5M concurrent users is a pretty big number. There are plenty of Node.js systems/frameworks which could scale to that size with minimal effort. SocketCluster ha

5M concurrent users is a pretty big number. There are plenty of Node.js systems/frameworks which could scale to that size with minimal effort. SocketCluster has been used in production to service hundreds of thousands of concurrent users and it can handle millions. I've had one report of a chat system (adult industry) which could handle 250K concurrent users using only two large servers. Also, I once did some consulting work for a popular cryptocurrency trading platform which handled tens of thousands of concurrent users/trading bots (with very high frequency of messages). After I was done, that company didn't talk to me for 6 months straight; it turns out that they hadn't had any issues with their pub/sub cluster since. Unfortunately SocketCluster doesn't get discussed very often among influential circles. I have no idea why because the feedback I get from users is essentially 100% positive. I guess Node.js doesn't get much hype these days.