Openstack：如何扩展nova host-describe命令？

I would like to extend the nova command host-describe by adding more columns with data from the database, however I fail to see how the initial request from shell.py (by the method "do_host_describe") travels to the database. I have searched for related documentation and also used PDB to follow along in the code. PDB showed me how the requests is built layer for layer, but I am unable to figure out where it is sent, and who catches it (whom also eventually send the correct data back). My investigations suggest the nova-api service should be involved, and that the path should be I would like to extend the nova command host-de