Re: Comments on the LDP Spec: Creating new Resources

Hi Nandana, thanks for responding.

Null URIs are actually very problematic, and (not null) relative URIs just make the problem worse. With the approach that we have: Generic Request URIs, hash URIs can be used in the same way:

Method: POST
URL: http://example.org/container/
Slug: miguel
Body:
@base <http://example.org/generic-requests/123123123123>.
<> a foaf:PersonalProfileDocument;
foaf:primaryTopic <#me>.

Is resolved to

<http://example.org/container/miguel> a foaf:PersonalProfileDocument;
foaf:primaryTopic <http://example.org/container/miguel#me>.

I honestly don’t see the case for using relative URIs (null or not null) at all. They bring many problems to the server and make the request document an invalid RDF document.




On Oct 9, 2014, at 1:55 PM, Nandana Mihindukulasooriya <nmihindu@fi.upm.es> wrote:

> Hi Miguel,
> 
> I guess the most common use case for the (not null) relative URIs is usage of hash URIs. For example, something like this.
> 
> <> a foaf:PersonalProfileDocument;
>    foaf:primaryTopic <#me> .
> 
> I think this case is less problematic because typically the profile document <> will become something like  <http://ex.org/container/miguel> and the <#me> becomes <http://ex.org/container/miguel#me>.
> 
> But if you have something like
> 
> <> a foaf:PersonalProfileDocument;
>    ex:property <anotherResource> .
> 
> This is a bit problematic because the resolution of it is a bit dependent of ending slash. The above snippet resolved against the base <http://ex.org/container/miguel> will become
> 
> (a) <http://ex.org/container/miguel> a foaf:PersonalProfileDocument;
>    ex:property <http://ex.org/container/anotherResource> .
> 
> and the same is resolved against the base <http://ex.org/container/miguel/> will become 
> 
> (b) <http://ex.org/container/miguel/> a foaf:PersonalProfileDocument;
>    ex:property <http://ex.org/container/miguel/anotherResource> .
> 
> However, I think LDP clients should never use the (a) with the slug to refer to itself because it can always use the null URI to refer to itself. We also discourage the use of dot segment relative URIs in the LDP BP. I wonder what are practical usages of non-hash relative URIs in POSTed content (before creation when the base of the document is unknown still). 
> 
> Best Regards,
> Nandana 
> 
> On Thu, Oct 9, 2014 at 6:25 PM, Miguel Aragon <miguel.aragon@base22.com> wrote:
> Hello to everyone
> Based on the design and implementation process that my team and I have experience, I've several comments about the LDP Spec that I'd like to share with you. But first lets make sure that we talk in the same language:
> 
> Concepts
> Note: Keep in mind that these are the concepts that are working for us. By no means I'm criticising the "Academic point of view"
> Relative URI: A relative URI that was not resolved to an absolute URI because the document didn't specified a base URI (@base).
> Null URI: an empty, relative URI.
> 
> Creation of LDP RDF Sources (LDPRS)
> There are several key points in section 5.1 Introduction that need to be considered:
> An LDPRS can be created by issuing a POST to an LDPC. 
> The client can specify a Slug header to provide a hint of the URI desired for the new resource.
> The examples show that a null URI can be used for the resource to be created. The resulting URI will be forged by the server.
> The LDP test suite goes beyond this and uses relative URIs in the resources that are POSTed to the server. (ex. <something> a ldp:RDFSource. ).
> At first we followed this approach, but when we started using JSON-LD as our main RDF format, we started encountering several problems with it:
> If non empty, relative URIs (ex. <something>) are accepted, it doesn't make much sense to support the Slug header. What would happen if both of them were used? 
> 
> Example: 
> Slug: something
> <somethingElse> a ldp:RDFSource.
> 
> By allowing the client to send both null URIs and non empty, relative URIs, a weird behaviour would be expected:
> If a null URI was used, forge a slug for the new resource and take the LDPC URI as a base for the URI of the resource to be created.
> If a non empty, relative URI was specified, treat that as a hint for the desired slug and use the LDPC URI as a base for the URI of the resource to be created.
> The logic needed for this behaviour will impose an unnecessary overhead for each request. 
> As far as we know, specifying relative URIs and not defining a base URI results in an invalid RDF document.
> If the server supported the creation of multiple resources on a single request, null URIs will overlap with each other.
> Common parsers (like Jena) don't treat null URIs and relative URIs consistently.
> Some of the possible approaches for addressing these problems are:
> The obvious solution would be to use fully qualified URIs on every request. But the client doesn't always know what the resulting URI will be.
> Another approach would be to use a placeholder, a fully qualified URI that the server knows it's acting just as a placeholder (Ex. <http://example.org/placeholder>). But that would mean the client is constantly specifying new triples for the same resource (in an academic point of view). And the problem of multiple resources on a single request wouldn't be solved by this approach.
> After some thought, we came with the concept of "Generic Request URI".
> 
> Generic Request URI
> A URI that has as a base, a known and never changing URI, and that ends with a slug that is different for every Generic Request URI created (in our case a timestamp).
> Example
> A template of the form: http://example.org/generic-requests/<timestamp> would create URIs like:
> <http://example.org/generic-requests/1412868212000>
> <http://example.org/generic-requests/1412868258000>
> <http://example.org/generic-requests/1412868262000>
> Using a Generic Request URI when creating resources covers the following problems:
> It standardises the URIs the server will receive.
> If the client wants to specify a hint, it would do so by passing a Slug header.
> Each request describes a unique resource and thus it is academically correct.
> Multiple resources can be created by declaring each one with a different Generic Request URI.
> 
> 
> So an LDP server would accept requests with the following forms:
> A resource with a fully qualified URI. In this case the client attempts to create a resource with a known URI so a Slug header isn't allowed and if the URI is already in use the server would respond with 409 Conflict.
> A resource with a Generic Request URI and no slug specified. The server would use the URI of the parent resource as a base and forge a slug for the new resource however the server is configured to do so.
> A resource with a Generic Request URI and a Slug header. The server would use the Slug header as a hint for the URI of the new resource to be created.
> I've more comments and concepts to share, but I will write another email for them.
> 
> -- 
> Miguel Aragón	
> Mobile: +52 (811) 798 9357
> Skype: miguel.araco
> Email: miguel.aragon@base22.com
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