keepalived? What do you mean?

It's a tool you tend to come across when you're building highly available network systems, https://github.com/acassen/keepalived .

You might use it to implement redundancy in the load balancer layer of your system, perhaps your firewall round-robins incoming connections between two IP:s where you have nginx proxying to share load between two mirrored clusters, and those IP:s are virtual and handled by keepalived that will shuffle in a backup virtual server if the one currently serving becomes unhealthy or needs to be switched out due to a config rollout or something.

It's a really neat way to be able to just throw more virtual servers at the problems in availability, redundancy, load balancing and so on. I think it does some ARP messaging to achieve this.

Edit: I've applied it with ProxySQL in one case, it was an application on a trajectory from a simple rig with one virtual web server, one virtual database server to a highly available and resilient system. When I left we had a master-master-cluster with ProxySQL in front, with three ProxySQL-machines keepalived, so in case one went out for some reason there where two more in the stack to fill in. When you aren't sure what kind of peak load you're going to handle it's nice to know that when the alarm comes you have one fresh machine buying you some time while you figure out what you need to do to the third before it is shuffled into service.

I think that OP really meant that in his/her mental model of things, answers already known and battle tested for numerous cases [by using Nginx] - making HA with keepalived? Check. Making logging to be buffered to save iops? Check. Implementing ratelimits and custom logic? Check.

Keepalived is not really bound to Nginx by any means and should perfectly fine work with Caddy too.

Yeah, precisely. I'm sure it works fine with Caddy too, it's just that I haven't had Caddy with me in the trenches, yet.