How to get caches_action to set expires headers when there is a hit in memcached?

Question

I have an action which I cache using

caches_action :my_action, :expires_in=>1.hours

and also set the expires headers in the action itself using

def my_action
   ...
   expires_in 1.hours
   send_data(...,:disposition => 'inline',:type => 'image/png',:filename => params[:title]+".png")
end

However, when I look at the cache control response header from a result that is coming as the result of a memcached hit, I get this:

Cache-Control: private, max-age=0, must-revalidate

The first time round, i.e. when there is nothing in the cache, it is what I expect, i.e.:

Cache-Control: max-age=3600, private

It looks like rails+memcached is neither caching the original response headers, nor setting appropriate headers itself. The result is that the client makes a request to the server each time even when the result (an image) hasn't changed. Though the action completes quickly as it gets a hit in the cache, it still winds up sending all the data again, which I'd like to avoid.

How do I get the headers to do the right thing so that the client either makes no request in the first place, or gets a 'not modified' response?

Answer 1

Make sure your environment is configured to support caching. So in config/environments/development.rb (or where ever) you should see:

config.action_controller.perform_caching = true

Also, the private cache tag tells intermediate cache servers not to store the content. This is secure by default. If you want to change this behaviour, just set your cache like this:

expires_in(1.hours, :private => false, :public => true)

In order to skip expensive processing on the server if the content has not changed, use:

if stale?(:etag => @model, :last_modified => @model.updated_at.utc)
  # Expensive stuff in here.
  respond_to do |format|
    ...
  end
end