The AD DC is actually is a bunch of core libraries and services. To make things easiest for our users, the services are linked into and started up by one binary, but internally each different task ends up in a forked process (if appropriate). But we do one better, and allow this to be controlled at runtime, so with '-M single' it essentially becomes a giant state machine, and can be handled with a single gdb. Inter-process communication is via a unix domain socket based messaging system or full DCE/RPC pipes.
External processes can register specific named pipes (when, as we do by default, we use smbd as the file server, this is actually a key part of the design), or DCE/RPC server modules can be loaded (the OpenChange project provides such a module).
We could discuss if more or less of Samba's internal communication should use one design pattern or another, but what is more interesting is that without fanfare or bother, some of those ideas, implemented pragmatically rather than dogmatically, have become an essential part of how Samba is implemented. That pragmatism has then brought us the AD DC that we are so proud to announce today.
I also love that the shared libraries that we now use internally make Samba much smaller as well, reducing the disk space overhead.
Finally, a surprising amount of the code is actually in modules on ldb, our ldap-like database at the core of the system.
I know you were hoping to troll with what has been a long-running design philosophy, but when you spend the time building the system, you find the pragmatism rules the day, and we use a variety of tools to get the job done, and to get it done is a way that is most seamless to our users.