awesome_cache.rb

require 'nokogiri'

class Cache
  def initialize
    @store = {}
  end
  
  def read key
    @store[key] 
  end
  
  def write key, value
    @store[key]  = value
  end
end

$cache = Cache.new

module Api
  class Post
    
    def self.first
      etag =  $cache.read('data:etag')
      response = fetch_first(etag)
      if response == :not_modified
        puts 'cache HIT'
        return $cache.read 'data'
      else
        puts 'cache MISS'
        puts 'Parse XML'
        post = new(response.last)
        data = yield post
        $cache.write 'data', data
        $cache.write 'data:etag', response.first
        return data
      end
    end
    
    def self.fetch_first(etag)
      if etag.nil?
        puts 'Fetch XML'
        ["1449ee0ec320e5bf5ed7a9949d4771d9", "<posts><post>Hello!</post></posts>"]
      else
        :not_modified
      end
    end
    
    def initialize(xml)
      @doc = Nokogiri.parse(xml)
    end
    
    def body
      @doc.children.first.text
    end
  end
end


class Post
  def initialize(post)
    @post = post
  end
  
  def self.first(&block)
    Api::Post.first do |post|
      puts 'wrap in domain layer'
      block.call new(post)
    end
  end
  
  def body
    @post.body
  end
end

def render_view 
  Post.first do |post|
    puts 'generate HTML'
    "<h1>#{post.body}</h1>"
  end
end

3.times do
  puts render_view
end

Output:

Fetch XML
cache MISS
Parse XML
wrap in domain layer
generate HTML
    <h1>Hello!</h1>
cache HIT
    <h1>Hello!</h1>
cache HIT
    <h1>Hello!</h1>