Introduction

Docker provides a bridge network on every service that defined in docker-compose file. On default, if we don’t specify the networks configuration, it will generate automatically based on project directory with _default suffix and each service will be in same network and can directly communicate. However, the problem comes when we have different container stack with new docker-compose file and we want to connect to existing services in the same network.

Docker Compose Networks

The idea is to connect different service in the pre-existing network is using this boilerplate configuration, and set the options external : true.

services:
  # ...
    networks:
      - app_network

networks:
  app_network:
    name: my-pre-existing-network
    external: true

Instead of attempting to create a network called [projectname]_default, Compose looks for a network called my-pre-existing-network and connects your app’s containers to it.

Demo

Assume we have 2 docker-compose file for different project.

Project A : docker-compose.yml 

services:
  db:
    image: mariadb:latest
    container_name: app-db
    ports:
      - '3306:3306'
    networks:
      - app-network

networks:
  app-network:
    driver: bridge
    name: project-A-network

This will create db service that loads mariadb image that expose port 3306 and connected in app-network named project-A-network.

Project B : docker-compose.yml

services:
  app:
    image: my-app
    container_name: main-app
    ports:
      - '80:80'
    networks:
      - app-network

networks:
  app-network:
    name: project-A-network
    external: true

This will create dummy app service that running our apps that expose to port 80. It will connect to project-A-network and can communicate each others with IP or service name.

Conclusion

This post has shown you how to connect various Docker Compose services together over a single network. They can join an already-established network or we can build a network from a service definition.

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