6 🔗 Docker Compose

🔗 Docker Compose

🌟 What is Docker Compose?

• A tool for defining and managing multi-container Docker applications
• Allows starting multiple containers in one go using a single YAML file (docker-compose.yml)

🤔 Why Use Docker Compose?

Instead of running multiple docker run commands manually, Docker Compose lets you define all services in a single configuration file and manage them together.

🛠️ Basic Structure of docker-compose.yml

version: '3.8'  # Docker Compose version
services: 
  app_service:  # Name of the service (container)
    image: my_app_image  # or ./vote # location to Dockerfile
    ports:
      - "8080:80"  # Port mapping (host:container)
    links:
      - db_service  # Deprecated: Was used to Linking to another service
    networks:
      - app_network  # Custom network
    depends_on:
      - db_service  # Ensure the database starts first
    environment:
      - APP_ENV=production
      
  db_service:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: example
    volumes:
      - db_data:/var/lib/mysql
      
networks:
  app_network:
  
volumes:
  db_data:

🛠️ Key Docker Compose Components

Component	Description
Services	Defines individual containers (like app and database)
Ports Mapping (ports)	Maps ports between host and container
Links (links) (Deprecated in v3+)	Adds entries to the /etc/hosts file to allow container communication
Depends On (depends_on)	Ensures one service starts after another
Networks (networks)	Creates isolated networks for secure communication
Volumes (volumes)	Persistent data storage between container restarts
Build Context (build)	When a Dockerfile is available, specify the build directory build: ./app_directory

🚀 Essential Docker Compose Commands

Command	Description
docker-compose up -d	Start containers in the background
docker-compose down	Stop running containers
docker-compose logs	View logs
docker-compose up --scale app_service=3	Scale services

🔗 Container Communication

Legacy Links (Deprecated)

--link redis:redis

• containerName:NameOfHostAppLookingFor
• Adds entry to the host file of the target container

Modern Networking Approach

Instead of using deprecated links, use custom networks:

services:
  voting-app:
    networks:
      - front-tier
      - back-tier
  
  redis:
    networks:
      - back-tier
      
networks:
  front-tier:
  back-tier:

🌐 Network Management

• Docker Compose automatically creates a default network for your application
• Services can communicate using service names as hostnames
• Custom networks provide better isolation and security

💡 Best Practices

• Use specific image tags instead of latest
• Define health checks for critical services
• Use environment files (.env) for sensitive data
• Leverage volumes for persistent data
• Use depends_on to define service startup order
• Always use networks instead of deprecated links