Joe Gregorio writes about “Deep ETags”, i.e. creating an etag value (which is (one of the factors) used to control HTTP caching) as “low” as possible. I’ve wondered about this a few days ago, and checked the Rails sources — Etag support has been added just recently:

Added that rendering will automatically insert the etag header on 200 OK responses. The etag is calculated using MD5 of the response body. If a request comes in that has a matching etag, the response will be changed to a 304 Not Modified and the response body will be set to an empty string.

So Rails does etags, which is good, but I believe it could offer much better support if this was connected to the lower layers (i.e. ActiveRecord). Should be easy to do …