The cache operates on a stale-while-revalidate model. This means that a document when viewed by a user on the client could be stale as revalidation only happens in the background and effects subsequent page views. While this is acceptable for most content, some sections of a page are critical or sensitive enough that it warrants always having the latest piece of information such as item pricing and rating. A good example of this scenario is in e-commerce and long tail product pages which might not be visited often but should have the correct price or quantity remaining when viewed.

Will post design doc shortly.