How to automatically restart a container if another container is recreated in docker-compose?

Question

I have two containers: A and B. Container B needs to be restarted each time container A is recreated to pick up that container's new id.

How can this be accomplished without hackery?

Answer 1

Not something I've tried to do before, but .. the docker daemon emits events when certain things happen. You can see some of these at https://docs.docker.com/engine/reference/commandline/events/#parent-command but, for example:

Docker containers report the following events:

attach commit copy create destroy detach die exec_create exec_detach exec_start export health_status kill oom pause rename resize restart start stop top unpause update

By default, on a single docker host, you can talk to the daemon through a unix socket /var/run/docker.sock. You can also bind that unix socket into a container so that you can catch events from inside a container. Here's a simple docker-compose.yml that does this:

version: '3.2'

services:

  container_a:
    image: nginx
    container_name: container_a

  container_b:
    image: docker
    container_name: container_b
    command: docker events
    restart: always
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro

Start this stack with docker-compose up -d. Then, in one terminal, run docker logs -f container_b. In another terminal, run docker restart container_a and you'll see some events in the log window that show the container restarting. Your application can catch those events using a docker client library and then either terminate itself and wait to be restarted, or somehow otherwise arrange restart or reconfiguration.

Note that these events will actually tell you the new container's ID, so maybe you don't even need to restart?