Comment 3 for bug 1755602

To expand on the "better workflow than we have today":

Right now, when nova puppet or ansible modules are running to set up nova-compute, they run discover_hosts after starting the service. However, if using ironic, and ironic isn't running, configured, or populated with hosts, then discover_hosts will not create any host mappings. That means that you can't create an empty nova deployment properly configured, pointing to ironic, and then start enrolling ironic nodes. It also means that you must run discover_hosts after adding ironic nodes, which is mixing nova and ironic tasks, and has been the source of much complaining from people using ironic.

If we allow discovery by service, then you can set up nova fully during the nova phase, and then move on to ironic without having to come back. A much better workflow.