Memcache Store with controlled expiration

gistfile1.rb

module ActiveSupport
  module Cache
    class AtomicMemCacheStore < MemCacheStore
      ALREADY_STORED = "STORED\r\n"
      cattr_accessor :grace_period
      @@grace_period = 3.minutes

      def read(key, options = nil)
        result = super
        
        if result.present?
          timer_key = timer_key(key)
          #check whether the cache is expired
          if @data.get(timer_key, true).nil?
            #optimistic lock to avoid concurrent recalculation
            if @data.add(timer_key, '', self.class.grace_period, true) != ALREADY_STORED
              #extend current cache key life
              @data.set(key, result, self.class.grace_period, options && options[:raw])
              #trigger cache recalculation
              return handle_expired_read(key,result)
            end
            #already recalculated or expirated in another process/thread
          end
          #key not expired
        end
        result
      end

      def write(key, value, options = nil)
        if expiry = expires_in(options)
          #extend write expiration period and reset expiration timer
          options[:expires_in] = expiry + self.class.grace_period
        end
        @data.set(timer_key(key), '', expiry, true)
        super
      end

    protected
      #simulate empty cache to trigger synchronous cache recalculation
      def handle_expired_read(key,result)
        nil
      end
      
    private
      def timer_key(key)
        "tk:#{key}"
      end
    end
  end
end