Hi As Nathan Fritz said, you'll need a small cluster. ejabberd, the server i'm aware of, is able to handle this load, but all depends on the type of load you expect.
having 200k concurrent users is not the relevant value for pubsub. the key numbers are: - expected number of nodes - expected number of publish/node/time - expected number of subscribers/node this will give you idea about in/out throughput, and also estimate lerver load. if your service handles lots of node creation/subscription/unsubscribe but few publishes, it's not the same load compared to lots of publishes with permanant subscriptions. here load is goes from cpu to network. the way you'll design the cluster depends on this.
