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With each new request that you make to port 80, HAProxy receives it and selects a server to handle it. HAProxy rotates through the list of servers, relaying the next request to the next server in line. You are now balancing the load across these servers.

Add health checks

https://www.haproxy.com/blog/how-to-enable-health-checks-in-haproxy

Resources

https://www.haproxy.com/blog/haproxy-configuration-basics-load-balance-your-servers