About one year ago our friends at Facebook brought an interesting issue to the IETF HTTP Working Group - a lot (20%!) of their transactions for long lived resources (e.g. css, js) were resulting in 304 Not Modified. These are documents that have long explicit cache lifetimes of a year or more and are being revalidated well before they had even existed for that long. The problem remains unchanged.
![Cache-Control: immutable](https://cdn-ak-scissors.b.st-hatena.com/image/square/5daa78324b8e72a03c9a7a9dbac43167b3b3483b/height=288;version=1;width=512/https%3A%2F%2Fblogger.googleusercontent.com%2Fimg%2Fb%2FR29vZ2xl%2FAVvXsEjVvmmXahJAbKKlvQ1B9sWwNmDWDg8O3ISDc9i6nz0yhJQMjXjfx2sYG4W_sH8CiQSm631oroFCwfb-x9OGltOY2FeHGKiTKbdTfaAOMDu7UTgb5BHy2IWZscrMQjC4zm8d1lUelVmrkFk%2Fw1200-h630-p-k-no-nu%2Fwithout-immutable.png)