title: scalability of tornado websocket chat
body: <p>In the tornado websocket chat example, participants are stored in the set (<a href="https://github.com/facebook/tornado/blob/master/demos/websocket/chatdemo.py#LC55" rel="noreferrer">link</a>)  , it is convenient in the case of a single server. But if run multiple instances of the application and nginx as a load balancer, how in this case, better to store the participants?</p>