i want to build a push notifications service like facebook with nodejs and socketio. i have a question: - how server can know what exactly client to send notification when many client connected to server? Thanks for help!
Associate sockets with users when they connect, so when you have to notify user "Test" just look at the user.sockets[] list and send him whatever info you want to send.
In other words, Node.js doesn't have to "know" about users, you should take care of that and talk with Node.js in "socket" terms.
You can use socket.io's room feature. You can subscribe each client to its unique room and then emit an event in that room so that user will be the only one to get that.