docker-compose.yml

version: '3'

services:
  app:
    # This defines the configuration options, including the context and dockerfile,
    # that will be applied when Compose builds the application image.
    build:
      # This defines the build context for the image build — in this case, the current project directory.
      context: .
      # This specifies the Dockerfile in your current project directory as the file
      dockerfile: Dockerfile
    image: app
    container_name: app
    # This defines the restart policy. The default is no,
    # but we have set the container to restart unless it is stopped.
    restart: unless-stopped
    env_file: .env
    ports:
      # This maps port from .env on the host to same port number on the container.
      - '$PORT:$PORT'
    links:
      - mongo

  mongo:
    # To create this service, Compose will pull the mongo
    image: mongo:4.2.5
    container_name: mongo
    restart: unless-stopped
    # This tells Compose that we would like to add environment variables
    # from a file called .env, located in our build context.
    env_file: .env
    environment:
      # MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD together create
      # a root user in the admin authentication database and ensure that authentication is enabled
      # when the container starts. We have set MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD
      # using the values from our .env file, which we pass to the db service using the env_file option.
      - MONGO_INITDB_ROOT_USERNAME=$DB_ADMIN
      - MONGO_INITDB_ROOT_PASSWORD=$DB_ADMIN_PWD
      - MONGO_INITDB_DATABASE=$DB_NAME
    ports:
      - '$DB_PORT:27017'
    volumes:
      - ./addons/init-mongo.js:/docker-entrypoint-initdb.d/init-mongo.js:ro
        # The named volume dbdata will persist the data stored in Mongo’s default data directory, /data/db.
        # This will ensure that you don’t lose data in cases where you stop or remove containers.
      - dbdata:/data/db

# Our top-level volumes key defines the volumes dbdata.
# When Docker creates volumes, the contents of the volume are stored in a part of the host filesystem, /var/lib/docker/volumes/, that’s managed by Docker.
# The contents of each volume are stored in a directory under /var/lib/docker/volumes/ and get mounted to any container that uses the volume.
# In this way, the data that our users will create will persist in the dbdata volume even if we remove and recreate the db container.
volumes:
  dbdata: