Etags vs. Last Modified Header

Hello ,
After reading online sources (Yahoo, Google etc.) about Etags, I’m still NOT sure I really understand about why Etags were created and what websites can gain from it. It would seem that the “Last-Modified” header should do the trick to determine when a static resource needs to be downloaded back to the client machine and cached again or NOT; so in what case does that method fails and Etags need/can to be used? Maybe there’s a scenario that I am missing that would be good to use Etags!???!

Thanks advance.


The main place they are helpful is if you have more dynamic or generated data rather than your typical static assets. You can hash or otherwise tag the result and if the hashes match you don’t need to transfer all of the data. It is a pretty specific case though so in “general” you shouldn’t use them or see them unless you know why you are using it.

Thanks make sense. I’m just not sure why Yahoo isn’t too clear about what you just stated.