Category Archives: RDF

http://www.w3.org/RDF/
skos:related http://crschmidt.net/blog/archives/author/crschmidt/skos.rdf#c8 RDF

Introducing SPO(G)

ARC, PHP, RDF, SPARQL

SPO(G) is not a new syntax, nor a format or a protocol. It is, however, a syntactic profile and a convention.

<sparql xmlns="http://www.w3.org/2005/sparql-results#">
  <head>
    <variable name="g"/>
    <variable name="s"/>
    <variable name="p"/>
    <variable name="o"/>
  </head>
  <results>
    ...
  </results>
</sparql>

I have previously written about exchange of named RDF graphs through the use of quads-in-zips, and while that approach works just fine, it needs to be implemented on both sides of the exchange.

This is also true for SPO(G), but with SPARQL implementations being widespread, the export side is already in place all around the web, and the import side is quite easily implemented — I have sent an implementation of SPARQLXMLResultLoader for ARC to bengee, and while he is also busy working on a streaming serialiser, it seems likely it’ll be a part of a coming release of ARC.

As can be seen from the example, SPO(G) is simply a constrained SPARQL Query Results XML Format: It needs to have three or four variables, s, p, and o must be present, with g being optional (making it YARS). For all results, all variables must be bound.

SPO(G) isn’t as compact as quads-in-zips, but there’s no reason for it not to be compressed during exchange, either through a manual process or via the usual gzip-encoding on-the-fly.

I should perhaps write it up properly, but I think I’d rather go off and implement it for Redland.

DOAP from the Bazaar

Python, RDF, Release

Last year, when I first released bzr-feed for generating an Atom feed for a Bazaar repository, I added an item to the TODO almost while writing the first lines:

* Add RDF/XML output with DOAP support

Just now, I removed that item, not because I updated bzr-feed, but because I have created a new Python script for generating DOAP using the same technique: bzr-doap.

Usage is quite simple — simply add something along the following lines to your .htaccess file, and you’re good to go (presuming the usual cgi-bin stuff is in order):

RewriteCond %{REQUEST_FILENAME} !-s
RewriteRule (.*).rdf$ bzr-doap.cgi?dir=$1

Output is generated based on the information present in the bzr branch, but it can be augmented/overridden through the use of a .doaprc in the current and/or parent directory, like this:

[Project]
short_desc: DOAP generator for a Bazaar repository branch.
description: A CGI script for automatically generating DOAP for a Bazaar repository branch.
programming-language: Python
license: http://usefulinc.com/doap/licenses/python
[Maintainer]
foaf_homepage: http://www.wasab.dk/morten/

There is of course a DOAP for bzr-doap, and as usual an Atom feed for you to follow its development in its Bazaar repository.

Named Graph Exchange

ARC, Perl, PHP, RDF, Redland, Release

Following up on Exchange of Named RDF Graphs and the rapidly developing ARC2 RDF system, I have written a PHP/ARC2 version of my script for parsing and serialising a graph archive, and repackaged the original version into a single script for Redland.

I will be using this for testing ARC2 (performance) with my photo database, to see if I can manage a simpler interface without sacrificing the excellent performance from Redland. So far, it seems parsing might be a bottleneck, but that isn’t really important, if the query handling is good (so far it looks great, I can implement a SPARQL endpoint in 6 lines of PHP) — I can do batch processing offline.

You can find the scripts and some example archives in its bzr repository: named-graph-exchange, and download the whole package in .zip– or .tgz-format.

Next in the pipeline is an implementation that talks to a SPARQL endpoint, only downstream for now, but possibly using SPARQL+ or SPARUL for remote updates in the future.

The scripts are licensed under the Eiffel Forum License, version 2, per sbp’s considerations.

Authorization by Codepiction

FOAF, Image Description, RDF, SPARQL

Over the last fours years and counting, I have been providing access to a triplestore with description of my photos through a facetted interface. Through those years I have received several requests for excluding people from the searchable interface, as they showed up quite prominently on Google.

It turned out to make the interface much less useful for me — and possibly for others as well — so not too long ago I ended up simply excluding photos depicting people — using SPARQL, of course:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?Photo 
WHERE { ?Photo foaf:depicts ?Person .
        ?Person a foaf:Person }

That, of course, didn’t really improve things from my personal perspective, so watching others experiment with OpenID and social networking, I decided to take the same route — with a twist.

I didn’t really want to maintain or discover a social network for this use, but then it dawned on me: My photos actually represent a social network, as codepiction describes relations between people even better than explicitly stated ones.

With that in mind, I got the most recent version of the PHP OpenID Library up and running, and now use SPARQL to extend the non-person subset created above with photos of codepictees for users logged in with their OpenID — assuming they match with the descriptions in the store:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?Photo 
WHERE { ?Photo foaf:depicts ?Friend .
        ?OtherPhoto foaf:depicts ?Friend .
        ?OtherPhoto foaf:depicts ?User .
        ?User ?ifp ?OpenID .
        ?ifp a owl:InverseFunctionalProperty }

Note that since I don’t restrict ?User to be distinct from ?Friend (and ?Photo from ?OtherPhoto), the query also returns photos depicting the user. The query also doesn’t explicitly look for foaf:openid, but rather any inverse functional property, and since I’ve used foaf:mbox/foaf:mbox_sha1sum quite a lot in the descriptions of the photos’ depictees, I have added the option to verify your e-mail address — it is not enough to trust what’s returned from an OpenID provider, as there is no validation, and I wouldn’t want everyone to create OpenID identities with my e-mail address.

All in all, you can now use OpenID to log into my facetted interface and — if you are recognized — get access to additional photos. Try it!

Quite slick use of codepiction, if I may say so myself…

NB: I cheated. Parts of the backend still use an old RDQL engine, so some of this was actually not done in SPARQL. Also, I supplemented the above codepiction query with queries regarding albums with codepiction and photographer information (mostly useful for myself, of course), meaning you will actually see more photos than you might expect.