Re: Comments on the LDP Spec: Creating new Resources

On Thu, Oct 9, 2014 at 4:24 PM, Miguel Aragón <miguel.aragon@base22.com> wrote:
> I’m sorry Alexandre, could you elaborate? I didn’t understand your response.

Sure, but let me ask you a question first. You seem to think that the
presence of the triple [[ <somethingElse> a ldp:RDFSource. ]] in what
you post should create the resource <somethingElse>, module relative
URI resolution.

Is that what you think?

Alexandre

>
> 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 20:27:29 UTC