Rails cache expire

Question

I have a rails application, in that I am using simple rails cache. My testing is as follows:

Rails.cache.write(\'temp\',Date.today,:expires_in => 60.seconds)
         


        

Answer 1

The :expires_in option only works with compatible stores (eg memcached) - not memory.

From http://guides.rubyonrails.org/caching_with_rails.html:

Finally, if you are using memcached or Ehcache, you can also pass :expires_in. In fact, all parameters not used by caches_action are sent to the underlying cache store.

Answer 2

After some search, I have found one possible reason why the cache is not cleaned after 60 seconds.

1. You call Rails.cache.write which is documented here.
2. It calls write_entry(namespaced_key(name, options), entry, options), where your option :expires_in is one part of the options argument.

3. The implementation of write_entry has the following condition:

   if expires_in > 0 && !options[:raw]
       # Set the memcache expire a few minutes in the future to support race condition ttls on read
       expires_in += 5.minutes
   end
   

So there are 5 minutes added to your 60 seconds. 2 possible solutions:

• Just live with it :-)
• Try to include the option :raw => true, perhaps this will skip the condition, so that your expiry works as suspected.

Answer 3

You should use fetch method with do block instead of Rails.cache.write. Even though the name is "fetch", it writes to the cache if you put inside do block. You also do not need to clear cache entry manually, just use the "fetch" with expires_in and race_condition_ttl. Keep the ttl value under expiry time and keep it as small as possible.

output_json = Rails.cache.fetch('temp',expires_in: 1.minute,race_condition_ttl:3) do
  my_expansive_operation().to_json
end