Hacking on RDF in Ruby

2010/02/28 by Ben

RDF.rb is easily the most fun RDF library I've used. It uses Ruby's dynamic system of mixins to create a library that's very easy to use.

If you're new at Ruby, you might know about mixins in other languages--Scala traits, for example, are almost exactly functionally equivalent. They're distinctly more powerful than Java interfaces or abstract classes. A mixin is basically an interface and an abstract class rolled into one. Rather than extend an abstract class, one includes a mixin into your own class. A mixin will usually require that a given class implement a particular method. Ruby's own Enumerable class, for example, requires that implementing classes implement #each. For that tiny bit of trouble, you get a ton of methods (listed here), including iterators, mapping, partitions, conversion to arrays, and more. (If you're new to Ruby, it might also help you to know that #method_name means 'an instance method named method_name').

RDF.rb uses the principle extensively. RDF::Repository is, in fact, little more than an in-memory reference implementation for 4 traits: RDF::Enumerable, RDF::Mutable, RDF::Queryable, and RDF::Durable. RDF::Sesame::Repository has the exact same interface as the in-memory representation, but is based entirely on a Sesame server. In order to work as a repository, RDF::Sesame::Repository only had to extend the reference implementation and implement #each, #insert_statement, and #delete_statement. Nice! Of course, implementing those took some doing, but it's still exceedingly easy.

RDF::Enumerable is the key here. For implementing an #each that yields RDF::Statement objects, one gains a ton of functionality: #each_subject, #each_predicate, #each_object, #each_context, #has_subject?, #has_triple?, and more. It's a key abstraction that provides huge amounts of functionality.

But the module system goes the other way--not only is it easy to implement new RDF models, existing ones are easily extended. I recently wrote RDF::Isomorphic, which extends RDF::Enumerable with #bijection_to and #isomorphic_with? methods. The module-based system provided by RDF.rb means that my isomorphic methods are now available on RDF::Sesame::Repositories, and indeed anything which includes RDF::Enumerable. This is everything from repositories to graphs to query results! In fact, query results themselves implement RDF::Enumerable, and thus implement RDF::Queryable and can be checked for isomorphism, or whatever else you want to add. This is functionality that Sesame does not have natively, and which I wrote for a completely different purpose (testing parsers). Every RDF::Enumerable gets it for free because I wanted to compare 2 textual formats. Neat!

For example, here's what it takes to extend any RDF collection, from RDF::Isomorphic:

require 'rdf'
module RDF
  ##
  # Isomorphism for RDF::Enumerables
  module Isomorphic

    def isomorphic_with(other)
      # code that uses #each, or any other method from RDF::Enumerable goes here
      ...
    end

    def bijection_to(other)
      # code that uses #each, or any other method from RDF::Enumerable goes here
         ...
    end
  end

  # re-open RDF::Enumerable and add the isomorphic methods
  module Enumerable
    include RDF::Isomorphic
  end
end

Of course, this just can't be done without monkey patching. Mixins and monkey patching together make for a powerful toolkit. To my knowledge, this is the first RDF library that takes advantage of these features.

It's possible to provide powerful features to a wide range of implementations with this. RDF.rb does not yet have a inference layer, but any such layer would instantly work for any store which implements RDF::Enumerable. Want to prototype some custom business logic that operates over existing RDF data? Copy it into a local repository and hack away. No need for the production RDF store to be the same at all, but you can still apply the same code.

As a counter-example, compare this to the Java RDF ecosystem. There are some excellent implementations (RDF::Isomorphic is heavily in debt to Jena), but they're all incompatible. Jena's check for isomorphism is not really translatable to Sesame, or anything else. RDF.rb, in addition to providing a reference implementation, acts as an abstraction layer for underlying RDF implementations. The difference is night and day--with RDF.rb, you only need to implement a feature once, at the API layer, to have it apply to any implementation. This is not a knock at the very talented people behind those Java implementations; making this happen is a lot of work in a language without monkey patching, and RDF.rb is only as good as it is because of the significant influences those projects have been on Arto's design.

The end result of the mixin-based approach is a system that is incredibly easy to extend, and just downright fun. It would be a fairly simple task to extend a Ruby class completely unrelated to RDF with an #each method that yields statements, allowing it to work in RDF::Enumerable. Voila, your existing classes now have an RDF representation. Along the same lines, if one is bothered by the statement-oriented nature of RDF.rb, building a system which took a resource-oriented view would not require one to 'break away' from the RDF.rb ecosystem. Just build your subject-oriented model objects and implement #each, and away you go--you can now run RDF queries and test isomorphism on your model. Build it to accept an RDF::Enumerable in the constructor and you can use any existing repository or query to initialize your model.

RDF.rb is not yet ready for production use, but it's under heavy development and already quite useful. Give it a shot. You can post any issues in the GitHub issue queue.


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