If you want to do a thorough reading of the code at the network/RPC level, I would suggest starting with greping the source for this string:
Angle bracket is important there. You’ll mostly find files with the names like
RequestStream instances will guide you through the RPC layer of what happens when different components of FDB receive different RPCs. Where those
RequestStreams are declared, which request types they are parameterized on, and how they are used inside actors themselves will be useful.
This isn’t a general answer to how to understand FDB completely, but there are a lot of files that are implementation details which you can skip over. If you follow the RPCs you can get a good idea of which components talk to each other, what they are sending around, etc.