Quiesce nova services

In order to make the deployment of openstack go better (in a non-live-upgrade) it is useful to be able to signal to the nova components (via a special file?, via a special os signal?) that an upgrade is about to occur, so that said processes can start to deny requests for additional work. For ex, nova-api being the first one that would be signaled and the other processes would also be signaled so that they would stop accepting work. This would allow for a graceful way to ensure that nova processes are not accepting work before they are shutdown, upgraded, and restarted. My guess is that many companies have these similar additions anyway (as patches? extensions? other?) so it would seem useful to formalize them and get them well supported upstream as a way to ensure that processes shutdown in a well defined order (until state-consistency & associated resumption is truly accomplished where then it will not matter what order they are shut-down in).