How to avoid the "Docker cannot link to a non running container" error when the external-linked container is actually running using docker-compose

Question

What we want to do:

We want to use docker-compose to link one already running container (A) to another container (B) by container name. We use "external-link" as both containers are started from different docker-compose.yml files.

Problem:

Container B fails to start with the error although a container with that name is running.

ERROR: for container_b  Cannot start service container_b: Cannot link to a non running container: /PREVIOUSLY_LINKED_ID_container_a_1 AS /container_b_1/container_a_1

output of "docker ps":

CONTAINER ID        IMAGE                          COMMAND                  CREATED             STATUS              PORTS                         NAMES
RUNNING_ID        container_a       "/docker-entrypoint.s"   15 minutes ago      Up 15 minutes       5432/tcp                      container_a_1

Sample code:

docker-compose.yml of Container B:

container_b:
  external_links:
  - container_a_1

What differs this question from the other "how to fix"-questions:

• we can't use "sudo service docker restart" (which works) as this is a production environment
• We don't want to fix this every time manually but find the reason so that we can
  • understand what we are doing wrong
  • understand how to avoid this

Assumptions:

• It seems like two instances of the container_a exist (RUNNING_ID and PREVIOUSLY_LINKED_ID)
• This might happen because we
  • rebuilt the container via docker-compose build and
  • changed the forwarded external port of the container (80801:8080)

Comment

• Do not use docker-compose down as suggested in the comments, this removes volumnes!

Answer 1

Docker links are deprecated so unless you need some functionality they provide or are on an extremely old version of docker, I'd recommend switching to docker networks.

Since the containers you want to connect appear to be started in separate compose files, you would create that network externally:

docker network create app_net

Then in your docker-compose.yml files, you connect your containers to that network:

version: '3'

networks:
  app_net:
    external:
      name: app_net

services:
  container_a:
    # ...
    networks:
    - app_net

Then in your container_b, you would connect to container_a as "container_a", not "container_a_1".

As an aside, docker-compose down is not documented to remove volumes unless you pass the -v flag. Perhaps you are using anonymous volumes, in which case I'm not sure that docker-compose up would know where to find your data. A named volume is preferred. More than likely, your data was not being stored in a volume, which is dangerous and removes your ability to update your containers:

$ docker-compose down --help

By default, the only things removed are:

- Containers for services defined in the Compose file
- Networks defined in the `networks` section of the Compose file
- The default network, if one is used

Networks and volumes defined as `external` are never removed.

Usage: down [options]

Options:
    --rmi type          Remove images. Type must be one of:
                        'all': Remove all images used by any service.
                        'local': Remove only images that don't have a custom tag
                        set by the `image` field.
    -v, --volumes       Remove named volumes declared in the `volumes` section
                        of the Compose file and anonymous volumes
                        attached to containers.
    --remove-orphans    Remove containers for services not defined in the
                        Compose file