code: public void signIn(String key, WebSocketConnection connection) {
          ConcurrentMap<WebSocketConnection, WebSocketConnection> bag = ensureConnectionList(key);
          bag.put(connection, connection);
      }
docs: Sign in a connection to the registry by key.
  
  Note multiple connections can be attached to the same key
  
  @param key
  the key
  @param connection
  the websocket connection
  @see #register(String, WebSocketConnection)