What HTTP client headers should I use to instruct proxies to refetch from origin, and cache the response?

Question

I'm currently working on a system where a client makes HTTP 1.1 requests of an origin server. I control both the client and the server software, so have free reign over HTTP headers set. Between the client are multiple, hierarchical layers of web proxy / cache devices (think, Squid or similar).

The data served up by the origin is usually highly cacheable, and I intend to set HTTP response headers to indicate this. Specifically, I plan to use Cache-Control: public, max-age=<value> . I understand that this will mean that intermediate proxies will cache the response up to the specified max-age , at which point they will revalidate against the origin (presumably with a Last-Modified header, looking for a 304 response).

The problem I have is that the client might become aware that the data held by caches might now be invalid. In this case, I need the client to make a request which instructs the caches to either fetch or revalidate their response with the origin. If the origin response is now different, the cache should store this new response. In my mind, this would involve the client making the request, and each cache in the chain should revalidate its response with the next upstream device, all the way back to the origin. The new response can then be served from the closest cache which actually has it.

What's the correct HTTP headers that need to be set on the client request to achieve this? At first I thought that setting Cache-control: no-cache in the HTTP request would make this happen, but reading the RFC, it seems that this will instruct the intermediate caches to both go back to the origin (desired) but also not cache the new response (not desired). I then saw an article in which an HTTP request header of Cache-control: max-age=0 would perhaps do this, but I'm not sure.

Will max-age=0 do what I need here, or do I need some other combination of HTTP headers?

Answer 1

I asked a similar question here: How to make proxy revalidate resource from origin . I since learned that proxy revalidate wasn't supported by nginx at the time of writing. It is scheduled for the 1.5 release .

Sending max-age=0 from the client should trigger this revalidate mechanism in the proxy, if the original response from the origin contained the right cache control headers.

But whether your upstream server(s) will respect these headers and revalidate with their origin is clearly not something you can just assume. If you have control over your upstream servers I think it could work.

Also etag is preferred over modified since headers afaik.

I found these to be helpful articles on the subject:

caching tutorial

cache control directives

http specs on validation

section 14.9.4 on this spec

[UPDATE] Nginx version 1.5.8 has been released since, and I can confirm that this mechanism is now working!