May 19, 2010

"Conjuring" as a Linked Data design pattern

Domain Modeling, Linked Data

Everywhere I look there are blobs laying around that nobody quite knows what to name except "database", "record", or 1, 2, 3, 4, 5... Oh sure, some of these blobs are XML and they're indexed. Some of them are even accessible via a Web API using hackable query string parameters. Still, though, nobody's quite sure what to name the blobs. If you find yourself in a similar situation, try this:

http://example.org/database/1 (303 redirect to...)
http://example.org/database/1/ (2XX returning the blob)

Got those URIs working? Now go into the code for the latter and add this:

if (request.getHeader("Accept") == "application/rdf+xml") {

 <rdf:RDF
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:owl="http://www.w3.org/2002/07/owl#">
    <owl:Thing rdf:about="http://example.org/database/1" />
 </rdf:RDF>

} else {
    blob
}

That unnamable thing you've been calling "1" now has a useful globally-unique HTTP identifier that establishes its presence on the Semantic Web as Linked Data.

Now comes the conjuring trick I saw my colleague Andrew Houghton performing. Free your mind of the idea that this blob is one "thing" and write an elevator speech describing the use cases for the blobs. If your blob is XML, look at the element names to help you write the speech. Circle the important nouns in the speech and peer into the blob looking for such a thing. If you think you see one, append the noun to the latter URI with a "#" like so:

http://example.org/database/1/#noun1
http://example.org/database/1/#noun2

etc.

Now that these meaningful names have actionable HTTP identifiers, we need to announce their existence inside the <rdf:RDF> like so:

<owl:Thing rdf:about="http://example.org/database/1/#noun1" />
<owl:Thing rdf:about="http://example.org/database/1/#noun2" />

You've just conjured up meaningful things out of the blob and given them Linked Data names using semantically-rich HTTP URIs. Granted, some of the semantics implied in the URI aren't reflected in the RDF, but that is easily remedied. And if somebody says you are lying about the existence of noun7 for record 53, update your code and/or the blob to fix the bug (it's not a "lie" since it wasn't intentional). Apologize and explain to them that conjuring always has been and always will be an inexact science. ;-)

This "Conjuring" pattern is a trustworthy starting point for making the important things lurking inside your blobs discoverable and available for unexpected reuse.