Copyright notice: This is an original article by xwi88, licensed under CC BY-NC 4.0. Commercial use is prohibited; please cite the source when reposting. Follow at https://github.com/xwi88

Common signals

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13

Some of the more commonly used signals:

 1       HUP (hang up)
 2       INT (interrupt)
 3       QUIT (quit)
 6       ABRT (abort)
 9       KILL (non-catchable, non-ignorable kill)
 14      ALRM (alarm clock)
 15      TERM (software termination signal)


 30      USR1 (user defined signal 1)
 31      USR2 (user defined signal 2)

Go Code

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

done := make(chan struct{})
ch := make(chan os.Signal, 1)
signal.Notify(ch, syscall.SIGHUP, syscall.SIGQUIT, syscall.SIGABRT, syscall.SIGKILL, syscall.SIGALRM, syscall.SIGUSR1, syscall.SIGUSR2, syscall.SIGTERM, syscall.SIGINT)

go func() {
    sgName := <-ch
    fmt.Printf("receive kill signal [%v], ready to exit ...", sgName)
    // resource release and other deals
    done <- struct{}{}
}

<-done

For a more convenient wrapper, see signal

kill testing

Run kill on the host to stop the app. There are two runtime environments: host and docker.

Host

kill -<signal_number | signal_name> <pid>

Tested — signals that can be caught normally and return 0:

• 1, 2, 3, 6, 14, 15, 30, 31
• HUP, INT, QUIT, ABRT, ALRM, TERM, USR1, USR2
  • case-insensitive
  • SIG prefix allowed, also case-insensitive

docker

docker kill -<signal_number | signal_name> <container_id>

Tested — signals that can be caught normally and return 0:

• 1, 2, 3, 6, 14, 15, 10, 12
  • In testing, docker kill only recognizes 10 and 12 as USR1 and USR2
  • Reason unclear; USR1 / USR2 not recommended
• HUP, INT, QUIT, ABRT, ALRM, TERM, USR1, USR2
  • case-insensitive
  • SIG prefix allowed, also case-insensitive

Conclusion

From the tests above, the commonly catchable, no-difference, return-0 signals are: 1, 2, 3, 6, 14, 15

By signal semantics, we recommend using:

• 2 SIGINT — trigger: CTRL+C or kill -2
• 15 SIGTERM

Graceful shutdown

1. Send kill with -15/-TERM/-SIGTERM
2. After the program catches the signal: remove the node from discovery, release resources, stop the program
3. After a configured interval, check whether it stopped; if not, fall back to kill -9/-KILL/-SIGKILL per your policy

k8s graceful shutdown

1. Remove the node
2. preStop hook optional delay, to drain in-flight traffic
3. Send kill with -15/-TERM/-SIGTERM
4. The program catches the signal, releases resources, stops
5. Force-killed on timeout via kill -9/-KILL/-SIGKILL
   • default 30s
   • pod: terminationGracePeriodSeconds to change it

Docker-compose kill issue

• Current Docker Compose version 2.2.2

docker-compose may fail to stop on CTRL+C; force quit with CTRL+\