How to monitor nginx proxy cache?

Question

I would like to see which objects get cached by my nginx reverse proxy (with an apache as a backend). So far I could not find a way, only the info that its not implemented yet.

The reason is that I would like to tweak my configuration for best performance without putting too much stress on the server, as the backend is a production system. I know benchmarking would be better, but its not an option right now. So I though an alternative measure would be to monitor the cache.

Is that possible, and if yes, how? (despite patching nginx with the patch mentioned in the link above)

Answer 1

Nginx saves the cached files at the Proxy_cache_path folder. The files are named by the md5 ooutput of the proxy url.

Please note that nginx saves the proxy header on the cached file. So it might be hard to check pictures.

Avi

Answer 2

• Nginx saves the cached files at the proxy_cache_path folder.
• The files are named after the md5 output of the proxy_cache_key (rather than the md5 output of the proxy url).

You can actually easily verify that by yourself: just open one of the files in your proxy_cache_path folder and run md5sum on the key:

$ cat /var/cache/nginx/d/be/54a355887f2e4391a6e0cb9150ae3bed
[...]
KEY: https://test/style.css
HTTP/1.0 200 OK
Strict-Transport-Security: max-age=31536000; includeSubDomains
[...]
$ echo -n https://test/style.css | md5sum
54a355887f2e4391a6e0cb9150ae3bed  -

Answer 3

Maybe slightly related. To see how many entries are in the cache zone, check how many entries are saved in the cache path (/tmp/nginx_cache in my example).

nginx@nginx-5d468f984d-trj8s:/tmp/nginx_cache$ find . -type f  | wc -l
18719

This shows 18719 entries saved in the zone (upper bound, that some could be stale or old).

The path for the cache zone is determine by the directive configuring the cache. See for example the nginx caching guide.