Cannot link to a running container started by docker-compose

Question

I am setting up my local development environment with docker containers. The docker-compose.yml is like following

version: '2'

services:
  db:
    image: mongo:3
  mq:
    image: rabbitmq:3
  api:
    build: .
    image: my_app/api
    ports:
      - "3000:3000"
    links:
      - db
      - mq
    environment:
      - NODE_ENV=development

It starts without error. And docker lists 3 running containers

docker-compose up -d
docker ps

e90e5a8b5d33        my_app/api    "/usr/local/bin/node "   0.0.0.0:3000->3000/tcp               my_app_api_1
42bfcd971b16        mongo:3       "/entrypoint.sh mongo"   27017/tcp                            my_app_db_1
a0685a816c47        rabbitmq:3    "/docker-entrypoint.s"   4369/tcp, 5671-5672/tcp, 25672/tcp   my_app_mq_1

However when I try to link to those running containers from another container

docker run --link my_app_mq_1:mq --link my_app_db_1:db -it worker 

I get error

 docker: Error response from daemon: Cannot link to /my_app_mq_1, as it does not belong to the default network.

I have also tried

 docker run --link my_app_mq_1:mq --link my_app_db_1:db -it --net default worker 

Same error.

So how can I link to a running container started by docker-compose?

Answer 1

Ok, found the answer to it. In case anyone else comes across the same problem, just do

docker network ls

This command lists all the docker networks. docker-compose will create a new network when you run docker-compose up. In my case, the network is named as myapp_default.

Note: Your app’s network is given a name based on the “project name”, which is based on the name of the directory it lives in. You can override the project name with either the --project-name flag or the COMPOSE_PROJECT_NAME environment variable. Networking in Compose

So the correct way to link to those containers is

docker run --link my_app_mq_1:mq --link my_app_db_1:db -it --net myapp_default worker 

Answer 2

my command to run links

docker run --network=YOUR_NETWORK --link SERVICE -t -i app command

Answer 3

When you use service definition version 2 and more, docker-compose creates user-defined network. Name resolution in a user-defined network works via Docker embedded DNS server. Here's related quote from documentation:

The Docker embedded DNS server enables name resolution for containers connected to a given [user-defined] network. This means that any connected container can ping another container on the same network by its container name.

Containers are also available by network aliases that docker-compose creates. It can be verified by command like:

docker inspect \
  -f '{{json .NetworkSettings.Networks.myapp_default.Aliases}}' my_app_db_1

It prints ["db","$CONTAINER_ID"].

Providing links with --link will not have any effect in case of existing user-defined network. You can make sure and look at /etc/hosts, which will not have the corresponding lines.

Thus the following command is sufficient:

docker run -it --net myapp_default worker