[PATCH 2 of 2] Cache: send conditional requests only for cached 200 OK responses
piotr at cloudflare.com
Mon Nov 24 22:40:41 UTC 2014
> That's up to a configuration.
You're right... It seems like a waste of disk space to me, but nginx
can be forced to do that.
This still leaves us with only 200 and 206.
> Which problem? You are trying to convince me that that
> conditional requests shouldn't be used to revalidate responses
> with non-200 status code because it may not work with some
> upstream servers. Sure, this can happen. Moreover, it can happen
> with responses with 200 status code, too.
No, I'm saying that it cannot work.
Maybe I shouldn't have tried to explain the reasoning behind
fd283aa92e04, because it looks that it just added confusion, so let's
1. client #1 requests "/logo.png" from website behind nginx,
2. nginx doesn't have "/logo.png" in cache, so it sends request upstream,
3. upstream server rate-limits nginx and replies with 503:
HTTP/1.1 503 Service Temporarily Unavailable
Last-Modified: Mon, 24 Nov 2014 22:00:00 GMT
4. 503 gets force-cached at nginx (due to configuration),
5. client #2 requests "/logo.png" from website behind nginx,
6. nginx has "/logo.png" in cache, but it's expired, so it tries to
GET /logo.png HTTP/1.1
If-Modified-Since: Mon, 24 Nov 2014 22:00:00 GMT
7. upstream replies with 304, accidentally revalidating 503:
HTTP/1.1 304 Not Modified
even though, the 304 is generated from:
HTTP/1.1 200 OK
Last-Modified: Mon, 10 Nov 2014 00:00:00 GMT
not from the rate-limiting.
This scenario describes perfectly behaving upstream server, the only
issue here is that nginx tries to revalidate response that cannot be
revalidated and applies 304 Not Modified response to it.
> Two trivial solutions include:
> - fix an upstream server;
There is no issue with the upstream server, only with nginx
revalidating non-200 status codes.
> - disable use of conditional requests, it's off by default.
So you're saying that just because it's off by default, it's fine for
it to be broken?
More information about the nginx-devel