Echo server, but the API are for the server, not the clients.
My specific case is: the app is open to operators, customers' and suppliers' users.
While the operators must see anybody on line, customers must only see the operators and that company's users online. Suppliers as well must only see operators and their own colleagues from the same supplier.
I tried to design a multiple presence channels system but since I don't know which companies or suppliers could be on line at a given moment I should have the operators' sessions subscribed to all the (potentially empty) companies' and suppliers' presence channels...