Rails 2.2’s eTAG functionality is great.

def show
  @article = Article.find(params[:id])
  if stale_record(@article)
    # do something expensive
  end
end

protected

  def stale_record?(record)
    fresh_when(:etag => [current_user, record], :last_modified => record.updated_at.utc)
    !request.fresh?(response)
  end

“304 NOT MODIFIED” all around!

But this determines freshness based on just the record.
If I deploy a new version of the “show” template eTAG-compliant browsers won’t refresh.

I could build my own way of handling this inside “stale_record?”
But Rails already has a method for dealing with this.

All cache keys are generated through the following code.

if ENV["RAILS_CACHE_ID"] || ENV["RAILS_APP_VERSION"]
  expanded_cache_key << "#{ENV["RAILS_CACHE_ID"] || ENV["RAILS_APP_VERSION"]}/"
end

So what does it mean to be the RAILS_APP_VERSION?
Well, I’m going to mis-use it, and say “it’s the current version of our app”

And Capistrano already gives this to us… it gives us /current/REVISION!!!

Perfect!

A quick “/config/initializers/capified_etags.rb” later

app_version_path = File.join(Rails.root, "REVISION")
if File.exist?(app_version_path)
  ENV["RAILS_APP_VERSION"] = File.open(app_version_path).read.strip
end

Nice.

Let me know if there are any other approaches out there.

Comments (View)

RudyGems

Expire eTAGs when you redeploy