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Serialization spec in the OA core

From: Bob Morris <morris.bob@gmail.com>
Date: Thu, 9 Aug 2012 11:10:17 -0400
Message-ID: <CADUi7O78K=gw+rNBBMZA7To_HDTcz0jH7ew8+OdAJxVjpgkp_Q@mail.gmail.com>
To: public-openannotation <public-openannotation@w3.org>
A claim and and a question about the core spec section 2.4

Executive summary: How is it determined if an implementation of OA
complies with Sec 2.4?

1. My claim: If an Annotation can be serialized as some form of RDF,
and the Annotation has an HTTP URI, then it is \always/ possible to
provide a resolution and dereference (in the sense of IETF STD 66
(http://tools.ietf.org/html/rfc3986 ) by use of a SPARQL SELECT query
submitted to  a SPARQL 1.1 endpoint for delivery of a serialization of
a graph which somehow "is" the Annotation graph.  This resolution and
dereference is in no way different from the typical HTTP URI
resolution and dereference in which the URI is first used as a key to
a local cache and then used as a parameter to a DNS service request
which returns an IP address that is then used as a parameter for an
HTTP protocol request to that IP address.

2. The question: In the core oa spec, what is the meaning of the
sentence Sec 2.4 (Serialization
http://www.openannotation.org/spec/core/#Serialization) "If the
Annotation has an HTTP URI, then when that URI is dereferenced, the
Annotation's serialized graph, and only the graph, MUST be returned in
an appropriate graph serialization format. " ?

As to 1, by resolution and hence dereference, STD 66 does not mean
that an HTTP URI must be the value of the argument to an HTTP GET request.
Indeed, as above, it rarely is.  It only means that some provision is
made to determine and apply an access mechanism:
    'URI "resolution" is the process of  determining an access
    mechanism and the appropriate parameters  necessary to dereference a
    URI; this resolution may require several   iterations.  To use that
    access mechanism to perform an action on the URI's resource is to
    "dereference" the URI.'   (From rfc3986, Sec 1.2.2).
Probably my claim is independent of whether the URI is an HTTP URI or
not, and probably so is the scheme of using a SPARQL endpoint to
provide dereferencing.

As to 2.,  The sentence in the spec is silent on the nature of the
resolution (in the sense of rfc 3986) of the URI before dereferencing.
Hence  to be compliant, the above SPARQL-based resolution, the only
requirement is that the SPARQL query, when launched return the Annotation's
serialized graph, and only that.  My problem is: by what criteria do I
measure whether this dereference returns "the Annotation's serialized
graph and only the graph"? In other words, how do I test compliance?
Perhaps this is really a question for an implementation more than the
spec. But doesn't
it still raise the question of how to decide that the implementation meets the
spec as to the "MUST"s in Sec 2.4?
As Stian has remarked elsewhere, and as is true even for vanilla HTTP
URI resolution, there is no expectation that the same resolution and
dereferencing will today produce the same graph as it did yesterday.

One possibility is that what's meant by returning "the Annotation's
serialized graph and only the graph" is that the dereferncing  returns
a rooted labelled digraph whose
root has an edge labelled rdf:type and taget node labelled
oa:Annotation and that root label is a  URI that is
the same as the "original" . Alas, just about anything could be such a
graph. Do we
instead mean that at the time of creation of the Annotation, the graph
is in some way immutably  persisted somewhere (a triple store? I think
not...)  and this "original"  is  graph-theoretically compared to the
dereference? What comparison would be useful if the SPARQL endpoint
supports adding assertions about resources referenced in the
Annotation--especially resources in the oa namespace, such as the
addition of another Target?

The above SPARQL mechanism is important to us because we require to
support a semantic pub/sub Annotation system.  Using a SPARQL query
(usually behind the scenes), users explicitly  express a semantic
interest in what kind of annotation about whose creation and
publication they wish to be notified.  We use SPARQLPuSH
(code.google.com/p/sparqlpush) to manage the subscriptions and
notifications. We use Jena Fuseki to provide the SPARQL endpoints.
Fuseki even supports invoking SPARQL queries via HTTP GET calls, so
the actual resolution can be to something which is even an HTTP URI,
though f rfc 3986 does not require that resolution be a mapping of one
HTTP URI to another.

Robert A. Morris

Emeritus Professor  of Computer Science
100 Morrissey Blvd
Boston, MA 02125-3390

IT Staff
Filtered Push Project
Harvard University Herbaria
Harvard University

email: morris.bob@gmail.com
web: http://efg.cs.umb.edu/
web: http://etaxonomy.org/mw/FilteredPush
The content of this communication is made entirely on my
own behalf and in no way should be deemed to express
official positions of The University of Massachusetts at Boston or
Harvard University.
Received on Thursday, 9 August 2012 15:10:51 UTC

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