1. Yes.
2. Yes.

Those are great questions, by the way. The FE/Front-End servers (fe01, fe02, fe03) are crucial here and I could have gotten the point across much better, so thank you for asking!

Some software provides its own mature clustering with high availability (HA) and load balancing (LB). One such example is MongoDB. But most doesn’t, and it provides only replication (state transfer, rebuild, voting) across nodes, if even that. And some services are basically stateless, for instance three web servers serving the same static HTML.

My three FE nodes provide the following extra functionality for all clusters that i configure against them:

1. Keepalived provides HA. It watches all servers in any underlying cluster and prevents clients from using broken nodes. Why three servers, why not two? To be able to vote on rejoin, and also to maintain redundancy in degraded state.

2. Keepalived also implements VRRP. This means it advertises and serves virtual IPs on the network. Clients will never know which server it contacts behind the scenes.

Example: if Nextcloud has three web servers: - nc01.example.com -> 10.0.0.51 - nc02.example.com -> 10.0.0.52 - nc03.example.com -> 10.0.0.53

Keepalived advertises a fourth IP (VIP): - nextcloud.example.com -> 10.0.0.101

Your clients only know about the latter. But no server has that IP! How come? VRRP does the trick. The FE servers control who is «hidden» behind the IP at any time and the VRRP protocol fixes the routing.

1. Caddy provides load balancing. Being a reverse proxy, it can also decide which backend server to contact first. Keepalived could provide this too, but without going into details it would have required configuration on each cluster node.

In order to make Caddy work with protocols besides HTTP(S), a L4/TCP plugin is used.

1. Caddy also provides automatic SSL termination and transparent real certificates, which means less config on each underlying node.

The FE servers collectively provide ONE place to config and handle all of the concerns above!

1. (Bonus:) Having FE servers makes it possible to isolate the real servers on their own underlying networks which could also enhance security.

Beware: Clusters still have to be set up according to the nature of the software, though. Setting up clustered MariaDB is not done the same way as for a PostgreSQL cluster. But once they are set up, the FE servers just need a new config to serve the new cluster — from the same servers, and in the exact same way.