Docker image: signal-demo

Handling signals in a docker container

Purpose

Demo/playground to play with signal handling in docker / kubernetes, additionally along with tmux or screen sessions. Some apps require a terminal, so it's easiest to run them in a tmux/screen session.

IMHO tmux is a bit easier to automate while I used screen a lot "interactively" (quite a while ago).

Try it

Docker

docker run -it --rm --name demo drpsychick/signal-demo
# will run and wait for signal

> docker stop demo # in another shell
--> got TERM signal

Kubernetes

kubectl apply -f k8s/signal-demo.yml
kubectl logs -f -l app=signal-demo
# will run and wait for signal

> kubectl delete pod -l app=signal-demo # in another shell (k8s will create a new pod)
--> got TERM signal

> kubectl delete -f k8s/signal-demo.yml # in another shell
--> got TERM signal

Sending signals

In interactive mode, the first signal to try is Ctrl-C, it will send an INT signal.

Other signals can be sent through the exec commands to PID 1 in the pod:

docker exec test kill -HUP 1
kubectl exec deploy/signal-demo-deployment -- kill -USR1 1

You'll see the corresponding messages as defined in the trap commands:

trap 'echo "--> got signal: TERM - shutdown."; tmux kill-server; screen -X kill' TERM
trap 'echo "--> got signal: INT - shutting down..."; tmux send-keys C-c exit C-m; screen -X kill' INT
trap 'echo "--> got signal: USR1 - stopping tmux"; tmux send-keys C-c exit C-m' USR1
trap 'echo "--> got signal: USR2 - stopping screen"; screen -X kill' USR2
trap 'echo "--> got signal: HUP - status report"; ps faxuwww' HUP
trap 'echo "--> got signal: QUIT/ABRT"' QUIT ABRT

What happens when you use exec

docker run --rm --name demo -e EXEC_DEMO=yes drpsychick/signal-demo

When you exec <cmd> then it takes over the running process and the shell script stops at that point. The command executed will be PID #1.

This can be very useful if you have an app that takes over signal handling and cleanup of children as well.