ETags are good, however normally they are generated based on strings. However, very often it is easier to pass in a complete model object as your ETag, or it's parametrized represenation (record id) together with the version. Or an array of objects (if you want to cache your object listing page and prevent it from spending time on template rendering). This module will take care of transforming any object into a stringified representation that is usable as an etag with minimum fuss.