Docker Compose: What Is It and How to Use It?

What is Docker Compose?

Docker Compose is a local orchestration tool that is part of the Docker ecosystem. It allows you to define all the services, networks, and volumes required by an application in a declarative YAML file, typically named docker-compose.yml. Rather than manually launching each container with lengthy docker run commands, Compose lets you start an entire application stack with a single command: docker compose up.

This tool addresses a fundamental need in modern development: environment reproducibility. When a project involves multiple components — a Python/Django web server, a PostgreSQL database, a Redis message broker, a Celery worker — each must be correctly configured and interconnected. Docker Compose formalises this configuration in a versioned file, ensuring every team member works in an identical environment.

At Kern-IT, Docker Compose is a central tool in our development workflow. Every Django project includes a Compose file that defines the application, database, and auxiliary services. This allows any developer to clone the repository, run docker compose up, and have a fully functional environment in minutes, without manually installing Python, PostgreSQL, or Redis on their machine.

Why Docker Compose Matters

In a context where applications are increasingly distributed and composed of multiple services, Docker Compose delivers concrete benefits at every stage of the software lifecycle.

• Guaranteed reproducibility: the docker-compose.yml file is versioned in Git. Every developer, whether working on macOS, Linux, or Windows, gets exactly the same environment. No more "it works on my machine" problems.
• Accelerated onboarding: a new developer joins the project? They clone the repository, run docker compose up, and start coding. No need for complex installation documentation or managing version incompatibilities.
• Service isolation: each service runs in its own container with its own dependencies. The PostgreSQL version used by one project does not conflict with another. This isolation eliminates side effects.
• Declarative configuration: environment variables, exposed ports, mounted volumes, and internal networks are described explicitly. The configuration is readable, auditable, and easily modifiable.
• Simplified integration testing: Compose can launch a complete stack for running integration tests in a CI/CD pipeline, then cleanly tear it down after tests complete.

How It Works

Docker Compose relies on a YAML file that describes the services composing the application. Each service corresponds to a Docker container, defined by an image (from Docker Hub or a local Dockerfile) and configuration parameters: ports, volumes, environment variables, and inter-service dependencies.

When you run docker compose up, Compose reads the YAML file, creates a dedicated Docker network for the application, builds any required images (if Dockerfiles are referenced), then starts the containers in the order defined by dependencies. Services communicate with each other via the internal network using their service names as DNS hostnames.

Compose also manages container lifecycle: docker compose down stops and removes containers and networks, docker compose logs displays logs from all services, docker compose exec runs commands inside a running container. For a Django project, this means running migrations, opening a shell, or executing tests without leaving the Compose ecosystem.

Concrete Example

At Kern-IT, we use Docker Compose for every client project. Consider a real estate management platform built with Django. The docker-compose.yml file defines four services: the Django server (with Gunicorn), a PostgreSQL database, a Redis broker, and a Celery worker for asynchronous tasks such as sending emails and generating PDF reports.

In development, developers run docker compose up and instantly have the entire stack available. Source files are mounted as volumes, enabling automatic Django server reload on every code change. In CI/CD, the same Compose file is used to run integration tests in an environment identical to production.

Implementation

1. Install Docker and Compose: Docker Compose is built into Docker Desktop (macOS, Windows) and available as a CLI plugin on Linux. Verify installation with docker compose version.
2. Create the docker-compose.yml file: define each service with its image, ports, volumes, and environment variables. Start with the main service (your application) then add auxiliary services.
3. Define dependencies: use depends_on to specify startup order. For a Django project, the web service depends on the database and Redis.
4. Manage environment variables: use a .env file for secrets and environment-specific configurations. Never version this file; provide a .env.example as a template.
5. Configure volumes: mount source code as a volume for development (hot reloading) and use named volumes for persistent data (database).
6. Test and iterate: run docker compose up --build to rebuild images after Dockerfile changes, and docker compose down -v to start from a clean state if needed.

Associated Technologies and Tools

• Docker: the containerisation engine on which Compose is built.
• Kubernetes: production orchestrator, where Compose handles local development.
• Nginx: reverse proxy often defined as a service in a Compose file.
• PostgreSQL: database frequently orchestrated via Compose for development.
• Redis: message broker and cache, a classic service in a Django Compose setup.
• Celery: asynchronous task worker, launched as an independent service in Compose.

Conclusion

Docker Compose is an essential tool for any multi-service project. By formalising environment configuration in a versioned YAML file, it guarantees reproducibility, accelerates developer onboarding, and simplifies integration testing. At Kern-IT, every Django project includes a Compose file that orchestrates the application, database, Redis, and Celery, allowing our team to focus on business code rather than infrastructure configuration. For Belgian companies looking to professionalise their development workflow, Docker Compose is a concrete and accessible first step towards infrastructure automation.