Last May, in Adding semantics to make data more valuable: the secret revealed, I showed how storing a little bit of semantics about the word "spouse"—the fact that it's a symmetric property (that is, that if A is the spouse of B, then B is the spouse of A)—let me look up someone's home phone number in my address book even if my entry for him there lacks his home phone number. I like this story because unlike biotech and some of the other popular domains for Semantic Web technology, everyone has an address book and understands the basic properties of an entry: first name, last name, email address, and so forth. (Because so many people have lived through the annoyances of moving their contact information from one email client or phone to another, address books also provide nice use cases for data integration issues.)

Back then, I wrote:

With software that understands an OWL expression stating that spouse is a symmetric property and a rule I define to say that spouses have the same home phone number, I can retrieve Leroy's home phone number...

OWL is great for defining the symmetry, but I glossed over the part about defining the fact that spouses have the same phone number. How do you define such a rule? n3 has a rules language, but I haven't seen it used much as the n3 subset known as Turtle (which leaves out such things) becomes more popular. Instead of defining a Semantic Web rules language, the W3C has decided to have the Rules Interchange Format Working Group standardize an interchange format between the many rules languages out there. (The W3C Rules Interchange Format Basic Logic Dialect PowerPoint presentation by WG co-chair Chris Welty provides good historical background.)

I can write a query that generates the triples I want to infer and call this query a "rule", but what do I do with it?

I've used a proprietary RDF rules language before, and was wondering if a standard one would come along. Some colleagues at TopQuadrant have shown me that we all have a straightforward, standardized RDF rules language right under our noses: SPARQL. I've been appreciating SPARQL's CONSTRUCT form more lately, and CONSTRUCT is the key here: like a SELECT statement, a CONSTRUCT statement defines conditions about which pieces of which triples to retrieve, but unlike SELECT, a CONSTRUCT statement assembles these into new triples. If we view a CONSTRUCT statement as the definition of a rule and the resulting new triples as the result of the execution of the rule, then we have a rules language and plenty of implementations of it available.

For example, the following SPARQL "rule" says that if ?person1 has the spouse ?person2 and the home telephone number ?phoneNum, then ?person2 also has the home telephone number ?phoneNum:

PREFIX  : <http://www.snee.com/ns/demo#>
PREFIX v: <http://www.w3.org/2006/vcard/ns#>

CONSTRUCT { ?person2 v:homeTel ?phoneNum . }
WHERE {
  ?person1 :spouse   ?person2 ;
           v:homeTel ?phoneNum .
}

When run with the following data (for the purposes of this demo, assume that the {:leroy :spouse :loretta} triple was generated by an OWL reasoner that saw {:loretta :spouse :leroy} and knew that :spouse was symmetrical),

@prefix  : <http://www.snee.com/ns/demo#> .
@prefix v: <http://www.w3.org/2006/vcard/ns#> .
:loretta :spouse   :leroy ;
         v:homeTel "434-923-9321" .
:leroy   v:workTel "434-932-5329" ;
         :spouse   :loretta .

It generates the triple {:leroy v:homeTel "434-923-9321"}.

OK, so I can write a query that generates the triples I want to infer and call this query a "rule", but what do I do with it? What makes it a rule about a particular set of data?

Holger Knublauch, a co-worker of mine who designed and developed the OWL plugin for Protégé before coming to TopQuadrant, recently wrote an RDF vocabulary called SPIN ("SPARQL Inferencing Notation"), which—among other things—can express associations between these rules and classes. So, for example, if the blank node rdf:_1 pointed to the query above, the following triple would associate this query rule to the v:Address class:

  v:Address spin:rule rdf:_1

To make the storage of the SPARQL rule in a triplestore even cleaner, Holger has implemented a way to store SPARQL queries as triples, and he's written the code to roundtrip between this and the standard text version. (See the SPARQL Text to SPIN RDF Syntax Converter for an online converter, and see spinrdf.org for more about what else the SPIN vocabulary can do, especially his blog entries as he developed it. I'm now finishing up a tutorial for the use of SPIN features in TopQuadrant products, and except for one optional step of the tutorial, it all works with the free version of TopBraid Composer.)

When you take it a little further, symmetrical properties and many other parts of OWL can also be implemented with SPARQL queries, and there's a lot going on among those who are doing this to find a sweet spot between RDFS and OWL Full that meets typical business needs without using a lot of processing power or dollars.