I set up 2 proxied server from 1 proxy server in NGINX. The 2 proxied servers simply return a HTML with a random number from Math.random() in Node.js.

My NGINX configuration is the following:

# ...
http {

    upstream backend {
        server IP_1:3000;
        server IP_2:3000;
    }
    proxy_cache_path /data/cache keys_zone=my_cache:10m;

    server {
         location / {
            proxy_cache my_cache;
            proxy_pass http://backend;
       }
    }
 
    # ...

}

When I go to DigitalOcean, I do find that a /data/cache directory was made, but it is empty. Now when I refresh the nginx server, I get a different random number every time as the /data/cache directory is empty. It's not the load-balancing method that is causing no caches to be written as I tried session-persistent load balancing methods such as ip_hash.

I tried the following:

Setting Cache-Control to max-age=21600.

What am I doing wrong?