The specific code in AbstractCommonSubscriber.java is as follows. When the yang notification is monitored, the websocket message is sent.

/**code**/

synchronized void post(final String data) {

     for (final WebSocketSessionHandler subscriber : subscribers) {

             final Optional<InetSocketAddress> remoteEndpointAddress = subscriber.getRemoteEndpointAddress();

             if (remoteEndpointAddress.isPresent()) {

                       subscriber.sendDataMessage(data);

                       LOG.debug("Data was sent to subscriber {} on address {}:", this, remoteEndpointAddress.get());

             } else {

                        // removal is probably not necessary, because it will be removed explicitly soon after invocation of

                       // onWebSocketClosed(..) in handler; but just to be sure ...

                       subscribers.remove(subscriber);

                       LOG.debug("Subscriber for {} was removed - web-socket session is not open.", this);

             }

       }

}

In the process of traversing the set, delete the element, will cause ConcurrentModificationException.