W3C home > Mailing lists > Public > public-hydra@w3.org > January 2015

Re: Mapping between a generic Web API and an LDP Server

From: Miguel <miguel.ceriani@gmail.com>
Date: Thu, 29 Jan 2015 00:28:10 +0100
Message-ID: <CALWU=RvKkREyaS=D3Tum83pGAArsWaV29hf4OBoA186xekhEBw@mail.gmail.com>
To: public-hydra@w3.org
Thanks Tomasz and Markus for your warm welcome and the feedback!

On Tue, Jan 27, 2015 at 10:36 PM, Markus Lanthaler
<markus.lanthaler@gmx.net> wrote:
>
> On 27 Jan 2015 at 22:08, Tomasz Pluskiewicz wrote:
>
> > On 2015-01-27 18:10, Miguel wrote:
> >> Hi everybody,
> >> I am working on a platform to define linked data applications using a
> >> dataflow language based on SPARQL.
>
> Could you elaborate a bit on this? Or provide some pointers to get a better idea of what exactly you are working on?

The language basically allows the developer to define pipelines of RDF
operators to build complex RDF transformations, that may also hold a
state (e.g., to define interactive applications). The behavior of most
of the nodes of the pipeline is defined through SPARQL 1.1 queries,
that may also be dynamically generated by other nodes.
It is still in a very experimental stage, but I was able to build some
simple interactive visualizations with it.
Some basics facts about the platform and links to publications and
code are on swows.org

> >> I have a problem that is not striclty in the scope of Hydra, but it
> >> seems to me quite related.
> >> I hope that I am not completely out of topic.
> >>
> >> Let's say I want to build a server that have to comply with a given Web
> >> API, that uses JSON,
>
> So basically you want to build a server which exposes some interface in a very specific way. For example you want to build something which does the same as Twitter's or Facebook's API and also looks exactly the same. Is that correct?

Exactly.

>
> >> and I want to implement it backed on an LDP server.
>
> Hmm.. Not sure how I should interpret that as LDP defines an interface, not a backend implementation.

Well, may be I should better say "backed on a server supporting the
LDP interface".
That is, the software I would like to generate from the mapping
described below is a proxy/adapter between a client "speaking" that
specific API (e.g., Twitter's API) and a server with an LDP interface.

> >> I can define a suitable Linked Data model and then map it from/to JSON
> >> with a JSON-LD profile.
> >> Then I have to map  somehow the operations supported by the Web API with
> >> operations on LDP (assuming that the operations offered by the Web API
> >> are simple enough to be mapped directly to LDP operations).
>
> In other words, it just supports the CRUD operations.. right?

Exactly

> > I don't exactly understand the work flow. Could you please give some
> > example?

A basic example is that I there is some existing client using a
specific Web API.
I want to (re)design the server based on a LDP interface for
interoperability, but at the same time I want to keep using that old
client.

> >> The question is: is there a standard way to do this second mapping?
>
> Hydra does that. We even had the basic CRUD operations in Hydra (well, we still have them in the spec but decided to drop them [1]) which allowed you to do that. Now you have to either define your own operations or leverage Schema.org Actions to describe the semantics of the operation.

Great, thank you for the pointer to the discussion.
Is this usage of Schema.org Actions documented more in detail somewhere?

> >> I can do it ad-hoc in different server-side languages, but ideally I
> >> would use some RDF vocabulary, like Hydra.
>
> So you basically want to describe the Web API in RDF and generate the implementation out of that?

Exactly

> >> As a fact Hydra seems very close to what I need, because it maps Web API
> >> operations to their Linked Data meaning.
> >> The main difference is that I would like to use this mapping to DEFINE
> >> the behaviour of a Web API, while (if I understood its purpose
> >> correctly) Hydra is used to DOCUMENT (to the client) the behaviour of a
> >> Web API.
>
> It should be quite straightforward to use a Hydra ApiDocumentation to auto-generate a server implementing the API - as long as the operations are simple enough. Effectlively it is the opposite of what the HydraBundle [2] is doing... even though it has some CRUD controller code generation which does most of the work for you.

Then I will definitely try it.

> > Hm, Hydra is used to describe a Web API using RDF terms, yes. So what do
> > you mean by define? Like define so that it can be later implemented?

In the sense of being able to generate the implementation out of that
definition.

> >> In general, it seems to me that such mapping could be useful to
> >> integrate existing Web APIs with linked data workflows.
>
> This probably needs a bit more explanation as well. What are "linked data workflows"?

Well, I also have still only a vague idea of how it may work in
general, but I will try to describe my view.

I used loosely the term "linked data workflows" to mean basically any
system or agent interacting with linked data through standard
protocols like the LDP interface, Hydra documented APIs, SPARQL
endpoints, Linked Data Fragments interface, etc.

JSON-LD has been designed to bridge the gap between "semantic-less"
JSON based applications and the linked data / RDF world. One great
thing of JSON-LD is that by defining a profile you enable a two way
conversion, from JSON to RDF and back.

If you could do the same kind of thing with a whole Web API, I guess
it would be much easier to "port" existing Web APIs to the "linked
data world".
Especially if from this mapping I could generate two adapters, one
converting from Web API calls to LPD interface calls (or linked data
version of the API, if there are operations more complex than CRUD),
the other converting calls the other way around.

If, as an example, I define such a mapping for Twitter's API, I could
then create a linked data application that integrates with the
standard Twitter client and the Twitter server.
In this "linked Twitter" application the tweets sent by the client go
through an adapter to an LDP container (that is just an example, I do
not know if the Twitter web client can be easily connected to a
different server).
Then some (linked data) agents do some fancy stuff, enriching my
tweets with all sort of Linked Open Data and finally put the tweets in
another LDP container. That container is just a fa├žade of another
adapter connected to the real Twitter server, so that my enriched
tweets got finally published.

Pushing this scenario further, this Twitter's API-linked data mapping
had to be defined just once to enable people to build their
linked-twitter agents or more complex pipelines.

> >> What do you think about it?
> >
> > This should be possible with relative ease. Of course a kind of proxy or
> > adapter would be necessary depending on your setup. Essentially if I
> > understand correctly such use case is the main selling point of JSON-LD.
> > First step would be to enhance each existing JSON response with
> > @context, @type and @id. Assuming a good REST design, building the Hydra
> > ApiDocumentation would then be accomplished as if the API was Linked
> > from the beginning. And of course incoming payloads must be converted to
> > JSON-LD if necessary and normalized by using the right @context.
> >
> > Is that roughly what you have in mind?

Yes, that is exactly what I have in mind.
But I would like somehow to generate this piece of software from a
purely declarative definition of the mapping that could possibly
consists only of:
- the JSON-LD profile
- the Hydra API documentation

Thanks again,
Miguel
Received on Wednesday, 28 January 2015 23:28:58 UTC

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