Hahahacking

after.rb

module EventSourcing
  class AggregateManager
    include Celluloid

    def instance_of(klass, id)
      # So, in order to remove it when terminated, now I use supervise_as(), 
      # But I have to remove the exlusive mode because it can't be used with
      # the blocks that supervise_as() uses under the hood.
      # But now I don't think access to this method is serialized as it was
      # with __exclusive__
      klass.supervise_as(id) unless Celluloid::Actor[id] && Celluloid::Actor[id].alive?
      Celluloid::Actor[id]
    end
  end
end

before.rb

module EventSourcing
  class AggregateManager
    include Celluloid
    exclusive
    
    #On this first version I do that with the __exclusive mode__.
    #The problem with this approach is that when I terminate an aggregate,
    #due to inactivity timeout for example, it isn't removed from the registry.

    def instance_of(klass, id)
      Celluloid::Actor[id] = klass.send(:new) unless Celluloid::Actor[id] && Celluloid::Actor[id].alive?
      Celluloid::Actor[id]
    end
  end
end

explanation.md

Explanation

The idea here is that aggregate instances (actors) are singletons. For a given aggregate id, only one actor can be running concurrently. So, I need to have a single actor that serializes access to aggregates, that's the AggregateManager.