nginx cache issue (http upstream cache: -5)

[Maxim Dounin]

Hello!

On Mon, Feb 05, 2018 at 04:08:33AM -0500, loopback_proxy wrote:

> I am new to nginx caching but have worked with nginx a lot. I tried enabling
> caching feature in our repository but it never worked so I thought I will
> pull a fresh copy of nginx and turn it on. I ended with the same issue. For
> some reason, nginx is not able to create the cache file in the cache dir.  I
> have already turned on proxy buffering and set full rw permission for all
> users on the cache dir. I also gdb'ed the code and it seems like it gets
> into ngx_open_and_stat_file from ngx_open_cached_file (
> http://lxr.nginx.org/source/src/core/ngx_open_file_cache.c?v=nginx-1.12.0#0144
> ) and it tries to open an non existent file in RDONLY mode if the of->log is
> not set (
> http://lxr.nginx.org/source/src/core/ngx_open_file_cache.c?v=nginx-1.12.0#0869
> ) 

The code in question is not related to your problem.

Instead, you should check what your backend returns.  There are 
number of cases when nginx won't cache a response, see description 
of the proxy_cache_valid directive for the details:

http://nginx.org/r/proxy_cache_valid

[...]

> And here's the debug log showing whats that nginx is failing to create the
> cache file. 
> 2018/02/05 08:57:26 [debug] 22509#0: *7 http cache key:
> "http://52.216.66.40"
> 2018/02/05 08:57:26 [debug] 22509#0: *7 http cache key:
> "/teddy-clip-art-teddy-md.png"
> 2018/02/05 08:57:26 [debug] 22509#0: *7 add cleanup: 00000000006D3560
> 2018/02/05 08:57:26 [debug] 22509#0: shmtx lock
> 2018/02/05 08:57:26 [debug] 22509#0: slab alloc: 120 slot: 4
> 2018/02/05 08:57:26 [debug] 22509#0: slab alloc: 00007FD2C31AA080
> 2018/02/05 08:57:26 [debug] 22509#0: shmtx unlock
> 2018/02/05 08:57:26 [debug] 22509#0: *7 http file cache exists: -5 e:0
> 2018/02/05 08:57:26 [debug] 22509#0: *7 cache file:
> "/tmp/nginx/cache/3/4f/ecbc29d001e852b40f09e913b5ced4f3"
> 2018/02/05 08:57:26 [debug] 22509#0: *7 add cleanup: 00000000006D35B0
> 2018/02/05 08:57:26 [debug] 22509#0: *7 http upstream cache: -5

Your assumption that it is "failing to create" is wrong.  Instead, 
this debug log snippet shows that nginx checked if there is a 
cached response available, and the result is negative.

-- 
Maxim Dounin
http://mdounin.ru/