JPz’log

I am giving a Ruby lecture tomorrow (some students happen to be quite lucky in fact). I wanted to present an example of a Ruby On Rails style meta programming technique. For example you can have a find_by_name or find_by_email method on your ActiveRecord objects. Those methods are in fact generated on the fly using a technique which looks like what follows. This has probably been explained a million times somewhere else on the web, but I don’t care

So let’s start with a very basic contacts book class which looks like:

class ContactBook
  def initialize
    @contacts = Hash.new
  end
 
  def add_contact(name, email)
    @contacts[name] = email
  end
 
  def contact_named(name)
    @contacts[name]
  end
end
 
book = ContactBook.new
book.add_contact "Pierre", "pierre@zzland.fr"
book.add_contact "Julien",  "julien@zzland.fr"
book.add_contact "Yoan",   "yoan@zzland.fr"
book.add_contact "Mr Bean", "info@mrbean.com"

Nothing impressive here, so now let’s get all the contacts whose email contain a given substring:

class ContactBook
  def email_containing(str)
    @contacts.select { |key, value| value.include? str }
  end
end
 
def display_contact(contact)
  puts "#{contact[0]} <#{contact[1]}>"
end
 
zzland = book.email_containing "zzland"
zzland.each { |contact| display_contact(contact) }

But what if we could have email_thestringtolookfor methods instead of this? Let’s do it by evaluating some code on the fly:

class ContactBook
  def method_missing(id, *args)
    method_name = id.to_s
    if method_name[0..5] == 'email_'
      str = method_name[6..method_name.length]
      to_eval = <<-END_FUNC
        def email_#{str}
          @contacts.select { |key, value| value.include? "#{str}" }
        end
      END_FUNC
      instance_eval to_eval
      return eval("self.email_#{str}")
    end
  end
end
 
zzland = book.email_zzland
bean  = book.email_bean
zzland.each { |contact| display_contact(contact) }
puts "-----"
bean.each { |contact| display_contact(contact) }

1. method_missing is invoked whenever a message is sent to the class instance, but no matching method name can be found.
2. If the name starts with email_, then we are in luck! (note that I should throw an exception if the name doesn’t start with this to comply with the Ruby semantics, but I was too lazy for that)
3. We generate a new method for the instance through evaluation.
4. We do not forget to invoke the new method through evaluation, as the original call would return nil since the method had not been found.

Nice isn’t it?

Today I had fun playing with Rinda, the Ruby implementation of the Linda distributed process coordination paradigm. I also had some time for blogging, something that I haven’t done for a few days because of too much work (and there still a bunch left to be done )

I won’t go through the theory behind Linda, but the gross idea is to use a shared memory space that implements the distributed blackboard paradigm : the Tuple Space. It can be summarized on the following figure:

Data is represented as sets of tuples which often have a limited lifetime. The tuple space provides the distributed processes the following primitives:

• write: writes a tuple to the tuple space
• read: reads a tuple from the tuple space
• take: reads and removes a tuple from the tuple space

The beauty of the model lies in the fact that the tuple space guarantees atomic operations and periodically cleans up old tuples. Also, reading operations match a tuple on:

1. the length of the requested and available tuples
2. the matching (exact value, regular expressions, …) of certain elements of the tuple.

I have put together a first Ruby program that reads text from the console, and writes tuples whenever a line is entered. The other program takes every new tuple that is emitted.

emitter.rb

#!/usr/bin/env ruby
 
require 'drb/drb'
require 'rinda/ring'
require 'rinda/tuplespace'
 
DRb.start_service
 
tuple_space = Rinda::TupleSpace.new
ring_server = Rinda::RingServer.new tuple_space
 
while true
  tuple_space.write [:message, gets]
end

receiver.rb

#!/usr/bin/env ruby
 
require 'drb/drb'
require 'rinda/ring'
require 'rinda/tuplespace'
 
DRb.start_service
 
tuple_space = Rinda::RingFinger.primary
 
while true
  puts tuple_space.take([:message, nil])[1]
end

What I really like is the simplicity of the code, thanks to the cleanness of Linda and the elegance of Ruby The other thing is that DRb is such a straightforward way to use distributed objects in Ruby…

BTW there is a Java implementation of this paradigm called JavaSpaces.