Your 99.5% target simplifies failover. As long as you don't do it very often, you can just drop sessions; it's not ideal, but it's within your target and just carefully schedule the maitenance (and take the failures when they happen).
If you can, the least equipment option would be to run ECMP from your application servers; no need for a load balancer at all. If you can run 10Gbps on 8 cores, a 16-core server (which is a small server) might do all you need; if so, you could do CARP/VVRP/IP takeover and have everything run through one box at a time.
Or, HAProxy will work fine in front. If you want to make it easier for HAProxy, don't terminate TLS on those, run in tcp mode and let the application server do TLS; but then you can't let HAProxy split requests. If that works for you, you could do better with something that can do Direct Server Return so your load balancer doesn't need to process outbound traffic, but I don't have experience running that kind of load balancer (only being behind one), so no software recommendations.
For HAProxy, especially tcp mode HAProxy, you want a power of two CPU cores that's no more than the number of NIC queues, hyper threads disabled. If you do it right, all the traffic stays on the same CPU, although it's tricky to align the backend connections.
Comments
Your 99.5% target simplifies failover. As long as you don't do it very often, you can just drop sessions; it's not ideal, but it's within your target and just carefully schedule the maitenance (and take the failures when they happen).
If you can, the least equipment option would be to run ECMP from your application servers; no need for a load balancer at all. If you can run 10Gbps on 8 cores, a 16-core server (which is a small server) might do all you need; if so, you could do CARP/VVRP/IP takeover and have everything run through one box at a time.
Or, HAProxy will work fine in front. If you want to make it easier for HAProxy, don't terminate TLS on those, run in tcp mode and let the application server do TLS; but then you can't let HAProxy split requests. If that works for you, you could do better with something that can do Direct Server Return so your load balancer doesn't need to process outbound traffic, but I don't have experience running that kind of load balancer (only being behind one), so no software recommendations.
For HAProxy, especially tcp mode HAProxy, you want a power of two CPU cores that's no more than the number of NIC queues, hyper threads disabled. If you do it right, all the traffic stays on the same CPU, although it's tricky to align the backend connections.