W3C home > Mailing lists > Public > semantic-web@w3.org > July 2005

Re: Atom for RDF transfer?

From: Mark Nottingham <mnot@mnot.net>
Date: Sat, 9 Jul 2005 22:05:44 -0700
Message-Id: <DACF56B8-90AA-4E04-968C-D6DE1172D279@mnot.net>
Cc: atom-syntax@imc.org, semantic-web@w3.org
To: Danny Ayers <danny.ayers@gmail.com>

Hi Danny,

What does Atom bring to the table here? Sounds like straight REST  
would do the trick admirably for you...


On 09/07/2005, at 3:18 AM, Danny Ayers wrote:

>
> While everyone's waiting for ratification of the Atom format, maybe
> there are a few brain-cycles that can be harnessed...
>
> What I (and others [1]) am looking for is a standard means of
> interfacing with an RDF store like Redland or Jena over HTTP. The
> operations required are:
>
> 1. query the store
> 2. add statements to the store
> 3. delete statements from the store
> 4. make an update to the store
> (5. sync stores)
>
> The first of these is reasonably well provided for already with
> SPARQL. There appears to be some commonality of aproaches to 2, 3 and
> 4, but no single standard seems yet to have emerged. 5. is something
> needed sometime before long, but could probably be fulfilled with the
> other points and an appropriate algorithm (possible approaches at
> [4],[5]).
>
> Now ideally all the operations would be done directly over HTTP, but
> there are one or two issues, and I'm wondering if the Atom
> format/protocol could form a consistent, easily implemented
> lightweight delivery mechanism as an alternative. It'd be tunneling,
> but without all the WS-* overhead. The interchange format specified
> for RDF is XML (i.e. RDF/XML) so that's the first hurdle hopped.
>
> This kind of thing is being covered to some extent by the W3C Data
> Access WG (DAWG) and Semantic Web Best Practices and Deployment WG,
> but they are tied to their charters. A good solution from left-field
> (built on good standards) could fast-forward development.
>
> So first the status quo, as far as I'm aware:
>
> 1. ask the store a query
> The SPARQL protocol and RDF query language [2] is emerging as the
> standard for queries, i.e. read-only operations. The query language
> itself is fairly SQL-like. The protocol as it currently stands has a
> generic WSDL 2.0 expression, but in practice *the* binding so far is
> to HTTP, using the query itself as a parameter in a GET:
>
> GET http://example.com/sparqlendpoint?query=...bunch of sparql...
>
> The results are returned as a XML doc in the response body, the format
> of which will depend on the nature of the query (there's a simple
> result-set format for SELECTs, RDF/XML for CONSTRUCT etc).
>
> The operations of 2, 3 and 4 can be covered by a protocol in a similar
> fashion: by supplying an RDF graph to add, a graph to delete, or a
> combined operation for update supplying a graph to delete followed by
> a graph to add. I'll just expand that a little before describing
> existing protocol support -
>
> 2. add statements to the store
> Data can be added to an RDF store by supplying a list of statements,
> that is the graph, as an RDF/XML doc. This is something all
> triplestores should support.
>
> 3. delete statements from the store
> This is a little trickier, there isn't any operation common to stores
> that says delete(graph). In practice this may mean listing and
> deleting the individual statements. I think there may be issues where
> the graph to delete matches a subgraph in the store where the nodes
> aren't sufficiently bound to URIs to make matching unambiguous.
> Frankly I'm not sure, but I don't think it would impact on the
> protocol.
>
> 4. make an update to the store
> A two phase operation, delete(graphA), add(graphB). It would be nice
> for this to happen as an atomic transaction.
>
> Ok, existing protocols/proposals include the NetAPI [6,7], and Joseki:
> RDF WebAPI [8] (I think this is currently moving over to SPARQL for
> queries, not sure about updates).
>
> The NetAPI use a two-part mime/multipart message with a HTTP POST, the
> first containing the graph to delete, the second containing the graph
> to add. Adding and deleting alone are special cases. (I seem to
> remember there being some issues relating to the use of mime/multipart
> around Atom, I can't for the life of me remember what they were).
>
> There's also URIQA, which can provide the operations listed, but is
> more aligned towards working with authoritative sources, i.e. where
> the host "owns" the resources identified. Technically it looks good,
> but involves the addition of extra HTTP methods, which has caused some
> controversy.
>
> So how might all this be done in Atom? I don't really know, beyond
> thinking perhaps that many of the interfacing operations with a
> triplestore may be exressed nicely as a sequence of entries, content
> as graphs, each entry representing an add/delete operation.
>
> Cheers,
> Danny.
>
> [1] http://lists.gnomehack.com/pipermail/redland-dev/2005-July/ 
> 001019.html
> [2] http://www.w3.org/TR/rdf-sparql-query/
> [3] http://www.w3.org/TR/rdf-sparql-protocol/
> [4] http://www.w3.org/DesignIssues/Diff
> [5] http://www.dbin.org/
> [6] http://www.w3.org/Submission/2003/SUBM-rdf-netapi-20031002/
> [7] http://www.wiwiss.fu-berlin.de/suhl/bizer/rdfapi/tutorial/ 
> netapi.html
> [8] http://www.joseki.org/protocol.html
> [9] http://sw.nokia.com/uriqa/URIQA.html
>
> -- 
>
> http://dannyayers.com
>
>
>


--
Mark Nottingham     http://www.mnot.net/
Received on Sunday, 10 July 2005 05:06:01 UTC

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