Docker-compose external_links not able to connect

Question

I have a couple of app containers that I want to connect to the mongodb container. I tried with external_links but I can not connect to the mongodb.

I get

Answer 1

Documentation at https://docs.docker.com/compose/compose-file/#/externallinks says

If you’re using the version 2 file format, the externally-created containers must be connected to at least one of the same networks as the service which is linking to them.

Ex:

Create a new docker network

docker network create -d bridge custom

docker-compose-1.yml

    version: '2'

    services:
      postgres:
        image: postgres:latest
        ports:
          - 5432:5432
        networks:
          - custom

    networks:
      custom:
        external: true

docker-compose-2.yml

    version: '2'

    services:
      app:
        image: training/webapp
        networks:
          - custom
        external_links:
          - postgres:postgres

    networks:
      custom:
        external: true