While optimizing the caching-behaviour of our website, I noticed that a whole lot of if-none-match-requests are sent to our site. As far as I understand caching, this should not be the case as long as the cache is still valid.
One particular request generates the following response-header:
HTTP/1.1 200 OK
Cache-Control: public, max-age=25920000
Transfer-Encoding: chunked
Content-Type: application/javascript; charset=utf-8
Content-Encoding: gzip
Expires: Thu, 04 Feb 2016 17:20:09 GMT
Last-Modified: Mon, 01 Jan 2001 23:00:00 GMT
ETag: W/"0"
Vary: Accept-Encoding
Server: Microsoft-IIS/8.5
X-AspNet-Version: 4.0.30319
Date: Fri, 10 Apr 2015 16:20:09 GMT
As you can see, the cache should be valid for 300 days. The way I understand it, the browser should use its cache directly during that period. Only after this period is over, it should issue a request with the header if-none-match.
But browsers seem to ignore that and send this if-none-match -request each and every time the page is loaded just to receive a 304-response ("Not Modified").
What do I need to change to keep browsers from sending these useless requests?
Yes, while the cache is fresh browsers should use a local copy without revalidation. However, this is not guaranteed. For example, when users use the Refresh button browsers make requests to the origin server anyway.
There is a
Cache-Control: immutable, max-age=…extension that tells browsers you really really mean they should use the cached resource without contacting the server.