All I/O in julia is non-blocking in the traditional sense of the word, because we don’t block threads on I/O activity. However, we do use tasks to simulate blocking I/O for ease of programming. The nb_available function isn’t super meaningful for sockets. It tells you how many bytes are available in the userspace buffer. It says nothing about now many bytes are available in the kernel, in various buffers along the network path or are in transit.