Std.Signal.listen

fn

listen: Bool

This is a callable function.

A request from outside the program to stop.

A supervisor — a container runtime, an init system, a terminal — stops a

service by asking rather than by killing: it sends a signal, waits a

configured moment, and only then kills. That moment is the whole

opportunity to finish the requests already in flight, close what is open,

and leave nothing half-written.

A program that cannot hear the request spends the moment unaware and is

killed in the middle of whatever it was doing. Every deployment then drops

live traffic, and nothing in the program says why, because from inside it

looks like being switched off at the wall.

Two signals are heard, and they mean the same thing here: the one a

supervisor sends, and the one a terminal sends when a person presses

Ctrl-C. The signal that cannot be heard by anything, which a supervisor

sends when the waiting is over, is not among them — which is why the

waiting is worth using.

The request is recorded, not acted on. What to do about it belongs to the

program: a server drains its connections, a worker finishes the job in

hand, a batch writes what it has. Acting inside the handler would mean

acting at a moment nothing chose, in the middle of any statement at all.

Begin listening, and say whether listening is possible here.

false means this platform has no signals of this kind, so no request will

ever arrive and requested will answer false forever. A service should

report that rather than announce a graceful shutdown it cannot perform.

Calling this more than once is allowed and does nothing after the first

time, so a component may make sure it is listening without knowing whether

something else already did.