>>> 
>>> If the RDF would support relative URIs things would be straight forward:
>>> 
>>>   var i = new NamedResource("") //Currently illegal and not supported by most toolkits!!!
>>>   graph.addTriple(i, RDFS.descrition, "This is the resource that will be created")
>>>   ..add more triples
>>>   var turtleToPost = graph.serializeAsTurtle()
>>> 
>>> Of course if RDF would support relative URIs we could use any serialization and also the problem I initially illustrated would be gone. 
>>> 
>>> The current question is not about using relative URIs or not but if the spec should be defined in terms of RDF or in terms of some particular serializations. The latter prevents RDF tools from being used, at least from being used  in a straight forward way.
>>> 
>>> Cheers,
>>> Reto
>> 
>> To be clearer, an actual RDF graph isn't comprised or relative URIs. Those URIs have to be absolute. Put differently, the final product of an RDF document processing pipeline has to be a graph comprised of absolute URIs. 
>> 
>> The problem is that with RDF tools you cannot create (unless you use a throwaway URI hack as described above) what needs to be posted against an LDP server.
>>  
>> 
>> There is still a subtle conflation of notation for describing an RDF graph and the actual serialization that manifests when said has been processed by a processor. For instance, a Turtle doc with relative URIs is basically an RDF description that a processor will transform into a final RDF document that's comprised of a graph with absolute URIs. 
>> 
>>  That's why I think that relying on turtle documents with undefined based-URI conflicts with the turtle spec that says: "A Turtle document defines an RDF graph composed of set of RDF triples"
>> 
>> Cheers,
>> Reto
> 
> As you can see, clearly, the habitual conflation of things in the world of RDF has struck again. 
> 
> The abstract syntax of RDF and notations for creating RDF document content are being conflated yet again. Basing any RDF based spec on a notation (rather than abstract model) simply repeats errors from the past. 
> 
> RDF specs should understand RDF and never undermine its abstract syntax. Likewise, Web based specs should understand HTTP URIs and their powers of denotation and de-reference. 
> 
> Turtle specificity, is causing problems here, clearly. 

The abstract syntax 1.1 says:
[[ 1.8 RDF Documents and Syntaxes
A concrete RDF syntax may offer many different ways to encode the same RDF graph or RDF dataset, for example through the use ofnamespace prefixes, relative IRIs, blank node identifiers, and different ordering of statements. While these aspects can have great effect on the convenience of working with the RDF document, they are not significant for its meaning.
]]

And then later: 

[[
Relative IRIs: Some concrete RDF syntaxes permit relative IRIs as a convenient shorthand that allows authoring of documents independently from their final publishing location. Relative IRIs must be resolved against a base IRI to make them absolute. Therefore, the RDF graph serialized in such syntaxes is well-defined only if a base IRI can be established [RFC3986].
]]

The abstract syntax clearly makes place for relative URIs. The whole point of what the LDPC spec is doing in defining a POST to an LDPC is to define a method for establishing the base URI without requiring the client to know it. 

If some syntaxes that do not support relative URIS have problems - and LDP does not and is not required to say something about those syntaxes -  this just shows that even though the meaning of the documents is defined in the end in terms of absolute URIs, there is a pragmatic loss for documents that do not support relative URIs. LDP is about the pragmatics ( protocol ) of publishing documents, and so  the loss does not show up in the meaning side of things.

Henry


> 
> -- 
> 
> Regards,
> 
> Kingsley Idehen	      
> Founder & CEO 
> OpenLink Software     
> Company Web: http://www.openlinksw.com
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> 
> 
> 
> 

Social Web Architect
http://bblfish.net/


Received on Monday, 31 March 2014 12:54:31 UTC

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