https://www.hivemq.com/blog/mqtt-essentials-part-7-persistent-session-queuing-messages/
And the usual long-lived tcp stuff.
by the way, load-balancing grpc (http2) like http wont work as expected. grpc is also a long lived tcp. So you might end-up in a situation where all NS connect to 1 AS, even if you have multiple instances, this depends in the boot order. How many AS are ready before the NS-instances come up.
I dont remember exactly how ecs works, and if you can avoid that by having an NS connecting to it’s “local” AS.