gistfile1.rb

class PostController < ApplicationController
  def search
    searcher.search(params[:keywords])
  end

  private

  def searcher
    Feature::Set.toggled_on?(:some_feature) ? SunspotPostSearcher.new : AnotherPostSearcher.new
  end
end

# Knows nothing about searching
class Post
  # ...
end

# Knows how to instantiate Posts from search results
class SunspotPostSearcher < SunspotSearcherWithInstrumentation
  def search(keywords, sort_order = default_sort_order)
    super(keywords, sort_order).results.map do |result|
      make_post(result)
    end
  end

  private

  # Instantiates Post from result
  def make_post(result)
      # ...
  end

  def default_sort_order
    {:field => :published_at, :dir => :desc}
  end

  # Overrides super-class method
  def search_type
    :sunspot_post_search
  end
end

# Like SunspotPostSearcher above, knows how to instantiate posts from
# whatever its backend returns
class AnotherPostSearcher < SomeOtherSearchBackend
  # ...
end

class SunspotSearcherWithInstrumentation
  def initialize(options = {})
    @instrumentation = options[:instrumentation] || Instrumentation.new
    @searcher        = options[:searcher]        || SunspotSearcher.new
  end

  def search(keywords, sort_order = nil)
    super(keywords, sort_order).tap do |documents|
      @instrumentation.register_event(search_type, keywords)
      @instrumentation.raise_alarm(search_type, 'empty!') if documents.empty?
    end
  end

  private

  # Can be overridden by subclasses
  def search_type
    :sunspot_search
  end
end

class Instrumentation
  def initialize(options = {})
    @stats_collector = options[:stats_collector] || StatsCollector.new
    @alarm_handler   = options[:alarm_handler]   || AlarmHandler.new
  end

  def register_event(event_type, event_data)
    @stats_collector.register_event(event_type, event_data)
  end

  def raise_alarm(type, description)
    @alarm_handler.raise(type, description)
  end
end

class SunspotSearcher
  def search(keywords, sort_order = nil)
    Sunspot.search do
      fulltext keywords
      order_by sort_order[:field], sort_order[:dir] if sort_order
    end
  end
end