Disable NGINX caching 304 Responses from Origin Server

[Peter Booth]

I can’t see an obvious issue, but I can say that there is no such thing as a simple web server setup where caching is involved.
I have gray hairs that appeared after working with a high traffic retail website that had seven levels of caching
(browser cache, CDN, hardware load balancer, nginx reverse proxy, servlets that write content, tangosol /oracle coherence, endeca caching)

I’m hoping that you are living in  a saner world than that one but I’m sure that you will have some craziness. 
I would encourage you to add $upstream_cache_status  to your log format
and/or add the directive  add_header X-Cache-Status $upstream_cache_status;
Instrumenting the cache can be a real life-saver when things go awry.

I’d also strongly encourage you to use redbot.org to check for aberrant behavior and webpagetest.org
to see how different browsers handle your site. 

Peter



> On Jul 26, 2017, at 3:29 AM, Ryan Barclay <ryan at rbftpnetworks.com> wrote:
> 
> The following config seems to work for the situation I discussed:
> 
> proxy_cache_valid 200 3M;
> proxy_cache_valid 304 0;
> proxy_cache_revalidate on;
> proxy_set_header If-Modified-Since $http_if_modified_since;
> proxy_ignore_headers Cache-Control Expires;
> 
> 
> ... can anybody see any problems with this config or future problems that may arise?
> 
> 
> On 24/07/2017 16:20, Ryan Barclay wrote:
>> We have a pretty simple setup with NGINX sitting on the front and a backend server (on a separate physical server) that provides the content.
>> 
>> Nginx then caches content based on the EXPIRES and Cache-Control headers set by the origin server.
>> 
>> We noticed that NGINX was not issuing 304 headers to images that were not in the local NGINX cache when the If-Modified-Since header was sent by the client. Instead, it would issue a 200 with the full data file.
>> 
>> To fix this, we applied:
>> proxy_set_header If-Modified-Since $http_if_modified_since
>> 
>> So then the If-Modified-Since header was passed to the backend and of course, it returned correctly with the 304 header - great.
>> 
>> But what we noticed was that NGINX would cache this 304 response and deliver future responses as 304 to clients even without the If-Modified-Since header.
>> 
>> How can we disable caching of 304 responses and fix this issue?
>> 
>> Thank you for your help, suggestions, and tips in advance.
> 
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