How processes react to UNIX signals

[nggit]

Tremolo's process manager doesn't have many features, yet resilience was the goal.

At the time this document was written, Tremolo is the only* ASGI server implementation that handles SIGKILL cleanly. You can try the following:

1. Run the ASGI server with 2 workers

python -m tremolo --port 8000 --worker-num 2 example:app

2. Find out the running PIDs based on the output

Tremolo main (pid 30120) is running 
[2025-02-15 13:20:15] Tremolo (ASGI) worker (pid 30121) is started at 127.0.0.1 port 8000
[2025-02-15 13:20:15] Tremolo (ASGI) worker (pid 30122) is started at 127.0.0.1 port 8000

Or check with pidof python3 || pidof python, then usually the main PID is rightmost in the output:

30122 30121 30120

3. Try to stop the main PID with SIGKILL

kill -9 30120

4. Now, what do you expect?

When running pidof python3 || pidof python again, we should see nothing. And this is how it happens in Tremolo.

*) In another implementations that I tested, they kept the child processes alive.

30122 30121

IMO this would make it a bit unpredictable.

Here is a summary table of how processes in Tremolo react to signals:

Signal	Tremolo (main)	Tremolo (worker)
SIGINT (Ctrl-C or kill -2)	Shutdown all children as well	Shutdown only this worker; main exits when no workers are alive
SIGTERM (kill -15 or just kill)	Shutdown all children as well	Shutdown only this worker; main exits when no workers are alive
SIGKILL (kill -9)	All children are stopped	Restart
Others	All children are stopped	Restart