Etag/Last Modified - the numpty question I've always wanted to know.

[p83822]

(11-02-2010 02:13 AM)aaronpeters Wrote:  

(11-02-2010 01:43 AM)pmeenan Wrote:  Do you have a reference for the Google recommendation? All I could find was the Page Speed optimization docs: http://code.google.com/speed/page-speed/...serCaching and they don't seem to say anything specific about having both strong and weak headers.

Pat, this sentence is on that page:
"It is important to specify one of Expires or Cache-Control max-age, and one of Last-Modified or ETag, for all cacheable resources."

So yeah, send 1 strong one (Expires or CC) *and* one weak one (Last-Modified/ETag).
Imo, this is a good best practice.

The weak one will only kick in if the Expires has .... expired. The browser will then have a file in cache but is not sure if it may use it. So it sends a conditional request to the server, asking the server "hey dude, may I use this file or not?" and the server will respond with either a 304 "yeah man, go ahead" or a 200 "nope, here is a new one, use that and throw away the old".

Thanks for going easy chaps. The thing that I think I don't like is that I can't work out whether the 304 "carry on" response itself persists. Is it just a "yeah man, carry on this time", or a "yeah man, carry on and BTW there's no need to bug my ass again for another n seconds"?

I suppose what I should really do is get off my backside and wireshark this stuff across browsers with various combinations of header :-D. (What I see in Firebug really confuses me - as jabubo finds as well, I think. I see conditional Gets *before* CC expiry, but only on every second request. Maybe that's what Google mean with the statments "the browser applies a heuristic to determine whether to fetch the item from cache or not. (The heuristics are different among different browsers.)").

BTW Pat, you recommend against Etags: 'ETag headers should generally not be used unless you have an explicit reason to need them'. Can you elucidate?

Thanks again everyone.

Neil