It's been incredibly important for me in both high-traffic network back-ends, as well as in GUI apps. When writing complex servers it's very problematic to hav

It's been incredibly important for me in both high-traffic network back-ends, as well as in GUI apps. When writing complex servers it's very problematic to have requests piling up waiting on something unresponsive (some other API, microservice, borked DNS, database having a bad time, etc.). Sometimes clients can be stuck waiting forever, eventually causing the server to run out of file descriptors or RAM. Everything needs timeouts and circuit breakers. Poorly implemented cancellation that leaves some work running can create pathological situations that eat all CPU and RAM. If some data takes too long to retrieve, and you time out the request without stopping processing, the client will retry, asking for that huge slow thing again, piling up another and another and another huge task that doesn't get cancelled, making the problem worse with each retry. Often threading is mixed with callbacks for returning results. The un-cancelled callbacks firing after the other part of the application aborted an operation can cause race conditions, by messing up some state or being misattributed to another operation.