If your clients have specific user IDs they need to send them to socket.io server. E.g. on client side you can do
// Browser (front-end)
<script>
const socket = io();
socket.emit('login',{userId:'YourUserID'});
</script>
And on server you will put something like
// server (back-end)
const users = {};
io.on('connection', function(socket){
console.log('a user connected');
socket.on('login', function(data){
console.log('a user ' + data.userId + ' connected');
// saving userId to object with socket ID
users[socket.id] = data.userId;
});
socket.on('disconnect', function(){
console.log('user ' + users[socket.id] + ' disconnected');
// remove saved socket from users object
delete users[socket.id];
});
});
Now you can pair socket ID to your user ID and work with it.