W3C home > Mailing lists > Public > public-hydra@w3.org > June 2014

Re: OLDM using the Hydra vocabulary for expressing property constraints

From: Benjamin Cogrel <benjamin.cogrel@bcgl.fr>
Date: Wed, 18 Jun 2014 11:12:07 +0200
Message-ID: <53A157E7.5020207@bcgl.fr>
To: "McBennett, Pat" <McBennettP@DNB.com>, "public-hydra@w3.org" <public-hydra@w3.org>
Le 18/06/2014 11:03, Benjamin Cogrel a écrit :
> Le 18/06/2014 09:53, McBennett, Pat a écrit :
>> Hi Benjamin,
>>
>> OLDM looks like an interesting project, and very clean - well done!
> Thank you!
>
>> I have a simple question though, one that I think applies to all the attempts I've seen at ORM-like mappers for RDF. Basically they all attempt to make things as easy as possible for developers (a good thing of course), and so they all try and make property access as 'programmer-friendly' as possible by using the dot notation.
>>
>> I know OLDM doesn't define a 'class' in the same way as other ORMs, but you still use the dot notation in your 'QuickStart' page examples [1]. For example, both 'alice' and 'bob' both have a property 'name', and the developer accesses that propery very simply, an intuitively, as 'alice.name' or 'bob.name'.
>>
>> But what if I need to allow 'alice' and 'bob' to have both a 'schema:name' [2] and a 'foaf:name' [3], and still be able to distinguish between the two of them? Is that even possible with OLDM? Surely I'm going to have to lose the intuitiveness of the current examples?
> I think OldMan provides a good answer to this problem: the mapping
> between attribute names and RDF properties is managed by the JSON-LD
> context (here is the one used in the QuickStart [1]). It allows you to
> map an arbitrary string (such as "emails") to RDF properties (like
> foaf:mbox).

Sorry: "map an arbitrary string to *one* RDF property".

>  So it is perfectly possible to give two different attribute
> names for the properties foaf:name and schema:name. You can even give
> different attribute names to the same RDF property as we did for the
> one-line bio to distinguish English and French values. JSON-LD is a
> pretty cool standard ;-)
>
> There is also a fallback mode for RDF properties that have no entry in
> the JSON-LD context: a "CURIE"-fied attribute name is generated (for
> instance foaf_name or schema_name). To conclude, when you see a short
> attribute name that maps to an RDF property, it means there is an entry
> in the JSON-LD context.
>
>> I know it's a fairly contrived example (i.e. you could argue why would you want to support both names when they 'mean' the same thing anyway), but for example on that same 'QuickStart' page you mention 'bob.types' as another example. But surely many vocabularies will want to define things they call 'types', so having multiple 'types' from multiple vocabularies within a single 'bob' or 'alice' instance is surely very important? How would OLDM support that?
> Good point.
> Actually, "types" must be considered as a reserved keyword (sorry, I
> have not documented it yet). "types" must be defined in the JSON-LD
> context as a JSON-LD term that refers to the "@type" keyword. It maps
> the rdf:type property.
>
> Thank you for your feedback, this will help me to update the documentation.
>
> Best
> Benjamin
>
> [1]
> https://github.com/oldm/OldMan/blob/master/examples/quickstart_context.jsonld
>
>> Cheers,
>>
>> Pat.
>>
>> [1] http://oldman.readthedocs.org/en/latest/quickstart.html
>> [2] http://schema.org/name
>> [3] http://xmlns.com/foaf/spec/#term_name
>>> -----Original Message-----
>>> From: Benjamin Cogrel [mailto:benjamin.cogrel@bcgl.fr]
>>> Sent: Monday, June 16, 2014 11:37 AM
>>> To: public-hydra@w3.org
>>> Subject: Re: OLDM using the Hydra vocabulary for expressing property
>>> constraints
>>>
>>> Le 14/06/2014 22:46, Markus Lanthaler a écrit :
>>>> On 13 Jun 2014 at 22:52, Benjamin Cogrel wrote:
>>>>> Le 13/06/2014 00:57, Markus Lanthaler a écrit :
>>>>>> This looks great. It's very intuitive. Great work! One thing that
>>>>>> isn't clear to me (by just looking at the documentation) is how you
>>>>>> do data validation. How does OLDM know that the email address isn't
>>> valid? Is that hard-coded for FOAF?
>>>>> Happy to hear that! Yes, you guess right, for the email address, we
>>>>> have a specific Python validator in Python that is registered for the
>>>>> foaf:mbox and schema:email properties. But the email address is just
>>>>> a special case and this practise should be avoided for regular properties.
>>>>> Currently, it checks:
>>>>>   - the XSD datatypes declared in the JSON-LD context,
>>>>>   - the container (set, list and language maps) when appropriate,
>>>>>   - the hydra:required, hydra:writeonly and hydra:readonly properties.
>>>>> This is clearly not enough to provide an alternative to what common
>>>>> ORMs do, so I have opened an issue for supporting new vocabularies [3].
>>>>> I will first give a try a SPIN-based data quality constraint ontology: [4] .
>>>> Do you intend to share this information with the client or will this just be
>>> used on the server side? In the latter case, I'm wondering whether it wouldn't
>>> be simpler to just create a mapping property-validator directly in code when
>>> you instantiate the ResourceManager or Model!?
>>>
>>> Yes, I intend to share this information with the client (except if the server
>>> wants to keep it secret). The client is then free to interpret it or not.
>>> What seems nice with SPIN templated constraints is that they allows us to
>>> define in the same time new RDF properties and their automatic
>>> transformations into SPARQL queries (please note that it does not imply the
>>> presence of SPARQL endpoint; a query can be simply applied to the RDF
>>> representation of a resource). I expect to generate new validators (and their
>>> mappings) automatically from the definition of these properties. Such
>>> declarative approach would kill two birds with one stone :-)
>>>
>>>
>>>>>> On the client side, in a lot of cases you have entities that look
>>>>>> different that those that the server gives you. So you probably want
>>>>>> something to map the representation returned by the server to your
>>>>>> local entities. You shouldn't require the client to use exactly the
>>>>>> same entities as the server as that would introduce tight coupling. From
>>> a Hydra point of view (and Web APIs in general) you probably also want
>>> some features that allow you to invoke operations.
>>>>>> Also, in most cases you probably can't assume that a SPARQL endpoint
>>> will be available.
>>>>>> So you have to navigate the resources expose by the server. Hydra
>>>>>> supports (and this will be
>>>>>> improved) some basic querying/filtering of collections. Ruben is
>>>>>> working on more sophisticated querying in a project he called Linked
>>>>>> Data Fragments [1]. He's also on this list and I'm sure he's more
>>>>>> than happy to answer any question you might have. Since this is
>>>>>> closely related to Hydra, those discussions would be very welcome on
>>>>>> this list btw. :-)
>>>>> Yes, I agree, on the client side we cannot always assume that (i) a
>>>>> SPARQL endpoint is available, (ii) the server will make no validation
>>>>> and (iii) accept our local representation. I will propose an
>>>>> interface to abstract the use of a SPARQL endpoint. For me,
>>>>> implementations of this interface (e.g. LD Fragments or Hydra
>>>>> clients) should in charge of mapping the client and server representations.
>>> What do you think?
>>>> The absence of SPARQL is one thing. The other thing I was talking about
>>> are entity representations themselves. A client might already have a Python
>>> class representing a person. When it retrieves the representation of a
>>> foaf:Person from the server, it somehow has to map the data it got to that
>>> class. Obviously that mapping has to be bidirectional.
>>> First, let me clarify a technical detail. By contrast with the other ORMs I
>>> know, there is no Python *class* representing a person in OldMan.
>>> There is one Model *object* that represents a RDFS class such as foaf:Person
>>> or my-example:LocalPerson [6]. A JSON-LD context + the schema describing
>>> a RDFS class + an IRI generator should provide enough information to
>>> generate a new Model object in most cases.
>>>
>>>
>>> Here is a recap about the scope of OldMan is and what it could be.
>>>
>>> The initial use case of this project is to provide an alternative to the ORMs
>>> used in Web frameworks like Django, Ruby on Rails or Symfony for building
>>> Web APIs. The relational database is replaced by a SPARQL endpoint that
>>> Web developers still control; the main point is to make them express most of
>>> the application logic declaratively in RDF and JSON-LD, not imperatively in
>>> Python. I like to think of this project as a transition tool for Web developers
>>> towards read-write Linked Data.
>>> Because adopting a declarative style is already a big move, I think it is
>>> important to preserve some reference points for this first step.
>>>
>>> As a second step, we can relax the assumption that the (main) Web API
>>> controls the SPARQL endpoint and see this Web API as the client of other
>>> independent (sub) Web APIs. OldMan, as an OLDM, is the module of the main
>>> Web API that is in charge of CRUD operations*. This OLDM can now be seen
>>> as the client of a datastore where the latter may use a different
>>> representation that the one of the main Web API** and may enforce its own
>>> data validation. I will propose an interface between the core of OldMan and
>>> client modules for interacting with SPARQL endpoints, LDF and LDP servers,
>>> Hydra Web APIs, etc. These client modules will be in charge of the mapping
>>> between local and remote representations you discussed.
>>> As agents, they will execute the CRUD "goals" assigned by the core part.
>>>
>>> You mentioned  the integration of non-CRUD hydra operations, this could be
>>> a third step.  Currently, the OLDM uses the Hydra description of the main
>>> Web API as the schema of its local representation. If non-CRUD operations
>>> should appear on Resource or Model objects, I think they should be
>>> operations provided (i) by the Web APIs the OLDM is client of or (ii) by a
>>> common abstraction of them. The latter abstraction would reduce the
>>> coupling. If I guess right, this would turn OldMan into a generic Hydra client
>>> library, isn't it? However, one thing still confuses me: how can we obtain nice
>>> Python methods (like [7]) from these Hydra operations?
>>>
>>>
>>> *: Please note that the CRUDController I mentioned my previous mail is just
>>> an extension that is useful only if you want to build a CRUD-like Web API.
>>> Even if your Web API is not a simple CRUD service, you may still need to use
>>> some CRUD operations internally.
>>> **: We assume that OldMan and the main WebAPI use the same
>>> representation.
>>>
>>>
>>>>>>> Also, in the future, I would like to support the "@reverse" JSON-LD
>>>>>>> keyword so I would be interested about having some "reversed
>>>>>>> supported properties".
>>>>>> We talked about this already. We might introduce a reversed flag
>>>>>> (similar to required) for
>>>>> supported properties to support it. This is already being tracked as ISSUE-
>>> 40 [2].
>>>>> Ok, I will give a try to the hydra:reverseOf property and see if it
>>>>> breaks my current design ;-)
>>>> Please share your insights on this as it will help us with the design of Hydra.
>>>>
>>>>
>>>>>> I find your project extremely interesting. What are your future
>>>>>> plans? What's still missing that you plan to add?
>>>>> Great, I hope it will be useful :)
>>>>> In addition to the points I mentioned, I have started a
>>>>> CRUDController [5] that manipulates resources having the same base
>>>>> IRI. This controller may be extended to implement the collection pattern.
>>>> Why did you decide to not expose those methods directly on your
>>>> models? You could also generalize this by evaluating the available
>>>> Hydra operations. This would then allow you to do things like
>>>>
>>>>    article.perform("LikeAction")
>>> The CRUDController is a module I have quickly written some time ago for
>>> having a better understanding of the scope of this project.
>>> As it will be a main component of some CRUD Web API implementations, the
>>> relation with some Hydra operations is something that should be clarified.
>>>
>>>> You should perhaps also use a term like "fragment-less IRI" or "hash-less
>>> IRI" instead of "base IRI" as the "base IRI" can have a fragment as well.
>>>
>>> You're totally right, thank you. Indeed, the "base IRI" term is confusing
>>> because it has a different meaning in JSON-LD and, for instance, in [8]. I have
>>> updated my code with the "hash-less IRI" term.
>>>
>>>> Keep up the good work,
>>>> Markus
>>>>
>>>>
>>>>
>>>>>> [1] http://linkeddatafragments.org/
>>>>>> [2] https://github.com/HydraCG/Specifications/issues/40
>>>>> [3] https://github.com/oldm/OldMan/issues/9
>>>>> [4] http://semwebquality.org/ontologies/dq-constraints
>>>>> [5] http://oldman.readthedocs.org/en/latest/oldman.rest.html
>>>> --
>>>> Markus Lanthaler
>>>> @markuslanthaler
>>>>
>>> Thank you Markus for this very interesting discussion,
>>>
>>> Benjamin
>>>
>>>
>>> [6]
>>> https://github.com/oldm/OldMan/blob/master/examples/quickstart_schem
>>> a.ttl
>>> [7]
>>> http://oldman.readthedocs.org/en/latest/oldman.html#oldman.model.Mod
>>> el.create
>>> [8] http://www.w3.org/blog/2011/05/hash-uris/
>
Received on Wednesday, 18 June 2014 09:12:42 UTC

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