I have used pdb (Python Debugger) to trace similar code path. Here is what I would do (considering this is a test system, where you can start services in debugger):

1. See what nova processes are running on your controller node. "systemctl | grep nova"
2. From nova-api.log, I grep'ed certain keywords to find the function in compute/api.py.
3. Add breakpoints in the code "/usr/lib/python2.7/site-packages/nova/<your path="" file="">", using import pdb; pdb.set_trace()
4. Start the process manually: eg: "systemctl stop openstack-nova-api.service"; /usr/bin/python /usr/bin/nova-api

Once you hit the break point you will be in pdb mode, it is very convenient to step through the code. You can do anything here, get backtrace, check variables, arguments and a lot more.

Horizon talks to Nova using the published REST API. It uses the compute api and the compute extensions. If you want to see what's happening, you can watch the Nova api log (often /var/log/nova/nova-api.log) on your controller, or consider something like tcpdump for traffic on port 8774 (which is the nova-api port).

Horizon uses the code in the python-novaclient module to communicate with Nova, so reading through the code and docs on that module might also help.

If you are looking for something more specific, it would be helpful if you could clarify that in your questions.