Exploring the source code to understand how requests are managed

Hi everyone,
I would like to implement a priority ordering to the requests sent by the clients, as part of my thesis.
I suppose there is some sort of message queue that connects the clients to the server, but I’m unable to find its implementation in this huge repository. Therefore, I need help going through the source code of the database and find out how the requests are managed.

Thank you very much for your assistance

I located the source files I’m interested in, but I don’t know which branch I should fork from to apply my changes: master, v4.2 or r4.2.0?