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Docker Compose

What is Docker Compose?

Docker Compose is a tool for defining and running multi-container Docker applications.
It uses a YAML file (docker-compose.yml) to configure application services, networks, and volumes.

With a single command (docker compose up), you can start all the defined services.

Why Use Docker Compose?

  • Simplifies running multi-container applications.
  • Configuration as code (docker-compose.yml).
  • Easy to share and reproduce environments.
  • Supports scaling services up and down.

Installing Docker Compose

Docker Desktop includes Compose by default.
Check version:

docker compose version

For Linux, install separately:
Install Docker Compose

Docker Compose CLI Basics

Command Description
docker compose up Start and attach to containers
docker compose up -d Start containers in detached mode (background)
docker compose down Stop and remove containers, volumes and networks
docker compose ps List running services (containers)
docker compose logs Show services logs
docker compose exec web sh Run a shell inside a service container
docker compose build Build or rebuild services
docker compose restart Restart running containers
docker compose stop Stop containers without removing them

docker-compose.yml Structure

A basic YAML file structure includes:

  • services: Application containers (e.g., web, db).
  • networks: Communication between services.
  • volumes: Data persistence.

Example:

version: "3.9"

services:
  web:
    image: nginx:alpine
    ports:
      - "8080:80"
    volumes:
      - ./html:/usr/share/nginx/html

  db:
    image: postgres:15
    environment:
      POSTGRES_USER: admin
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: mydb
    volumes:
      - db-data:/var/lib/postgresql/data

volumes:
  db-data:

Example: Multi-Container App

docker-compose.yml

version: "3.9"

services:
  backend:
    build: ./backend
    ports:
      - "5000:5000"
    environment:
      - DATABASE_URL=postgres://admin:secret@db:5432/mydb
    depends_on:
      - db

  frontend:
    build: ./frontend
    ports:
      - "3000:3000"

  db:
    image: postgres:15
    restart: always
    environment:
      POSTGRES_USER: admin
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: mydb
    volumes:
      - db-data:/var/lib/postgresql/data

volumes:
  db-data:

Commands

docker compose up -d      # Start all services in background
docker compose ps         # Check running services
docker compose logs db    # View logs of db service
docker compose down       # Stop and clean up

Overriding and Extending Compose Files

  • Default file is docker-compose.yml.
  • You can use multiple files for environments (e.g., docker-compose.override.yml).
docker compose -f docker-compose.yml -f docker-compose.prod.yml up -d

Scaling Services

You can scale services with a single command:

docker compose up -d --scale web=3

This runs 3 replicas of the web service.

Best Practices

  • Keep secrets outside YAML files (use .env files).
  • Use version control for your Compose files.
  • Separate development and production configurations.
  • Use named volumes for persistent data.

References