Creating Distributed Social Networks
with Linked Open Data and FOAF+SSL

[or “What’s your favourite cheese?”]

Toby Inkster BSc (Hons) ARCS <http://tobyinkster.co.uk/>

March 2009

The #swig Cheese Lovers' Club has decided to set up its own web site, allowing people to join the club and to share information about their favourite cheeses.

Despite all sharing a passion for cheeses, the cheese lovers are all very busy people, so don't have time to type in their favourite cheeses. Therefore they want to hook into the growing collection of linked open data, and simply select cheeses from DBpedia, the semantic web version of Wikipedia.

Also, despite all being very clever, the cheese lovers are very forgetful people, and don't like having to set up new user names and passwords for every online service they use. Therefore, they want to use an emerging technology, FOAF+SSL for identification.

The Database

Rather than use a traditional, relational database management system, the club opt to use a triple store for their information. As only a few people will be expected to visit the web site each day, it is decided that a flat N-Triples file will be adequate storage. If site traffic exceeds expectations there is a clear route to scale up to a bigger system, such as Seseme, Jena or OpenLink Virtuoso. As well as powering the web site, the club can directly expose the N-Triples file to the web, allowing it to act as the web site's (read-only) API.

The club itself can be represented in the database as a foaf:Group resource with foaf:member links to foaf:Person resources for each club member.

A person's cheese preferences will be represented by a foaf:topic_interest link to a cheese. (Technically this doesn't say that a person likes a cheese, but just that the person has expressed an interest in a cheese. That is close enough for the purposes of an example, but in real life, we'd probably want to define a more specific predicate.)

Populating the Database with Linked Open Data

The database is populated with an initial list of cheeses by performing the following SPARQL query on DBpedia.

SELECT DISTINCT ?resource ?label ?page ?depiction ?countrylabel
WHERE {
	?resource
		<http://dbpedia.org/property/wikiPageUsesTemplate>
			<http://dbpedia.org/resource/Template:infobox_cheese> ;
		<http://www.w3.org/2000/01/rdf-schema#label> ?label ;
		<http://xmlns.com/foaf/0.1/page> ?page .
	OPTIONAL {
		?resource <http://xmlns.com/foaf/0.1/depiction> ?depiction . 
	}
	OPTIONAL {
		?resource <http://dbpedia.org/property/country> ?country .
		?country <http://www.w3.org/2000/01/rdf-schema#label> ?countrylabel .
	}
	FILTER ( lang(?label) = "en" )
	FILTER ( lang(?countrylabel) = "en" ) 
}

Note that the dbprop:wikiPageUsesTemplate predicate is used as a proxy for the rdf:type query that might be expected in there, as in practice it seems to return a more useful set of data.

Authentication using FOAF+SSL

FOAF+SSL is a simple semantic-web-based authentication system. There are no user names and passwords to remember. You obtain a FOAF+SSL client certificate, install it into your browser, and then any FOAF+SSL-enabled site can identify you. Enabling FOAF+SSL for an HTTPS web site using Perl is a doddle. Simply install the CGI::Auth::FOAF_SSL module from CPAN and follow the instructions.

As soon as someone visits the club's home page, we know who they are and we can check our database to see if they're a member. Members get to explore the site; non-members are given the opportunity to join.

When someone opts to join, there are no forms to fill out: the sign-up process is as simple as clicking "Join". Using FOAF+SSL and a little SPARQL, the site can determine their name, homepage, photograph and so forth from their FOAF file. This data is cached in the site's database. (The site should rebuild its cache of personal data occasionally to ensure up to date information, but that feature has not been built yet.)

Because there is no long sign-up process acting as a barrier to becoming a member, potential members can start using the site straight away. (Indeed, if we wanted to we could remove the sign-up step altogether and simply add a person as a member as soon as they've picked a cheese to add to their favourites list.) Simple sign-up processes tend to encourage more members to join than arduous ones do.

Icing on the Cake

Cheesecakes tend not to have any icing, but here are some other noteworthy features:

Ideas for Further Development

Links

FOAF+SSL

DBpedia

#swig Cheese Lovers' Club

Acknowledgements

Thank you to the subscribers of the foaf-protocols mailing list, especially Henry Story, not only for the development of FOAF+SSL, but also for helping beta-test the cheese club website.