Forwarding request when serving from NGINX cache - Is this possible?

Question

I have enabled caching with Nginx. I followed the official docs and it works fine. What happens is, when the request comes in for the first time, it is proxy passed to a server. The response is cached.

From there on, all requests are served from the Nginx cache until it expires. But I want an additional thing.

Here is what I want:

When the request is served from the cache, I want to make an API request to my server so that I do not loose on the number of hits (counter)

How can I do this?

I had tried the mirror command, but it does not get called when the page is served from cache. It only gets called when the cache expires. Here is the config snapshot that I tried. Is this even possible? Any other way to do this?

EDIT Cache directive as highlighted in the second screenshot.

Do you even get cache hits? There are no nginx caching directives here. — Michael Hampton, Aug 05 '20 at 11:36
@MichaelHampton Updated my screenshot. `proxy_cache` directives are there. — Suhail Gupta, Aug 05 '20 at 11:39
@GerardH.Pille I want to call a custom API when the cache hit takes place. Is this possible? — Suhail Gupta, Aug 05 '20 at 11:43
Please add configuration information as text, not as screenshots. Screenshots are difficult to use. — Tero Kilkanen, Aug 05 '20 at 15:52

score 1 · Accepted Answer · answered Aug 05 '20 at 11:52

1

This doesn't appear to be possible. The mirror directive is processed as a subrequest to the original request, so if nginx does not process the original request, the mirror request will also not be processed.

As a workaround, you can add a second custom access_log directive, which uses a custom log_format. Your program can then read this log to determine when a cache hit occurred and to which URL. In fact, it could just use that log to count all hits, cached or not.

answered Aug 05 '20 at 11:52

Michael Hampton

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Could you please share a small example – Suhail Gupta Aug 05 '20 at 11:54
I got it. Does this file ever get purged by Nginx? – Suhail Gupta Aug 05 '20 at 12:09
@SuhailGupta No, you have to rotate logs yourself, e.g. with logrotate. – Michael Hampton Aug 05 '20 at 13:11
Is there a service that streams nginx logs? I want to continuously read logs into my application and put those into database. – Suhail Gupta Aug 05 '20 at 13:59
@SuhailGupta Something like logstash? – Michael Hampton Aug 05 '20 at 14:00

Forwarding request when serving from NGINX cache - Is this possible?

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