Migrating traditional applications to Docker containers is a fundamental step to leverage the efficiency and scalability that Docker offers. In this chapter, we'll explore how to adapt existing applications to run in containers and cover best practices for a successful migration.

Benefits of Migrating Applications to Docker

Migrating applications to Docker provides several benefits, such as portability, scalability, and development environment consistency. By containerizing an application, it is possible to reduce dependencies and simplify deployment in diverse environments.

This image shows how migration to Docker is done

Assessment of the Application for Migration

Before migrating, it's important to evaluate the application's architecture to ensure it can adapt well to a container environment. Check the following aspects:

• Dependencies: Identify the application's dependencies and determine if they are available in Docker images.
• Persistent State: Determine if the application uses persistent data and consider moving storage to volumes.
• Configuration: Assess if configuration files can be externalized using environment variables.

Creating the Dockerfile for the Application

Once the application is assessed, the next step is to write a Dockerfile that defines the runtime environment. Below is an example Dockerfile for a Node.js application:

dockerfile

Configuring Volumes for Persistent Storage

Applications that require data storage should be configured to use volumes. This ensures that the data persists even when containers are restarted or deleted.

bash

Externalizing Configuration with Environment Variables

To maintain deployment flexibility, it is advisable to externalize configuration using environment variables. This allows changing application parameters without modifying the code.

bash

Testing the Application in a Container Environment

It is essential to conduct thorough testing to ensure the application functions correctly in a container environment. Docker makes it easy to run tests in isolated environments.

Testing Example

To run tests in a container, use the following command:

bash

Deployment of the Migrated Application

Once the application has been migrated and tested, it is ready to be deployed. This can be done using Docker Compose if the application has multiple services or directly using the docker run command.

Deployment with Docker Compose

Example of a docker-compose.yml file for an application with a database:

yaml

Conclusion

Migrating applications to Docker containers is an excellent way to optimize the deployment and maintenance of applications. By following the steps and best practices presented in this chapter, it is possible to transform traditional applications into scalable and portable solutions, ready to run in container environments.