Given the same problem, I'd look at a reactor pattern MQTT mosquitto like pubsub protocol in Go, Erlang or something JVM/CLR. The beauty of publish subscribe of many low bandwidth clients is that they need tiny bits low latency work in a sizable bandwidth app. Could run BINC/CBOR RFC 7409 over zeromq on lower-end infiniband between boxen. Millions of IM users are just similar enough to tons of smart meters that the solution overlap is enough, but obviously people are more bursty with jagged sinewave daily traffic loads.

Let the clients handle end-to-end encryption with something zk DH/OTR/SMP.

I'm curious how they later monitored this and did other common SRE stuff. Also deployments.