By default, Rails will append all asset
paths with that asset‘s timestamp. This allows you to set a
cache-expiration date for the asset far into the future, but still be able
to instantly invalidate it by simply updating the file (and hence updating
the timestamp, which then updates the URL as the timestamp is part of that,
which in turn busts the cache).
It‘s the responsibility of the web server you use to set the
far-future expiration date on cache assets that you need to take advantage
of this feature. Here‘s an example for Apache:
# Asset Expiration ExpiresActive On
ExpiresDefault "access plus 1 year"
Also note that in order for this to work, all your application servers must
return the same timestamps. This means that they must have their clocks
synchronized. If one of them drift out of sync, you‘ll see different
timestamps at random and the cache won‘t work. Which means that the
browser will request the same assets over and over again even thought they
didn‘t change. You can use something like Live HTTP Headers for
Firefox to verify that the cache is indeed working (and that the assets are
not being requested over and over).