Ask HN: please share your favourite firewall/loadbalancer failover setups

I am looking at expanding our deployment infrastructure for high availability (pivoting into the B2B market, negotiating a second contract with an SLA), and will need to make a hardware purchases soon. I am trying to find sources of good information regarding the various failover configurations possible, and their pros/cons. Looks like running two *BSD machines for the firewall/loadbalancer, with failover provided by CARP, is a pretty simple solution. The Linux side of things looks a little more neglected. Are there any decent linux firewall/loadbalancer failover solutions?

A redundant, scalable back end is a topic that has been documented extensively, and there is no lack of information or options when it comes to scaling, in this case, a Rails application. However, I am failing to find good information on best practices for redundant front end load balancing. An example question I have: since we are on a budget, is it "acceptable" to dual-purpose the redundant hardware on the front end? Being able to run our cluster of background workers on the firewall/loadbalancer failover would obviously save resources, though I am not sure if it's a good practice.

If anyone has experience in this category, please speak up! Anyone with relevant links, I would love to see them.