Re: Comments on the LDP Spec: Creating new Resources

The normative definition of an LDP RDF Source is:
    An LDPR whose state is fully represented in RDF, corresponding to __an
RDF graph__.
(emphasis mine)

I can see why, reading the specification, one could come to the conclusion
that RDF Sources are RDF graphs.

The, admittedly non-normative, introduction starts with TimBL's 4 rules for
Linked Data. Given the name "Linked Data Platform", I can also see why one
could come to the conclusion that resources with URI identifiers should
return information in RDF about themselves, rather than something else.

Rob


On Thu, Oct 9, 2014 at 1:32 PM, Miguel Aragón <miguel.aragon@base22.com>
wrote:

> In our server resources are basically named graphs that contain a resource
> that shares the same name as the graph and additional resources that extend
> that resource (think, # resources). So for us, if you send a document like:
> [[ <somethingElse> a ldp:RDFSource. ]] to the URI:
> http://example.org/container/ we take that as if you are trying to create:
>
> graph <http://example.org/container/somethingElse> {
>         <http://example.org/container/somethingElse> a ldp:RDFSource.
> }
>
> That’s the way our platform works (not the way I think).
>
> On Oct 9, 2014, at 3:27 PM, Alexandre Bertails <alexandre@bertails.org>
> wrote:
>
> > 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
> >>>> CONFIDENTIALITY NOTICE: This e-mail message, including any
> attachments, is
> >>>> for the sole use of the intended recipient(s) and may contain
> confidential
> >>>> and privileged information. Any unauthorized review, use, disclosure
> or
> >>>> distribution is prohibited. If you are not the intended recipient,
> please
> >>>> contact the sender by reply e-mail and destroy all copies of the
> original
> >>>> message.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>
>
>
>


-- 
Rob Sanderson
Technology Collaboration Facilitator
Digital Library Systems and Services
Stanford, CA 94305