HTTP Caching

comments_controller.rb

class CommentsController < ApplicationController
  layout false
  
  def show
    headers['Cache-Control'] = 'must-revalidate'
    item = Item.find(params[:item_id])
    if stale?(:etag => item.comments.count, :public => true)
      @comments = item.comments.all(:include => :user)
    end
  end
  
end

default.vcl

#Called at the beginning of a request, after the complete request has been received and parsed.
sub vcl_recv {
  if (req.request == "GET" && req.url ~ "^/(items)/") {
    # Remove the cache control header in the incoming request, otherwise it can bypass the proxy
    unset req.http.Cache-Control;
    # Remove their cookie if set, varnish won't check the cache if the cookie is present
    unset req.http.cookie;
  }
}

#Called after a document has been successfully retrieved from the backend.
sub vcl_fetch {
  if (req.request == "GET" && req.url ~ "^/(items)/") {
    # Remove any cookie that was set by Rails for the page we are caching, you don't want to make someones cookies public
    remove beresp.http.Set-Cookie;
  }
}

items_controller.rb

class ItemsController < ApplicationController
  def show
    @item = Item.find(params[:id])
    expires_in 3.hours, :public => true, 'max-stale' => 0
  end
end

show.html.erb

<div class="item_container">
  <!-- Item stuff -->
</div>
<div id="comment_section">
  <h2 class="comment_header">Comments:</h2>
  <div class="comment_list">
    <%= link_to "View Comments", item_comments_path(@item), :class => "commentable" %>
  </div>
</div>
<script type="text/javascript">
  $(document).ready(function(){
    $.fn.loadComments = function() {
      var self = this;
      var comment_list = $(this).parents('.comment_list');
      $.get($(this).attr('href'), function(data) {
        var parent = comment_list;
        parent.html(data);
        parent.show();
      });
    }
    $(".commentable").loadComments();
  });
</script>