Day-09.md

Day 9: Docker Compose - Advanced Features & Best Practices

Concepts:

• depends_on revisited: Understanding its limitations (only ensures start order, not service readiness).
• Healthchecks: Defining commands that Docker can run periodically to check if a service is healthy, allowing Compose to wait for services to be truly ready.
• Environment Variables: Passing configuration to services (using environment and .env files).
• Building Images with Compose: The build key for building images on the fly.
• Scaling Services: Using docker-compose up --scale to run multiple instances of a service.
• profiles (Compose v3.4+): For running different subsets of services in development vs. testing vs. production.
• extends (Compose v2/3): Reusing common configurations across multiple Compose files.

Examples:

1. Healthchecks (improving depends_on):

version: '3.8'
services:
  web:
    build: ./backend # Build from backend directory
    # Removed ports mapping here, as Nginx will handle external exposure
    volumes:
      - ./backend:/app # Mount host's backend directory to container's /app
    environment:
      REDIS_HOST: redis
    networks: # Connect 'web' service to 'app_network'
      - app_network
    depends_on:
      redis: # Ensure 'redis' service starts before 'web' (does not wait for readiness)
        condition: service_healthy # Wait for redis to be healthy

2. Using .env files for environment variables: Create a file named .env in the same directory as docker-compose.yml:

DB_USER=myuser
DB_PASSWORD=mypassword

Then, in docker-compose.yml:

version: '3.8'
services:
  db:
    image: postgres:13
    environment:
      POSTGRES_USER: ${DB_USER} # Uses variable from .env
      POSTGRES_PASSWORD: ${DB_PASSWORD} # Uses variable from .env

Docker Compose will automatically load variables from a .env file.

3. Scaling a service:

Adding nginx to docker-compose.yaml

services:
  nginx: # New Nginx service to act as a reverse proxy/load balancer
    image: nginx:alpine # Use a lightweight Nginx image
    volumes:
      - ./nginx:/etc/nginx/conf.d # Mount custom Nginx configuration
    ports:
      - "8080:80" # Expose Nginx's port 80 to host's port 8080
    networks:
      - app_network
    depends_on:
      web: # Nginx depends on the 'web' service to be running
        condition: service_started # Nginx can start as soon as web containers are started

mkdir nginx
cd nginx

# default.conf
upstream web_app {
    # This tells Nginx to load balance requests across all instances
    # of the 'web' service. Docker Compose's internal DNS handles resolution.
    server web:5000;
}

server {
    listen 80; # Nginx listens on port 80 internally

    location / {
        # Proxy all requests to the 'web_app' upstream group
        proxy_pass http://web_app;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

docker-compose up -d --scale web=3 # Runs 3 instances of the 'web' service
docker-compose ps # Observe multiple web containers

Then scale back down:

docker-compose up -d --scale web=1

4. profiles for development vs. production services:

version: '3.8'
services:
    app:
        build: .
        profiles: ["app"] # Only starts with 'app' profile
    db:
        image: postgres
        profiles: ["app"]
    test_runner:
        image: my-test-image
        profiles: ["test"] # Only starts with 'test' profile

Run with specific profile: ddocker-compose --profile app up -docker-compose --profile app up -d

Challenge 9: Robust Application with Healthchecks and Secrets

• Take your multi-tiered application from Challenge 8.
• Add a healthcheck to your PostgreSQL service to ensure it's truly ready before your Flask app attempts to connect.
• Use a .env file to manage sensitive information like database credentials, rather than hardcoding them in docker-compose.yml.
• Experiment with scaling your web service using docker-compose up --scale. Observe how Docker handles the multiple instances.