How to display the age of an nginx cached file in headers

Question

I've set up a caching server for a site through nginx 1.6.3 on CentOS 7, and it's configured to add http headers to served files to show if said files came from the caching server (HIT, MISS, or BYPASS) like so:

add_header X-Cached $upstream_cache_status;

However, i'd like to see if there's a way to add a header to display the age of the cached file, as my solution has proxy_cache_valid 200 60m; set, and i'd like to check that it's respecting that setting.

So what i'm looking for would be something like:

add_header Cache-Age $upstream_cache_age;

I'm unable to find anything of the sort though, can you help?

Thanks

Answer 1

The nginx documentation is quite exhaustive — there's no variable with the direct relative age of the cached file.

The best way would be to use the $upstream_http_ variable class to get the absolute age of the resource by picking up its Date header through $upsteam_http_date.

add_header X-Cache-Date $upstream_http_date;

For the semantic meaning of the Date header field in HTTP/1.1, refer to rfc7231#section-7.1.1.2, which describes it as the time of the HTTP response generation, so, basically, this should accomplish exactly what you want (especially if the backend runs with the same timecounter).