[mac01021]

It can be unacceptable sometimes and other times just wasteful to block a whole host OS thread just because a read or a write on one socket out of 1000s is not ready.

We solve this problem with APIs for asynchronous or nonblocking IO.

But such APIs must be cleverly designed if they are to permit you to propagate backpressure from the downstream end of your program's dataflow to the upstream end. And handle errors in a sane way, etc.