I used to be really concerned about this, but one day I sat down and did the math and realized that the worst case scenario is that I'd have no more than two ti

I used to be really concerned about this, but one day I sat down and did the math and realized that the worst case scenario is that I'd have no more than two times as many messages, which means this isn't a scalability issue: if a topic has N subscribers, at bare minimum you are going to have N+1 messages to send a given event (one to receive it into the cluster, and N to dispatch it to the subscribers); if everyone is connected to some random machine, then you need to take the incoming event and send it to the right internal machine that is storing that topic (which is one extra message) and then that machine will need to indirectly send the message to the connecting computers of the subscribers (which is one extra message per subscriber), so you end up with 2N+2 messages. The alternative, where users cluster themselves based on topic with their entry connections, is that these individual end users have to maintain a ton of separate connections for inbound messages for each of the topics they are interested in, which is often going to work out to be much worse for everyone involved.