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

From:   Reto Gmür <reto@apache.org>
To:     "public-ldp@w3.org" <public-ldp@w3.org>, 
Date:   03/25/2014 07:28 AM
Subject:        Practical issues arising from the "null relative 
URIs"-hack



Hi,


More than one year has passed since Pierre-Antoine Champin explained why 
specifying LDP using the concept of "null relative URI" is problematic 
[1]. Unfortunately the concept of "null relative URI" is still in the 
latest version of the spec. This ties the LDP spec to some RDF 
serializations and probably violates RFC3986 according to which the 
*sender* is responsible for making sure that a base URI for the relative 
references can be established.

But the main point that LDP is no longer defined in terms of the abstract 
RDF syntax shows to be problematic when using higher level abstraction 
frameworks such as JAX-RS (the java standard for REST) to implement and 
LDP server or client.

A method that returns an RDF representation of a Resource would typically 
be defined like this:

@GET
public Graph getResourceDescription();

The JAX-RS runtime (more specifically so called MessageBodyWriters) will 
take care of serializing the returned graph into the format preferred by 
the client.

One would define a method that handles post requests with an RDF Graph as 
message body like this:

@POST
public Response postResourceDescritption(Graph graph);

Unfortunately this doesn't work to handle LDP POST requests as the message 
body cannot be converted to an RDF Graph until some application logic 
defined the URI for the new resource. All work around are quite horrible. 
One would be to have a type RelativeGraph to which text/turtle can be 
deserialized without a base URI, another one would be to take a String as 
argument and take care of the deserialization in the application code.


Pierre-Antoine original solution proposal included the usage of BNodes. As 
some people have strong feelings against BNodes this elegant approach 
might have been precociously discarded by some. 

A quick fix would be to simply define "null relative URI" (which is 
currently undefined both in LDP as well as in RFC 3986/3987) as 
http://www.w3.org/ns/ldp/null-relative/.


Cheers,
Reto



1. http://lists.w3.org/Archives/Public/public-ldp-wg/2013Mar/0077.html