The proxy is having a list of backends and their status itself. If a connect to a backend fails with a network error we take it out of the rotation for a while and try again later.
We don't have to do something on failover to a standby-proxy. It will just get requests and handle them as there is no state to be transfered.
http://jan.kneschke.de/projects/mysql/mysql-proxy-ha.png explains it as picture.
You can also use LVS, VRRP|CARP, ... to do it on the IP level if you prefer it.
Jan
