Re: Comments on the LDP Spec: Creating new Resources

I’m sorry Alexandre, could you elaborate? I didn’t understand your response.

On Oct 9, 2014, at 3:21 PM, Alexandre Bertails <alexandre@bertails.org> wrote:

> On Thu, Oct 9, 2014 at 4:18 PM, Miguel Aragón <miguel.aragon@base22.com> wrote:
>> Hi Dave,
>> 
>> I’m ok with that, I just don’t want the LDP Spec to force us to support null
>> URIs and relative URIs without a base.
>> 
>> If that’s the case then the LDP test suite needs to be modified because it
>> sends both null URIs and relative URIs without a base (which may not be
>> allowed by all servers).
> 
> Miguel, you just do not control what resource get created using RDF,
> nor you control its interaction model...
> 
> Alexandre
> 
>> 
>> On Oct 9, 2014, at 3:14 PM, David Wood <david@3roundstones.com> wrote:
>> 
>> On Oct 9, 2014, at 16:04, Miguel Aragón <miguel.aragon@base22.com> wrote:
>> 
>> The approach that I’m offering allows applications to be moved from one
>> service to another. The problems with relative URIs are these:
>> 
>> 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. The base of the document needs to be: (parent’s URI)
>> + (slug created)
>> If a non empty, relative URI was specified. The base of the document needs
>> to be: (parent’s URI) <- making sure that it ends in a “/"
>> 
>> 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.
>> 
>> 
>> Hi Miguel,
>> 
>> Thank you for clarifying your position.
>> 
>> I think the thing that you are missing here is that the server always has
>> the final say. It is up to the server to decide what to do with a Slug or
>> when a base URI is missing. It might reject the request, use what it has or
>> something else. This is in accordance with Web Architecture.
>> 
>> For example, this issue report records what we (Callimachus Project) decided
>> to do:
>>  https://github.com/3-Round-Stones/callimachus/issues/163
>> 
>> Still, if LDP wants to specify this more tightly to assist interoperability,
>> it will need to be careful. Deciding quickly could break a lot of services
>> that are close to LDP compliance now.
>> 
>> Regards,
>> Dave
>> --
>> http://about.me/david_wood
>> 
>> 
>> 
>> 
>> On Oct 9, 2014, at 2:58 PM, David Wood <david@3roundstones.com> wrote:
>> 
>> On Oct 9, 2014, at 15:51, Miguel Aragón <miguel.aragon@base22.com> wrote:
>> 
>> You say you like them, but you haven’t addressed the problems that I
>> described. I’m not saying they should be prohibited, I’m saying it shouldn’t
>> be mandatory to support them.
>> 
>> 
>> 
>> Hi Miguel,
>> 
>> I honestly don’t see the problem you say you outlined and I did in fact give
>> you a use case since you said you don’t have one. Relative resolution of
>> URIs to the base allows portability in both data and applications built on
>> that data.
>> 
>> Why is it difficult to support the generation of a URI based on the
>> concatenation of a base URI and a relative URI? I am not trying to be
>> difficult, I just don’t understand why that is hard.
>> 
>> Regards,
>> Dave
>> --
>> http://about.me/david_wood
>> 
>> 
>> On Oct 9, 2014, at 2:43 PM, Andrei Sambra <andrei@w3.org> wrote:
>> 
>> 
>> 
>> On 10/09/2014 03:42 PM, David Wood wrote:
>> 
>> On Oct 9, 2014, at 15:11, Steve Speicher <sspeiche@gmail.com> wrote:
>> 
>> On Thu, Oct 9, 2014 at 3:05 PM, Miguel Aragón <miguel.aragon@base22.com>
>> wrote:
>> 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.
>> 
>> I believe this is a general misconception, the base URI to use for
>> resolution just instead carried outside the entity body.  Many RDF libraries
>> allow you to supply the absolute base URI to use for resolution when handing
>> off the model, this topic was discussed on the list some time ago [1].
>> 
>> Since it is a common stumbling block and not that clear, I would suggest we
>> include additional guidance in the best practices and guidance document [2].
>> 
>> 
>> 
>> For what it is worth, we just love relative URIs. This is because they allow
>> us to easy move applications from one service to another. We would be quite
>> unhappy if we could not both use relative URIs and be LDP compliant.
>> 
>> 
>> +1
>> 
>> Relative URIs are incredibly useful.
>> 
>> -- Andrei
>> 
>> 
>> Regards,
>> Dave
>> --
>> http://about.me/david_wood
>> 
>> 
>> 
>> - Steve
>> 
>> [1]: http://lists.w3.org/Archives/Public/public-ldp-wg/2014Apr/0008.html
>> [2]:
>> https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp-bp/ldp-bp.html#use-relative-uris
>> 
>> 
>> 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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>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>>

Received on Thursday, 9 October 2014 20:24:58 UTC