Image from Load Balancing with HAProxy Service Discovery Integration.
Consul Service Discovery for HAProxy - HAProxy Technologies
HAProxy TechnologiesNick Ramirez

Back in December 2019 I wrote a post about Dynamic Upstreams in NGINX w/ Consul and here’s the summary:

I find combination of Consul for service-discovery and NGINX with basing its upstream configuration on it extremely powerful. As I also mentioned multiple times already – I think Upsync is one of the most elegant solutions for combining these two lovely pieces of technology together. I hope that this article will push you into direction of having less things hardcoded and plenty more dynamic and discoverable.

I still find NGINX with Upsync combination to be one of the cleanest and most elegant solutions to the problem of having upstreams provided dynamically. That said, HAProxy just up the ante by building in support for Consul, quote:

When you register more instances of the same service, HAProxy will fill in these disabled server slots, which lets you scale up or down in many cases without a reload.

And that’s the beauty. The entire control over the backends being in and out is under Consul control and HAProxy just routes through it dynamically. Yet again raises as a viable alternative to a fully-blown HTTP server. Respect.