Containers are not linked with docker-compose version 2

Question

I have a docker-compose file that I upgraded from version 1 to version 2.

It set ups a simple Selenium hub with a firefox node.

When I set it up as version 1 it launches fine. When I set it up with version 2 the ff container returns "Not linked with a running Hub container" and exits.

As I researched it and understood it , is that the linkage between the containers somehow suffers.

Is there a solution ?? Am I missing something ??

version: '2'
services:
  hub:
    container_name: hub
    image: selenium/hub 
    ports:
      - "8080:4444" # HOST:CONTAINER
    expose:
      - "4444" 

  ff:
    container_name: ff
    image: selenium/node-firefox 
    links:
      - hub
    expose:
      - "5555" 

Answer 1

Add an environment variable to your ff section of the Docker Compose file (and you can remove the link):

ff:
  container_name: ff
  image: selenium/node-firefox
  environment:
    - HUB_PORT_4444_TCP_ADDR=hub
  expose:
    - "5555"

Compose version 2 uses a different style of networking. From the upgrading guide:

environment variables created by links have been deprecated for some time. In the new Docker network system, they have been removed. You should either connect directly to the appropriate hostname or set the relevant environment variable yourself, using the link hostname.

From the networking documentation:

links are not required to enable services to communicate - by default, any service can reach any other service at that service’s name.

The Selenium dockerfile uses version 1 style networking by ENV variable. Here in the code, if that variable isn't set (which Docker used to do) the entry_point.sh command exits. Providing the variable explicitly solves this.