Simple question about proxy cache

[Maxim Dounin]

Hello!

On Sun, Mar 17, 2013 at 08:08:39PM +0000, John Moore wrote:

> I've used nginx as a reverse proxy server for a long while but I've not 
> tried out the proxy cache until today, and I have to say I'm a little 
> bit confused by what I'm seeing in the cache log, and I'm wondering 
> whether I've set things up correctly. My requirements are actually 
> pretty simple. I have a couple of locations which I want to proxy to 
> another server and cache the results. Thus:
> 
>            location /media/house_images/{
>              proxy_pass http://backend;
>              proxy_cache one;
>           }
> 
>           location /media/boat_images/{
>              proxy_pass http://backend;
>              proxy_cache one;
>           }
> 
> 
> Apart from this, I don't want any cacheing of responses to be done. I am 
> assuming that the default is NOT to cache unless a cache zone is 
> specified (at the server or location level, presumably), so either 
> omitting a proxy_cache or specifying 'proxy_cache off' should be 
> sufficient to achieve this, should it not?

Yes, without proxy_cache (or with "proxy_cache off") configured in 
a location cache won't be used.

> Two things are puzzling me, though. Firstly, in the cache log, I'm 
> seeing the URLs of all kinds of requests which SHOULD NOT be cached, and 
> I'm wondering whether all requests are logged whether they're cached or 
> not - I certainly hope this is the case and it's not actually cacheing 
> these responses. I would definitely prefer to only see entries in the 
> log for requests matching locations for which a cache has been 
> specified. I presume this is possible?

You can configure logs for a specific location, see 
http://nginx.org/r/access_log.

> Secondly, the very requests which I would expect to be cached are all 
> showing up in the log with the word 'MISS' in the $upstream_cache_status 
> column.

This usually happens if your backend doesn't specify allowed cache 
time (in this case, proxy_cache_valid should be used to set one, 
see http://nginx.org/r/proxy_cache_valid) or if backend responses 
doesn't allow cache to be used (either directly with 
Cache-Control/Expires headers, or indirectly with Set-Cookie 
header, see http://nginx.org/r/proxy_ignore_headers).

-- 
Maxim Dounin
http://nginx.org/en/donation.html