Environment variables are a powerful way to configure your applications and services without hardcoding values into your codebase. Docker Compose allows you to define and manage environment variables in a flexible and efficient manner. In this section, we will cover the following topics:
- Introduction to Environment Variables
- Defining Environment Variables in Docker Compose
- Using
.envFiles - Overriding Environment Variables
- Practical Examples
- Exercises
- Introduction to Environment Variables
Environment variables are key-value pairs that can influence the behavior of running processes. They are commonly used to:
- Configure application settings (e.g., database URLs, API keys).
- Control the behavior of services (e.g., debug mode, logging levels).
- Pass sensitive information securely.
- Defining Environment Variables in Docker Compose
In Docker Compose, you can define environment variables in several ways:
a. Using the environment Key
You can specify environment variables directly within the docker-compose.yml file using the environment key.
version: '3.8'
services:
web:
image: my-web-app:latest
environment:
- DEBUG=true
- DATABASE_URL=postgres://user:password@db:5432/mydatabase
db:
image: postgres:latest
environment:
- POSTGRES_USER=user
- POSTGRES_PASSWORD=password
- POSTGRES_DB=mydatabaseb. Using the env_file Key
You can also specify environment variables in an external file and reference it in your docker-compose.yml file using the env_file key.
version: '3.8'
services:
web:
image: my-web-app:latest
env_file:
- .env
db:
image: postgres:latest
env_file:
- .env
- Using
.env Files
.env FilesThe .env file is a simple text file containing key-value pairs of environment variables. It is typically placed in the root directory of your project.
Example .env file:
DEBUG=true DATABASE_URL=postgres://user:password@db:5432/mydatabase POSTGRES_USER=user POSTGRES_PASSWORD=password POSTGRES_DB=mydatabase
- Overriding Environment Variables
Docker Compose allows you to override environment variables defined in the docker-compose.yml file or .env file by specifying them in the shell environment when running docker-compose commands.
In this example, the DEBUG and DATABASE_URL environment variables will override the values defined in the docker-compose.yml file or .env file.
- Practical Examples
Example 1: Using Environment Variables for Configuration
version: '3.8'
services:
web:
image: my-web-app:latest
environment:
- DEBUG=${DEBUG}
- DATABASE_URL=${DATABASE_URL}
db:
image: postgres:latest
environment:
- POSTGRES_USER=${POSTGRES_USER}
- POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
- POSTGRES_DB=${POSTGRES_DB}Example 2: Using an .env File
Create a .env file with the following content:
DEBUG=true DATABASE_URL=postgres://user:password@db:5432/mydatabase POSTGRES_USER=user POSTGRES_PASSWORD=password POSTGRES_DB=mydatabase
Reference the .env file in your docker-compose.yml:
version: '3.8'
services:
web:
image: my-web-app:latest
env_file:
- .env
db:
image: postgres:latest
env_file:
- .env
- Exercises
Exercise 1: Define Environment Variables
- Create a
docker-compose.ymlfile for a simple web application and a database. - Define environment variables for the database connection string and debug mode directly in the
docker-compose.ymlfile.
Solution:
version: '3.8'
services:
web:
image: my-web-app:latest
environment:
- DEBUG=true
- DATABASE_URL=postgres://user:password@db:5432/mydatabase
db:
image: postgres:latest
environment:
- POSTGRES_USER=user
- POSTGRES_PASSWORD=password
- POSTGRES_DB=mydatabaseExercise 2: Use an .env File
- Create an
.envfile with the necessary environment variables. - Modify the
docker-compose.ymlfile to use the.envfile.
Solution:
Create a .env file:
DEBUG=true DATABASE_URL=postgres://user:password@db:5432/mydatabase POSTGRES_USER=user POSTGRES_PASSWORD=password POSTGRES_DB=mydatabase
Modify the docker-compose.yml file:
version: '3.8'
services:
web:
image: my-web-app:latest
env_file:
- .env
db:
image: postgres:latest
env_file:
- .envConclusion
In this section, we explored how to use environment variables in Docker Compose to configure and manage your services. We covered defining environment variables directly in the docker-compose.yml file, using .env files, and overriding environment variables. By mastering these techniques, you can create more flexible and maintainable Docker Compose configurations.
Docker: From Beginner to Advanced
Module 1: Introduction to Docker
- What is Docker?
- Installing Docker
- Docker Architecture
- Basic Docker Commands
- Understanding Docker Images
- Creating Your First Docker Container
Module 2: Working with Docker Images
- Docker Hub and Repositories
- Building Docker Images
- Dockerfile Basics
- Managing Docker Images
- Tagging and Pushing Images
Module 3: Docker Containers
- Running Containers
- Container Lifecycle
- Managing Containers
- Networking in Docker
- Data Persistence with Volumes
Module 4: Docker Compose
- Introduction to Docker Compose
- Defining Services in Docker Compose
- Docker Compose Commands
- Multi-Container Applications
- Environment Variables in Docker Compose
Module 5: Advanced Docker Concepts
- Docker Networking Deep Dive
- Docker Storage Options
- Docker Security Best Practices
- Optimizing Docker Images
- Docker Logging and Monitoring
Module 6: Docker in Production
- CI/CD with Docker
- Orchestrating Containers with Docker Swarm
- Introduction to Kubernetes
- Deploying Docker Containers in Kubernetes
- Scaling and Load Balancing