How to link multiple docker-compose services via network

4 min read  •  2020-06-11  •   how-to   devops  •   #docker   #networking

This scenario came from a question I was asked docker-compose and network connectivity between services defined in different docker-compose.yml files.

The desired result was to be able to define a docker-compose.yml in one file, and in a second docker-compose.yml have the ability to reach the first service via service or container name for development purposes.

Default scenario: Two separate docker-compose.yml and two separate default networks🔗

Let’s take a simple docker compose file.

version: '3' 
services: 
  service1: 
    image: busybox 
    command: sleep infinity

When it starts up, a default network is created. Its name is based on the service name and the directory name of the docker-compose.yml file.

$ pwd
/tmp/docker-example/compose1

$ docker-compose up -d
Creating network "compose1_default" with the default driver
Creating compose1_service1_1 ... done

Second docker compose file🔗

version: '3' 
services: 
  service2: 
    image: busybox 
    command: sleep infinity

Starting services in a second docker compose file, we see the same behavior. A new default network is created and used.

$ pwd
/tmp/docker-example/compose2

$ docker-compose up -d
Creating network "compose2_default" with the default driver
Creating compose2_service2_1 ... done

A side-effect of these isolated networks are that the containers are unable to ping one another by service name or container name.

Test: From Service 1 ping Service 2🔗

# By service name
$ docker exec -it compose1_service1_1 ping service2
ping: bad address 'service2'

# By container name
$ docker exec -it compose1_service1_1 ping compose2_service2_1 
ping: bad address 'compose2_service2_1'

Test: Service 2 ping Service 1🔗

# By service name
$ docker exec -it compose2_service2_1 ping service1
ping: bad address 'service1'

# By container name
$ docker exec -it compose2_service2_1 ping compose1_service1_1 
ping: bad address 'compose1_service1_1'

New scenario: Sharing a network between services🔗

If you want define services in multiple docker-compose.yml files, and also have network connectivity between the services, you need to configure your services to use the same network.

To create an external network, you can run docker network create <name>. -- where <name> can be a single string without spaces.

Creating the network🔗

$ docker network create external-example
2af4d92c2054e9deb86edaea8bb55ecb74f84a62aec7614c9f09fee386f248a6

Modified first docker-compose file with network configured🔗

version: '3' 
services: 
  service1: 
    image: busybox 
    command: sleep infinity 

networks: 
  default: 
    external: 
      name: external-example

Restarting the services

$ pwd
/tmp/docker-example/compose1

$ docker-compose up -d
Creating compose1_service1_1 ... done

Modified second docker-compose file with network configured🔗

version: '3' 
services: 
  service2: 
    image: busybox 
    command: sleep infinity 

networks: 
  default: 
    external: 
      name: external-example

Restarting the services

$ pwd
/tmp/docker-example/compose2

$ docker-compose up -d
Creating compose2_service2_1 ... done

After running docker-compose up -d on both docker-compose.yml files, we see that no new networks were created.

$ docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
25e0c599d5e5        bridge              bridge              local
2af4d92c2054        external-example    bridge              local
7df4631e9cff        host                host                local
194d4156d7ab        none                null                local

With the containers using the external-example network, they are able to ping one another.

Test: Service 1 ping Service 2🔗

# By service name
$ docker exec -it compose1_service1_1 ping service2
PING service2 (172.24.0.3): 56 data bytes
64 bytes from 172.24.0.3: seq=0 ttl=64 time=0.054 ms
^C
--- service2 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.054/0.054/0.054 ms

# By container name
$ docker exec -it compose1_service1_1 ping compose2_service2_1
PING compose2_service2_1 (172.24.0.2): 56 data bytes
64 bytes from 172.24.0.2: seq=0 ttl=64 time=0.042 ms
^C
--- compose2_service2_1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.042/0.042/0.042 ms

Test: Service 2 ping Service 1🔗

# By service name
$ docker exec -it compose2_service2_1 ping service1
PING service1 (172.24.0.2): 56 data bytes
64 bytes from 172.24.0.2: seq=0 ttl=64 time=0.041 ms
^C
--- service1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.041/0.041/0.041 ms

# By container name
$ docker exec -it compose2_service2_1 ping compose1_service1_1
PING compose1_service1_1 (172.24.0.3): 56 data bytes
64 bytes from 172.24.0.3: seq=0 ttl=64 time=0.042 ms
^C
--- compose1_service1_1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.042/0.042/0.042 ms

As a note, you can configure your services to use a custom container name by declaring the container_name key under each service (i.e., at the same level as image).

Link to Docker-compose docs - container_name

Takeaway🔗

You can connect services defined across multiple docker-compose.yml files.

In order to do this you’ll need to:

  1. Create an external network with docker network create <network name>
  2. In each of your docker-compose.yml configure the default network to use your externally created network with the networks top-level key.
  3. You can use either the service name or container name to connect between containers.