5.3.1. Kubernetes Concepts and Docker Compose

Docker Compose — Multi-Container Applications:

Compose defines multi-container applications in a single YAML file. It's designed for development and single-host deployments where you need multiple containers working together.

# docker-compose.yml
version: '3.8'

services:
  web:
    image: nginx:1.25
    ports:
      - "80:80"
    volumes:
      - ./html:/usr/share/nginx/html:ro
    depends_on:
      - app

  app:
    build: ./app                   # Build from ./app/Dockerfile
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/mydb
    restart: unless-stopped

  db:
    image: postgres:15
    volumes:
      - pgdata:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: "${DB_PASSWORD}"  # From .env file

volumes:
  pgdata:                          # Named volume managed by Compose

docker-compose up -d               # Start all services (detached)
docker-compose down                # Stop and remove containers
docker-compose ps                  # Status of services
docker-compose logs -f app         # Follow logs for 'app' service
docker-compose exec app bash       # Shell into running container
docker-compose build               # Rebuild images
docker-compose pull                # Pull latest images

Kubernetes Concepts:

Kubernetes (K8s) is the dominant container orchestration platform. Key objects:

# kubectl — Kubernetes CLI
kubectl get pods                           # List Pods in default namespace
kubectl get pods -n kube-system            # List Pods in kube-system namespace
kubectl get all                            # All objects in namespace
kubectl describe pod mypod                 # Detailed info including events
kubectl logs mypod                         # Container logs
kubectl logs -f mypod                      # Follow logs
kubectl exec -it mypod -- /bin/bash        # Shell into Pod
kubectl apply -f deployment.yaml           # Create/update from manifest
kubectl delete -f deployment.yaml          # Delete resources in manifest
kubectl scale deployment myapp --replicas=5  # Scale Deployment
kubectl rollout status deployment/myapp    # Watch rolling update
kubectl rollout undo deployment/myapp      # Rollback to previous version

⚠️ Exam Trap: kubectl scale (like kubectl edit) changes only the live object. The manifest file — or the Git repository, under GitOps — still holds the old value, so the next kubectl apply of that file, or the next reconciliation, puts it back. To stay declarative, change the manifest and apply it, or update the manifest immediately after an emergency imperative change.