Nginx testing proxy_cache_valid parameter

Firstly, I created proxy pass for the site. Test location is there for testing cache.

# the site
http {
    ...
    proxy_cache_path /tmp/nginx-cache levels=1:2 keys_zone=my_cache:10m max_size=1g inactive=60m use_temp_path=off;
}

server {

    listen 8080;

    ...

    access_log /var/log/nginx/access.log;
    location /test {
        return 200 “It’s now $time_local\n”;
    }

}
# the proxy
server {
    listen 80 default_server;

    root /var/www/;

    index index.html index.htm;

    server_name example.com;

    charset utf-8;

    location / {
        include proxy_params;

        proxy_pass http://localhost:8080;

        proxy_cache my_cache;

        proxy_cache_revalidate on;

        proxy_cache_min_uses 3;

        proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;

        proxy_cache_background_update on;

        proxy_cache_lock on;

        proxy_cache_valid 200 15s;

    }

}
proxy_params
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;

The cache server is already confgured to cache responses with status code 200 for 15 seconds. This typically results in cache updates every 15 or 16 seconds.

proxy_cache_valid 200 15s;

Once per second, we send an HTTP request to the address. The response does not change until the caches on the server expire. This happens every 15 or 16 seconds

$ while sleep 1; do curl http://example.com/test; done
...
It’s now 23/Oct/2019:15:11:12 +0000
It’s now 23/Oct/2019:15:11:12 +0000
It’s now 23/Oct/2019:15:11:12 +0000
It’s now 23/Oct/2019:15:11:28 +0000
It’s now 23/Oct/2019:15:11:28 +0000
It’s now 23/Oct/2019:15:11:28 +0000
^C

We can also inspect the logs on the origin server to confrm that it is receiving
a request only every 15 or 16 seconds

/var/log/nginx/access.log
...
127.0.0.1 - - [23/Oct/2019:15:11:12 +0000] "GET /test HTTP/1.0" 200 38 "-" "curl/7.58.0"
127.0.0.1 - - [23/Oct/2019:15:11:28 +0000] "GET /test HTTP/1.0" 200 38 "-" "curl/7.58.0"

useful link
https://serversforhackers.com/c/nginx-caching