CODEWITHSUNDEEP
dockerdocker-compose
Rafael Marques
Rafael Marques

Reputation: 1445

Test restart policy - how to crash a container such that it restarts

I have a docker-compose file that creates 3 Hello World applications and uses nginx to load balance traffic across the different containers.

The docker-compose code is as follows:

version: '3.2'
services:
  backend1:
      image: rafaelmarques7/hello-node:latest
      restart: always
  backend2:
      image: rafaelmarques7/hello-node:latest
      restart: always
  backend3:
      image: rafaelmarques7/hello-node:latest
      restart: always
  loadbalancer:
      image: nginx:latest      
      restart: always
      links:
          - backend1
          - backend2
          - backend3
      ports:
        - '80:80'
      volumes: 
        - ./container-balancer/nginx.conf:/etc/nginx/nginx.conf:ro

I would like to verify that the restart: always policy actually works.

The approach I tried is as follows:

I was expecting that after the 3rd step (stop/kill the container. this makes it exist with code 137), the restart policy would kick in and create a new container again.

However, this does not happen. I have read that this is intentional, as to have a way to be able to manually stop containers that have a restart policy.

Despite this, I would like to know how I can kill a container in such a way that it triggers the restart policy so that I can actually verify that it is working.

Thank you for your help.

Upvotes: 6

Views: 3988

Answers (2)

Eldad Assis
Eldad Assis

Reputation: 11045

You can docker exec into the running container and kill processes. If your entrypoint process (pid 1) starts a sub process, find it and kill it

docker exec -it backend3 /bin/sh
ps -ef

Find the process that pid 1 is its parent and kill -9 it.

If your entrypoint in the only process (pid 1), it cannot be killed by the kill command. Consider replacing your entrypoint with a script that calls your actual process, which will allow you to use the idea I suggest above.

This should simulate a crashing container and should kick the restart process.

NOTES:

  1. See explanation in https://unix.stackexchange.com/questions/457649/unable-to-kill-process-with-pid-1-in-docker-container
  2. See why not run NodeJS as pid 1 in https://www.elastic.io/nodejs-as-pid-1-under-docker-images/

Upvotes: 5

David Maze
David Maze

Reputation: 159428

If you run ps on the host you will be able to see the actual processes in all of your Docker containers. Once you find a container's main process's process ID, you can sudo kill it (you will have to be root). That will look more like a "crash", especially if you kill -13 to send SIGSEGV.

It is very occasionally useful for validation scenarios like this to have an endpoint that crashes your application that you can enable in test builds and some other similar silly things. Just make sure you do have a gate so that those endpoints don't exist in production builds. (In old-school C, an #ifdef TEST would do the job; some languages have equivalents but many don't.)

Upvotes: 5

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