Comment 2 for bug 1535254

The doc is http://docs.openstack.org/liberty/config-reference/content/list-of-compute-config-options.html
and also you can get the illustraion in https://github.com/openstack/nova/blob/master/nova/notifications.py#L44 which is different from actual implementation.

Currently, if 'notify_on_state_change' is set to 'vm_state' or 'vm_and_task_state', all the changes will cause a compute.instance.update notification. I think we'd better introduce another valid value 'all_state' to get all the compute.instance.update notifications. And if 'vm_and_task_state' is set, we only send update notifications when vm state or task state have been changed. The same with 'vm_state'.