Networking Between Multiple Docker Compose Services

Sometimes you may need to communicate between services defined across multiple compositions I.E services that Docker will start on isolated networks. To enable our services to talk one another as if they had all been defined in one docker-compose.yml we can leverage Docker’s user-defined networks, in this case the Bridged network.

Demonstrating The Problem

To illustrate the issue, let’s assume we have two docker-compose.yml, front/docker-compose.yml and back/docker-compose.yml.

front/docker-compose.yml

version: '2'

services:
  front:
    image: tutum/curl:alpine
    command: curl back

back/docker-compose.yml

version: '2'

services:
  back:
    image: nginx
    command: nginx -g 'daemon off;'

Start the services with a docker-compose up, back service first and then the front service; you should see the following output:

Recreating front_front_1 ...
Recreating front_front_1 ... done
Attaching to front_front_1
front_1  | curl: (6) Couldn't resolve host 'back'
front_front_1 exited with code 6

The problem is quite clear, usually we can rely on Docker to resolve the container names for us, but in this instance our services are running on isloated networks and thus do not support automatic service discovery.

A Solution

user-defined networks to the rescue! As the official Docker documentation says:

"It is recommended to use user-defined bridge networks to control which containers can communicate with
each other, and also to enable automatic DNS resolution of container names to IP addresses"

Creating a custom bridge network

Create the network

   $ docker network create -d bridge my-custom-network
   $ docker network ls

You should see my-custom-network in the list

Now update both docker-compose.yml as follows:

front/docker-compose.yml
```
 version: '2'

 services:
   front:
     image: tutum/curl:alpine
     command: curl back
 networks:
   my-custom-network:
     external: true
```
back/docker-compose.yml
```
 version: '2'

 services:
   back:
     image: nginx
     command: nginx -g 'daemon off;'
 networks:
   my-custom-network:
   external: true
```
We’ve just specified the network we would like our docker-compose to use, my-custom-network.

NOTE: external:true specifies that the network (my-custom-network) has been created outside of compose and therefore docker-compose should not attempt to create it. It will also throw and error if the network does not exist.

Now instruct the services to use this network