W3C home > Mailing lists > Public > public-ldp@w3.org > March 2014

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

From: Reto Gmür <reto@apache.org>
Date: Mon, 31 Mar 2014 22:55:45 +0200
Message-ID: <CALvhUEWUYTwSbm-NFSY2VDOeetmf8XxK3CPCKoR6RPXftRy5-w@mail.gmail.com>
To: "public-ldp@w3.org" <public-ldp@w3.org>
(somehow the thread moved to the other lists where I cannot post, so
forwarding to here)


On Mon, Mar 31, 2014 at 4:12 PM, Kingsley Idehen <kidehen@openlinksw.com>wrote:

>  On 3/31/14 8:53 AM, henry.story@bblfish.net wrote:
>
>
>  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 <http://www.w3.org/TR/rdf11-concepts/#dfn-rdf-graph>composed of set of
> RDF triple <http://www.w3.org/TR/rdf11-concepts/#dfn-rdf-triple>s"
>
>  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<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-concrete-rdf-syntax> may
> offer many different ways to encode the same RDF graph<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-rdf-graph>
>  or RDF dataset<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-rdf-dataset>,
> for example through the use ofnamespace prefixes<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-namespace-prefix>,
> relative IRIs, blank node identifiers<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-blank-node-identifier>,
> and different ordering of statements. While these aspects can have great
> effect on the convenience of working with the RDF document<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-rdf-document>,
> they are not significant for its meaning.
> ]]
>
>  And then later:
>
>  [[
> *Relative IRIs:* Some concrete RDF syntaxes<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-concrete-rdf-syntax>
>  permit relative IRIs as a convenient shorthand that allows authoring of
> documents independently from their final publishing location. Relative IRIs
> must be resolved against <http://tools.ietf.org/html/rfc3986#section-5.2>
>  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<http://tools.ietf.org/html/rfc3986#section-5.1>
>  [RFC3986<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#bib-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
>
> Henry,
>
> I understand the excerpts above, if you look closer you'll notice we have
> "abstract syntax" and "concrete syntax" which is utterly confusing. What we
> have in reality are:
>
> 1. A language -- RDF
> 2. Notations -- Turtle, RDF/XML, RDFa, HTML+Microdata, TriG, TriX etc..
>
> An abstract RDF graph isn't comprised of entity relations where
> participants are denoted using relative IRIs, they are always absolute
> IRIs. Basically, if you base a spec on notations you basically skew the
> entity relations semantics expressed and represented by abstract RDF
> statements (triples) .
>
> The fact that RDF (the language -- signs, syntax, and semantics) is
> loosely coupled with notations (so called concrete syntaxes) doesn't imply
> that one builds an RDF specification atop specific notations, and then
> expect no confusion as a consequences of subtleties across RDF's many
> associated notations.
>
> The spec has to address the problem that's arising here by enabling
> implementers understand the fact that Turtle (as a notation) can be used to
> *declare* an RDF graph (which is where relative HTTP URIs come into play,
> for instance) as well has being an RDF graph serialization format (where
> the serialized RDF graph is never comprised of relative URIs). A Turtle
> processor understands these fundamental rules.
>

The spec could be clearer and say that for the POST turtle is not used to
serialize a graph but that here it serializes a graph template[1] (which
is, IIUC what refer to as graph declaration). But such increased clarity
wouldn't change anything to the fact that the spec is built on specific
notations, abusing their relative IRI feature to build such RDF templates.


> If we can be a little clearer, the confusion can be addressed.
>

To me the problem is not confusion, but the inability to use every
serialization (e.g. serializations that allow signing of the sent graph)
and  RDF tools in a straight forward manner. This problem arise from the
spec being (for these part) designed for the notation rather than for RDF.

The spec could promote the use of relative URIs consistently against the
URI of the HTTP request whenever the serialization supports this. But the
actual normative part should be based on RDF. I think this would better
satisfy both the RDF as well as the REST[2] goal of the charter.


Cheers,
Reto

1. https://twitter.com/AndySeaborne/statuses/450574666936360961
2. http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-drivensays
"A REST API should spend almost all of its descriptive effort in
defining the media type(s) used for representing resources and driving
application state, or in defining extended relation names and/or
hypertext-enabled mark-up for existing standard media types." however LDP
is spending effort in specifying how an already defined media type should
be interpreted.

 --
>
> Regards,
>
> Kingsley Idehen	
> Founder & CEO
> OpenLink Software
> Company Web: http://www.openlinksw.com
> Personal Weblog: http://www.openlinksw.com/blog/~kidehen
> Twitter Profile: https://twitter.com/kidehen
> Google+ Profile: https://plus.google.com/+KingsleyIdehen/about
> LinkedIn Profile: http://www.linkedin.com/in/kidehen
>
>
>
>
Received on Monday, 31 March 2014 20:56:11 UTC

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