Today I was poking around in the rails tmp/cache directory and I saw an entry I did not expect. It appeared that a controller action was being cached but I was not using, page, action or fragment caching.
I was setting an expiration header on the action response and as it turns out there is a handy gem in rails now called rack-cache. Rack-cache will create cache entries for items that you set future a expiration for. Since this isn’t mentioned in the Rails Caching Guide it took me a little while to track it down.
Here’s a quick example of it in action
Create a dummy app
rails new rack-cache cd rack-cache bundle exec rails generate controller RackCache cache_this
Edit the cache_this method
class RackCacheController < ApplicationController
def cache_this
render :text => Time.zone.now.to_s
end
end
Fire it up in production mode so caching is enabled
RAILS_ENV=production bundle exec rails s -p6666
When you hit the page in the browser and hit reload you’ll see the time change
http://localhost:6666/rack_cache/cache_this
2012-03-29 00:40:09 UTC 2012-03-29 00:40:29 UTC
Now lets change the action slightly
class RackCacheController < ApplicationController
def cache_this
expires_in(5.minutes, :public => true)
render :text => "The time is #{Time.zone.now.to_s}"
end
end
Now each time you reload the page you get the same time
The time is 2012-03-29 00:58:07 UTC
The time is 2012-03-29 00:58:07 UTC
Without a reload the browser won’t bother asking for the resource for another 5 minutes. With the reload we get a 304 message.
Normally a simple clear of your browser cache would get you a new time but Rack-Cache also cached this on the server. So clear all you want, you’ll not get an update until 5 minutes after the time in the window. If there are 10 folks hitting this page they will all see the same time. The 1st person to hit it after expiration will update the time.
The time is 2012-03-29 01:04:01 UTC
So where is the output cached? By default Rails used the ActiveSupport::Cache::FileStore which lives in tmp/cache — configurable via config.cache_store.
ls -tr tmp/cache/ assets B15 2A5 A92 B2C B31 # B31 is newest dir find tmp/cache/B31 tmp/cache/B31 tmp/cache/B31/A20 tmp/cache/B31/A20/1c4fa8db52f778f10f535fe2253f3a63f92fd87b
The cached response is in
tmp/cache/B31/A20/1c4fa8db52f778f10f535fe2253f3a63f92fd87b
cat tmp/cache/B31/A20/1c4fa8db52f778f10f535fe2253f3a63f92fd87b o: ActiveSupport::Cache::Entry :@compressedF:@expires_in0:@created_atf1332983041.303097: @value[I"(The time is 2012-03-29 01:04:01 UTC:EF
This is a pretty powerful caching option that Rails developers should understand and use when appropriate. In some cases, not knowing this feature was in place has broken applications. Maybe I’ll edit the Rails Guide to include this information.
I noticed that rack-cache was filling our apache error log with entries
cache: [GET /somepath] miss cache: [GET /anotherpath/logo?1333186605] fresh cache: [POST /yet/another/path] invalidate, pass
Turn off the verbose option by adding this to your production.rb
config.action_dispatch.rack_cache = {:metastore=>"rails:/", :entitystore=>"rails:/", :verbose=>false}
To disable rack-cache altogether just do this
config.action_dispatch.rack_cache = nil
Some useful links