Does If-None-Match header overrules Cache-Control: max-age?

Question

I have a webapp that serves an image:

I'm clearly setting the Cache-Control header properly, and the web server is setting the Etag.

The problem is that the resources is being If-None-Matched every single time effectively ignoring the Cache-Control header. (The framework is returning 304 accordingly but the roundtrip exists).

I've read that the Etag header may overrule Cache-Control and still go to check the match even if the resource is local and valid. This makes no sense to me.

Anyone knows what's going on? What am I missing here?

PD: The web server is a Play! application

Answer 1

Finally got it. Some browsers (Google Chrome and Firefox for sure) ignore the local cache when you reload the page with CMD+R or F5.

In my case, no matter how high I set the max-age, a page reload will always force the If-None-Match check and get a 304 back. Navigating through the website by clicking links will not trigger this behaviour and caches will work just fine.

Note: I didn't noticed this since in a single page webapp (like mine) there's no "link clicking navigation", since most of them are loaded at the beginning.

Answer 2

You may be getting hit by proxies.

The max-age directive is intended for browsers, while the s-maxage directive is intended for proxies.

You seem to be missing the s-maxage directive in your Cache-Control header.

You may also want to add the public directive. (even though according to the specification this should be the default, it doesn't hurt to set it explicitly)

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.3

Also, keep in mind that some proxies could very well choose to ignore all this, so you won't stop receiving requests completely even if you have everything perfectly configure on you end.

I have all 3 directives set in a service that serves images (that also uses ETag and Last-Modified) for a CDN and I haven't noticed abnormal behaviour in my tests.