api_post.rb

class Api::Post
  def self.first
    etag = $cache.read('data:etag')
    response = fetch_first(etag)
    if response == :not_modified
      puts 'cache HIT'
    else
      puts 'cache MISS'
      $cache.write 'data:etag', response.first
      $cache.write 'data', response.last
    end
    new(etag)
  end
  
  def self.fetch_first(etag)
    if etag.nil?
      puts 'Fetch XML'
      ["1449ee0ec320e5bf5ed7a9949d4771d9", "<post>Hallo!</post>"]
    else
      :not_modified
    end
  end
  
  attr_reader :cache_key
  def initialize(etag)
    @cache_key = etag
    @document = nil
  end
  
  def body
    parse if @document.nil? # Lazy-loading
    @document.children.first.text
  end
  
  def parse
    @document = Nokogiri.parse($cache.read('data'))
  end
end

show.html.erb

<% cache [@post.cache_key, 'author'] %> 
 <h1><%= @post.author %></h1> 
<% end %> 

Hello, <%= current_user.name %>, this is not cached. 

<% cache [@post.cache_key, 'body'] %> 
<p><%= @post.body %></p> 
<% end %>