[nulltype]

I haven't used Pusher or Socket.IO, so I don't know specifically what they offer. However, I have made a simple unidirectional websocket-type server and it was not a terrible hassle.

With a go server on a GCE n1-standard-2 you should be able to handle over 100k connections per server. I haven't done any benchmarks for throughput, but using redis as the messaging backend you can do pretty well.

You take 100k connections on each of the endpoint servers and make a single redis connection, using redis pubsub to subscribe to channels. Publishers can connect to the same redis and publish to those channels.

If you wanted bi-directional it could work a similar way I think, I'd guess the specific architecture depends on your requirements.