Heartbeats

A delivery company sends a driver on a six-hour route and asks the driver to phone the dispatcher every 30 minutes. Most calls say nothing more than "still driving". Their value is in the call that does not come. When 45 minutes pass with no call, the dispatcher knows something is wrong and can send help at once.

A long-running goroutine has the same problem. From outside, a worker busy with a slow job and a worker stuck on a call that never returns look exactly alike. Neither sends a result, and neither returns