Multiton

Multiton design pattern ensures only one object is allocated for a given state.

The ‘multiton’ pattern is similar to a singleton, but instead of only one instance, there are several similar instances. It is useful when you want to avoid constructing objects many times because of some huge expense (connecting to a database for example), require a set of similar but not identical objects, and cannot easily control how many times a contructor may be called.

  class SomeMultitonClass
    include Multiton
    attr :arg
    def initialize(arg)
      @arg = arg
    end
  end

  a = SomeMultitonClass.new(4)
  b = SomeMultitonClass.new(4)   # a and b are same object
  c = SomeMultitonClass.new(2)   # c is a different object

How It Works

A pool of objects is searched for a previously cached object, if one is not found we construct one and cache it in the pool based on class and the args given to the contructor.

A limitation of this approach is that it is impossible to detect if different blocks were given to a contructor (if it takes a block). So it is the constructor arguments only which determine the uniqueness of an object. To workaround this, define the class method ::multiton_id.

  def Klass.multiton_id(*args, &block)
    # ...
  end

Which should return a hash key used to identify the object being constructed as (not) unique.