1. Set up a live server and a backup server.
2. start a message sender to send messages to the live server and start a message receiver to receive it.
3. during the send-receive process, kill -9 the live server and backup server takes over
4. then messages are sent to the backup server but the messages won't be delivered to the receiver.
It's look like a reconnection failed from client point of view. But investigation shows that the connection is established but the messages are not delivered for some reason yet to be found.
So far I observed that the session reattachment seems to be the cause.