Doesn't work for datacenters. Also is implemented with round-robin DNS to nodes (1-N, check X-Forwarded-For) in each AZ, which then handle balancing.
Also worth noting that unless you turn on cross-region balancing, if an AZ doesn't have a node in it and the RR DNS points them at that AZ, they'll be turned away. Additionally, without it you need to scale by multiples of AZs you run in otherwise you'll have unbalanced traffic.
On another note, I've always been curious if they're just abstractions around HAproxy at the per-node level.
Very true. Cross-AZ load balancing works quite well. I believe Amazon has said it's RR across the servers with the least connections, but degenerates to a simple RR without many nodes per AZ.
