The C10K problem
Dan Kegel’s 1999 write-up of why a server could not handle ten thousand simultaneous connections, and what would have to change.
It is a historical document now - ten thousand connections is unremarkable - but it is the clearest explanation of why every modern runtime is built the way it is. Thread-per-connection does not scale because threads cost memory and context switches. That single constraint produced epoll, kqueue, async I/O, event loops, and eventually goroutines and virtual threads.
Read it and a lot of “why is it like this” questions about Node, Go and Netty answer themselves.
See also: