Connecting 2 Docker Compose projects on the same network

Lately, I have been loving using Docker for almost everything. Recently at work, I needed a docker-compose project to join an existing one’s network, mainly to access the existing project’s MySQL instance. If you’re looking for the same, look no further!

Specify project name

First, you would want to setup a project name for your docker-compose project, the project whose network you would want other project(s) to join; instead of docker setting up one based on the folder the compose file exists in. This step is optional, but I recommend doing this. To set a project name, all you need to do is:

docker-compose -p potato_app up

This creates a network named potato_app_default for the services defined in your compose file. Now, we can configure the compose file of the other project to connect to this network!

Configuring the compose file for the other project

Let’s look at the following docker-compose.yml file:

version: "3"
services:
  app:
    container_name: my-app
    build: .
    restart: always
    environment:
      MYSQL_DB_ADAPTER: ${MYSQL_DB_ADAPTER}
      MYSQL_DB_DATABASE: ${MYSQL_DB_DATABASE}
      MYSQL_DB_USERNAME: ${MYSQL_DB_USERNAME}
      MYSQL_DB_PASSWORD: ${MYSQL_DB_PASSWORD}
      MYSQL_DB_HOST: ${MYSQL_DB_HOST}
      MYSQL_DB_PORT: ${MYSQL_DB_PORT}
    command: bash -c "node index.js"
    ports:
      - "80:8000"
networks:
  default:
    external:
      name: potato_app_default

The main part to look at here is the networks key. What the above configuration does is that it instructs Docker to join an external network named potato_app_default by default, instead of creating its own network. Here onwards, the service app can communicate with other services on the network named potato_app_default.

Notice the ports section? This maps the internal port 8000 of the container to port 80 of the host, like it normally should!