We have an internal service discovery that supports server push when service instances are added/removed. The current Subscriber interface is inherently pull-based, which works with the assumption that load balancer always asks for available endpoints for every RPC call, but that's not how some RPC frameworks are implemented, e.g. in TChannel I can update the Peers in the connection object, but connection never asks for them itself. Currently the only way to use sd.Subscriber with such connection is to have some bg loop that polls it periodically.

If I am not mistaken, the sd.zk.Subscriber is also based on the push model internally, so it would be good to expose push model via the top level interface.