A 503 page gets written to my proxy cache, overwriting the 200

[Maxim Dounin]

Hello!

On Thu, Jan 16, 2014 at 09:02:36AM -0500, rge3 wrote:

> Hi,
> 
> I'm trying to use the proxy cache to store regular pages (200) from my web
> server so that when the web server goes into maintenance mode and starts
> returning 503 nginx can still serve the good page out of cache.  It works
> great for a few minutes but then at some point (5 to 10 minutes in) nginx
> will overwrite the good 200 page in the cache with the bad 503 page and then
> start handing out the 503.  Looking at my config I don't understand how a
> 503 could ever get written to cache but it is.  And the 200 page was brand
> new (written 10 minutes before) so it shouldn't be the "inactive" time on
> the proxy_cache_path setting causing nginx to delete the good file.  Can
> anyone tell me what I'm missing?  Here are the relevant pieces of my
> config:

[...]

>                 # don't cache pages with php's session cookie
>                 proxy_no_cache $cookie_$GotSessionCookie;
> 
>                 # bypass the cache if we get a X-NoCache header
>                 proxy_cache_bypass $http_nocache $cookie_$GotSessionCookie;
> 
>                 proxy_cache_use_stale http_500 http_503 error timeout
> invalid_header updating;
>         }
> 
> I can't imagine how a 503 would ever get cached given those
> proxy_cache_valid lines but maybe I don't understand something.  Thanks for
> any ideas!

An exiting cache can be bypassed due to proxy_cache_bypass in your 
config, and 503 response can be cached if it contains 
Cache-Control and/or Expires which allow caching.

-- 
Maxim Dounin
http://nginx.org/