There has been a lot of community discussion in the past few months about the scope of Linked Data and whether it is just limited to RDF. Paul Miller asked Does Linked Data need RDF? and opened an intense debate.
Recently Ed Summers posted this example of a SKOS concept modelled in Atom
Can this be considered Linked Data? Is it just a serialisation of RDF (RDF/Atom perhaps) and sits alongside RDF/XML, Turtle, RDF/JSON and NTriples? When people say Linked Data is RDF do they mean RDF the model or RDF the syntax?
Let's assume that beside the need of URIs, to be linked data you need a way to decompose it to triples, well, to map it to the RDF abstract syntax (you don't actually need a particular serialisation to manifest somewhere in the process).
Then I would say that if the transformation to RDF is not explicitly defined, it is not linked data, as you need to "sniff" the content and "know" how to transform/extract it.
But, once somebody puts a GRDDL "hint" in the Atom namespace document, all the atom feeds would become automatically linked data, right?
To answer the question, a particular Atom document is not linked data if there is explicitly defined mapping/transformation to RDF (in the document itself or in the namespace document); but if Atom becomes an agreed/standardised (controlled?) serialisation, then it would became linked data as the mapping to the RDF's abstract syntax would be "known".
Then I would push your question to even web pages with microformats (a html page with RDFa is linked data already, I hope) — if you have the proper html @profile defined, that means that you can follow links towards how to GRDDL the data? is it linked-data?
I would say yes. As long that you can follow an explicitly defined trail towards the data embedded there (via its extraction service).
Then what is not linked data? well, when you have to guess it, to use heuristic methods to "extract" it, that would not be linked data.
Twilight zone: a web page with a link rel="alternate" type="application/rdf+xml" towards its equivalent data, is it linked data? I would say not quite, as it points to a different document and since they were separated at birth, they may evolve independently.
answered 06 Nov '09, 16:32
Considering that there is no "RDF the syntax" (there's RDF/XML, n3/turtle, RDFa, SPARQL query results format...) I consider Linked Data as RDF to refer to the data model, i.e. a series of machine-readable triples that each consist of the identification of a resource and a property name/value pair about that resource. With its clear identification of bits of metadata about specific resources, treating an Atom file as a series of triples is pretty easy, and I know I've seen code to convert it somewhere, so I'd consider it to qualify as linked data.
answered 06 Nov '09, 16:14
This raises its head again with http://dowhatimean.net/2009/11/whats-in-a-name-and-the-linked-data-police ...
Atom can be parsed into triples, and it can link to URIs that dereference to more data from other places on the web.
But, if you are serious about publishing "linked data", about interoperability, about data becoming exponentially more useful the more it is connected with other data, I doubt you would choose Atom as your sole means of publishing that data, because the majority of RDF tools won't work with it.
In fact, it seems few people who are serious about "linked data" in any quantity, rest easy until they have provided 3 or 4 different serialisations of their data.
answered 21 Nov '09, 23:28
I put up my opinion on FriendFeed recently. I think Atom is too tied to the mental model of weblogs and 'streams' of data rather than graphs of data. There's nothing wrong with doing it that way, but I'm sceptical about whether it encourages the same kind of thing that's good about RDF.
There are far too many people who think that if they produce an Atom feed, they are now open and have satisfied their openness ambitions. Except, the stuff after the first 20 items on the feed gets neglected. It's much harder to do that in RDF-land. This seems to map up with a philosophical preference for the new: the archives don't matter, so long as the new stuff is open.
By saying this, I don't mean to suggest that people like Ed Summers and others who are applying Linked Data principles to Atom are Doing It Wrong. I'm saying that the knock-on effect is that many people are likely to think "oh, I've published an Atom feed. Huzzah! I'm doing it right!"
From a purely political perspective, this feels like brand dilution.
answered 21 Nov '09, 20:35
If the Atom uses universal identifiers in common with other sites, then yes. Otherwise no.
Rule of thumb: If it's linked to other bits of data (via URIs), it's linked data, otherwise it's just data.
answered 02 Dec '09, 02:14
Laurian: in the last paragraph you are conflating 2 things - if the content is DRY (that is, if the 2 representations can evolve separately) and if the content is linked data.
[link rel="alternate" type="application/rdf+xml"] itself is web document metadata and an RDFa statement (e.g. see this example). therefore, the document contains linked data.
the document that rel="alternate" points to is also a part of linked data ecosystem, as it provides some RDF data about the document and is linked to from the document.
P.S. re. DRY - it is trivial to construct an RDFa example where the HTML part and the RDFa part say different things. happens in "real life" as people make mistakes. so an RDFa webpage can get out of sync just as easily.
answered 13 Nov '09, 11:38
Datalinkyness comes in degrees; the more we make it sound like an all-or-nothing affair, the sillier we sound to the outside world.
Let's ask more practical questions: eg. If I publish my SQL database using Atom, is it hard to consume with RDF tools, mix with other RDF data and query with SPARQL? what's the best RDF/Atom combination these days? (Atom-OWL? Yahoo DataRSS? Something with GRDDL? does GRDDL get much real world use? when is it considered secure, is GRDDL much used with XSLT2, XQuery? etc ...?)
answered 26 Dec '09, 22:54
I agree completely with Christopher Gutteridge and DanBri. Something it's linked data if it uses URIs to identify data and links to other data via URIs. Linked data doesn't refer to semantics it only refer to links. Tim Berners-lee explains it here very clearly: