use "start_dependenies" as readiness probes as in Kubernetes

[ozbillwang]

Thanks to share the idea on how to set the dependencies between services.

What I want to do with it is, use the service start_dependencies as readness probes as we used in Kubernetes.

So the ideal docker-compose file could be:

version: '2'
services:
  the_database:
    image: ubuntu:14.04
    command: >
      /bin/bash -c "
        sleep 5;
        nc -lk 0.0.0.0 5432;
      "
  another_service:
    image: ubuntu:14.04
    command: >
      /bin/bash -c "
        sleep 8;
        nc -lk 0.0.0.0 5555;
      "

  start_dependencies:
    image: dadarek/wait-for-dependencies
    depends_on:
      - the_database
      - another_service
    command: the_database:5432 another_service:5555

  the_web_server:
    image: ubuntu:14.04
    depends_on:
      - start_dependencies
    command: >
      /bin/bash -c "
        nc -z the_database 5432 &&
        echo Connected to DB and started!
      "

The only change I made is in session of the_web_server, change

    depends_on:
      - the_database
      - another_service

to

    depends_on:
      - start_dependencies

So service the_web_server will only start when start_dependencies reports the other two services are health.