How to balance and sync chat on nodejs?

Question

Node.js

Hello!
There is a chat on nodejs + socket.io. I read a couple of articles on the net about this, there are options using cluster, nginx. Actually, everything is clear here, but it is not clear how to synchronize clients from all workers? Let's say 10 people on one worker, and the other part on another, how to send messages to all active clients?
When a client connects, I create an object with some flags and its socket, and add it to the array with all other clients.