It was in C, but the process was expected to run for months without crashing or running out of memory; if it had an abnormal exit, you'd (the owner as set in some code thing) get an email with the core backtrace. If it had a memory leak, ops would be on you quickly. Ops was considered a primary driver of requirements to dev, and absolutely everything could be reloaded into a running server without restarting it. There was a TCP control port where a TCL interpreter inside the service was exposed and generally you wrote the (very simple, CS 101 style) TCL commands to manage the server. It was a "No Threads Kernel", scaled to the dozens or hundreds of physical machines communicating over a very well managed network, and most 1 process per core, and 1 core for the OS. The 200 or so unix developers (as we were called) had a common understanding of how the framework worked and if you were just writing app code it was basically impossible to write slow services. We had technical writers that would interview the developers and write books that could be handed to outside developers and lead to a successful integration with no developer time spent.
The NTK was primarily for sending msgs over the network - we had a principle to never write to disk (which were pretty slow in the 1990s), so everything was just a server to get network messages and then send out other messages in reponse and then assemble the replies/timeouts and send back a msg to the caller. All done over persistent connections established by the infrastructure, the applications just registered callbacks for msg type 'X' which would present one with the caller information, the msg as a buffer, and potentially a "user word" which would be a way to keep server state around between calls.
The layering, from the main select loop thru different layers of TCP connection, SSL handling (if used, not 100% was SSL in the 90s), thru persistent link handling, application msg handling, timers, memory allocation, etc. was done with such art that I felt it to be a thing of beauty.