Selfhosted

SLAs?

You're going to need a redundant ISP and a generator. You've left the territory where it's economical to self host something if that's what you're looking at. You still have several other single points of failure.

And I'll be honest, your setup isn't ready for an SLA either. Just having a second machine is such a small part of what you need to do before doing any guarantees. Are you using a Dynamic DNS service? What's the networking setup look like? Router to Compute?

From the sounds of it, you're not a professional. It might be time to engage an expert if you want to grow this.

This is a rabbit hole that's going to be very expensive. Caddy isn't going to do what you are wanting. You likely need enterprise systems which are complex and require at least 3 machines.

I would use AWS or Azure instead

My personal opinion, as soon as you’re charging and providing SLAs you’ve exceeded what you should be doing on a residential ISP.

I’d really recommend putting your app in a real cloud solution, which can provide actual load balancing via DNS natively for regional failover if you desire.

I do this with HAProxy and keepalived. My dns servers resolve my domains to a single virtual ip that keepalived manages. If one HAProxy node goes down, the other picks right up.

And this is one of the few things I’ve got setup with ansible, so deploying and making changes is pretty easy.

Keepalived to set up a floating IP between two proxy hosts. The VIP is where the traffic points to, the two hosts act as active/passive HA.

Additional SPoFs: Your upstream internet connection, your modem/router, electricity supply, your home (not burning, flooded, collapsed, etc.). And you.

No, the router being the SPOF (single point of failure) is totally avoidable.

At mny home (no SaaS services offered, but critical "enough" for my life services) i have two different ISPs on two different tecnologies: one is FTTC via copper cable (aka good old ADSL successor) plus a WFA 5G (much faster but with data cap). Those two are connected to one opnSense router (which, indeed, is a SPOF at this time). But you can remove also this SPOF by adding a second opnSense and tie the two in failover.

So the setup would be:

• FTTC -> ISP1 router -> LAN cable 1 to port 1 of opnSense n.1
• FTTC -> ISP1 router -> LAN cable 2 to port 1 of opnSense n.2
• FWA -> ISP2 router -> LAN cable 1 to port 2 of opnSense n.1
• FWA -> ISP2 router -> LAN cable 2 to port 2 of opnSense n.2

Then in both opnSense i would setup failover multi-WAN and bridge them together so that one diyng will trigger the second one.

edit: fixed small errors

Congrats, you're officially at the point where you should probably looking at kubernetes. Highly available, failover, and load balancers. It's a steep learning curve, but if you're looking for this level of availability you're probably ready for it

So you have 2, or 3 spof, your home internet, your home router, and your reverse proxy container.

You can solve most of that with a second internet connection on its own router and some k3s/k8s

Current router points to one container then you have your second router point to the other container. You can use DNS load balancing to share the connections over your 2 internet connections.

Depending on your monitoring system you if a connection goes down you could then trigger a DNS update to remove the offline connection from DNS. You will have to set the ttl of the record to facilitate the change more rapidly.

OPNsense and HAproxy might be a place to start, they work well together. You can define a backend pool of servers for roundrobinning, and if you buy a block of IPs you can roundrobin the incoming requests as well. I run OPNsense as a VM so that I can use Proxmox's high availability service for the router and it'll failover or manually livemigrate if I'm doing maintenance. You can VLAN the servers off from the rest of the network as well with OPNsense, and set up VPNs there for clients if needed, or use the SDN functions in the hypervisor to segregate servers if you're running them on the hypervisor.

The term you're looking for is load balancing. DNS load balancing will work fine for your purposes. Use a DNS host that supports health checks to the endpoints, and you're all set. If one goes down, DNS will not be returned when querying the record for the downed host.