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Discover how to allow communication between containers in different Docker Compose applications running simultaneously with this step-by-step guide.
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Enabling Network Communication Between Docker Compose Applications

In the world of Docker, applications are often deployed in containers, which are isolated units that run your applications. However, there are instances when you may need containers to communicate with each other across different Docker Compose applications. This guide delves into how you can enable this network communication between two separate Docker Compose applications running simultaneously.

The Challenge

Consider two Docker Compose applications, Application A and Application B, both running their own database services. Application A requires access to a database in Application B. The main question is:

How can a container in Docker Compose Application A communicate with a container in Docker Compose Application B?

You may have a scenario where you have delivered Application A as a docker-compose.yml file, which relies on an external database (Postgres) that is running in Application B. It’s essential that both applications be able to connect to each other seamlessly for successful operation.

Example Scenario

Application A: This includes a service (Keycloak) that uses a Postgres database.

Application B: This acts as a demo infrastructure, providing an isolated Postgres database for development and showcasing purposes.

The Solution: Creating an External Network

The solution to enabling communication between these two Docker Compose applications is to create an external network that both applications can connect to. Here's a step-by-step explanation of how to set this up.

Step 1: Create an External Network

First, you will need to create a dedicated external network that will allow both applications to talk to each other.

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Step 2: Modify Docker Compose Configuration

Next, you will update the docker-compose.yml files of both applications to join this external network.

For Application A

At the end of the docker-compose.yml file for Application A, you will add the external network configuration like this:

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For Application B

Similarly, you will make the same addition to the docker-compose.yml file for Application B:

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Step 3: Update Environment Variables

In your terminal setup, ensure that your environment variables for Application A accurately reference the service in Application B. For instance, you might have:

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Step 4: Starting the Applications

Now you can start the applications. First, navigate to the directory of Application B and execute:

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Then, in a new terminal, navigate to Application A and run:

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With both applications running on the same external network, the containerized services should now be able to communicate effectively.

Conclusion

Creating an external network in Docker is a straightforward yet powerful method to facilitate communication between containers across different Docker Compose applications. This method allows you to maintain the independence of your applications while ensuring they can still interact when necessary.

By following the steps outlined above, you can overcome the challenges of container communication without needing more complex Docker configurations. Now, you can deploy and demonstrate your applications seamlessly!