oslo.service implement different service launcher and PorcessLauncher is one of them. ProcessLauncher can be used in two way 1. with fork 2. without fork. ProcessLauncher with fork use the cotyledon module which take care of the signal handling (for example SIGTERM etc) but ProcessLauncher without fork run the service on main thread and does not implement the signal handling. Signal hanlder for SIGTERM is needed for graceful shutdown to work.

We need to implement those signalhandler for no_fork case which will be done in oslo.service which will be used  during the graceful shutdown of the Nova compute service.