Scaling socket.io broadcast

Question

I want to broadcast a 1Kb message with socket.io (node.js framework), every 3 seconds to a large number of users. What is the best way to scale it (1 user = 1 'listener' with socket.on('periodicMessage',callback) )?

There is no other CPU usage (one read of an external database which is filled by an other external module every 3 seconds), so i am trying to know if a simple heroku server can broadcast a message to 10 000, 100 000, 1 million or more users.

Answer 1

We have easily scaled to tens of thousands of 'listeners' on a single node.js process. I am not sure how many you actually can scale to, given that each socket is a file descriptor, and the plain vanilla kernel can have 65K fd's for each process, no more.

CPU would not be a problem. If at all, upload bandwidth would be (1KB * 50K users / 3 sec = 50M/3sec = 16MB/s upstream. I never measured Heroku, so don't know if they sustain this. I suppose they do, but maybe they limit you, since they are paying Amazon for this, after all).