How to Network Connect Two Containers With Docker-compose

In This Tutorial, You Will Learn:

• How to define a network in your docker-compose.yml file
• How to connect multiple services to the same network
• The benefits of using Docker networks for container communication

Software Requirements and Linux Command Line Conventions

How to Connect Two Containers With Docker-Compose

Connecting containers in Docker Compose involves creating a docker-compose.yml file that specifies the services and the network they will communicate through. Let’s dive into the setup.

Step-by-Step Instructions

1. Create a docker-compose.yml File: Start by creating a new directory and a docker-compose.yml file within it.

   $ mkdir my_project && cd my_project

   $ touch docker-compose.yml

   This file will define the services and networks for your application.

2. Define Your Services: Open the docker-compose.yml file and define the services that you want to connect. You have two options for networking: using an explicit network or allowing Docker to create a default network.
   

   Option 1: Explicit Network

   This method gives you more control over networking by explicitly defining a custom network named my_network.

   version: '3'
   services:
     web:
       image: nginx
       networks:
         - my_network
     db:
       image: mysql:5.7
       environment:
         MYSQL_ROOT_PASSWORD: example
       networks:
         - my_network
   networks:
     my_network:

   In this setup:

   • A custom network my_network is explicitly created.
   • The web and db services are both attached to my_network.
   • This isolates services from other containers unless they are manually added to the same network.

   Option 2: Default Network (Automatically Created)

   If you do not define a network, Docker Compose automatically creates a network named your_project_default. Containers can still communicate using service names.

   version: '3'
   services:
     web:
       image: nginx
     db:
       image: mysql:5.7
       environment:
         MYSQL_ROOT_PASSWORD: example

   In this setup:

   • Docker Compose automatically creates a network named your_project_default.
   • Both web and db are attached to this default network.
   • Containers can still communicate using service names (e.g., db).

   Key Differences Between Explicit and Default Networks

   Feature	Explicit Network	Default Network
   Network Name	User-defined (e.g., my_network)	Automatically created (your_project_default)
   Service Name Resolution	Yes	Yes
   Multiple Networks	Yes (can assign services to multiple networks)	No (all services are in one default network)
   Isolation	Yes (only containers in the same custom network can communicate)	Yes (only containers in the same project communicate)
   Manual Cleanup Required?	Yes (network is removed when containers stop)	Yes (default network is removed when containers stop)

   Which Option Should You Choose?

   • Use the explicit network if you need better network management, multiple networks, or service isolation.
   • Use the default network if all services should communicate automatically within the same project, without extra configuration.
3. Run Your Docker Compose Setup: With the docker-compose.yml file in place, launch your services using the following command.

   $ docker-compose up -d

   This command starts your services in detached mode, allowing them to run in the background.

4. Test Container Communication: To see if the containers can communicate, you can enter the web container and ping the database container using its service name.

   $ docker exec -it my_project_web_1 /bin/bash

   Then Inside the docker container run:

   root@container:/# apt update && apt install -y iputils-ping
   root@container:/# ping db

   

Conclusion

Connecting containers with Docker Compose is a straightforward process that enhances the communication between services in your applications. By using Docker networks, you can ensure that your services are logically grouped and can communicate effectively with one another. This tutorial provided a practical example of setting up a basic web and database service connection. As you build more complex applications, understanding these networking concepts will be vital.

Frequently Asked Questions (FAQ)

1. What is Docker Compose?

   Docker Compose is a tool for defining and running multi-container Docker applications. It allows you to configure your application’s services in a single file.

2. How do I access a service from another container?

   You can access a service from another container using the service name defined in your docker-compose.yml file as the hostname.

3. Can I connect a container to multiple networks?

   Yes, you can connect a single container to multiple networks by specifying additional networks in your service definition.

4. What happens if I don’t define a network?

   If you don’t define a network, Docker will create a default bridge network for your containers, but it’s