Rails page caching with separately cached global elements.

I'm trying to build an optimized caching setup for my app.

Large parts of the app will use full page caching (using something like Ben Scofield's Progressive Enhancement approach http://www.slideshare.net/bscofield/page-caching-resurrected.)

The issue is: I'd like to include some global elements on all pages, where the global element changes much more rapidly that most other pages on the site, and I don't want to expire nearly all the pages on the site because a minor global element needs to be updated.

An example of this is "10 most recent blog posts." shown on every page. The RailsEnvy tutorial on caching (http://www.railsenvy.com/2007/2/28/rails-caching-tutorial ) uses this example as a case where you _would_ need to clear the whole cache, so maybe I'm being a little too creative here.

I was hoping to do this with SSI as apache serves the cached pages, as in: <!--#include virtual="/includes/ten_newest_posts.html" -->

This doesn't work because: 1: On the rails side, if page caching is turned off, the include is ignored and the element is empty when the page is rendered dynamically.

2: If the page is cached, but the element included via ssi doesn't exist on the file system, it results in a routing error.

Google says some people have tried this, but I haven't seen a howto or successful report of this approach. After fumbling around and searching for a couple hours, I get the sense that this is trickier than it sounds: * Knowing when the SSI include statement should be processed by rails or by apache2 * Getting apache to ssi include a file that doesn't exist and needs to be generated on the fly * Getting rails to include the ssi content served to the client, by not including the ssi for the content cached to the file system

So, I'm looking for alternative approaches or more input on this approach. Is something like this even possible now? One thing that may make this work is if I can get the ssi to only conditionally include the file if the file exists.

Something like this:

  <!--#if expr=' -e /global_elements/top10.html' -->     <!--#include virtual="/global_elements/top10.html" -->   <!--#else -->      Non-dynamic text to go here.   <!--#endif -->

But SSI doesn't seem to support file tests or some similar workaround that I've found

I *could* (and may) use a javascript layering approach on the client side, but then the content in the global elements would not be visible to spiders used to build search indexes, which is not really desirable.

Thanks for any feedback.