Unit 3: Networking, Volumes & Docker Compose

Part G: Volumes, Storage & Networking

1. Volumes (Persistent Data)

Named volumes are Docker-managed persistent stores. A container can be deleted/recreated while important database data remains safe.

docker volume create mydata
docker run -d --name db -v mydata:/data IMAGE

Hinglish: Production data ko container ki writable layer par depend mat karo; persistent storage/volumes use karo.

2. Bind Mounts

A bind mount maps a host path/file into a container. It is useful for local development to sync code.

docker run -it --rm -v ${PWD}:/app IMAGE sh

3. Port Mapping & The localhost Rule

Syntax is -p HOST_PORT:CONTAINER_PORT. Example -p 8080:80 means host 8080 forwards to container 80.

Inside a container, 'localhost' means that same container. If your app and database are separate containers, the app must connect using the database's service/container name (e.g., 'db'), NOT 'localhost'.

Part H: Docker Compose Complete

Compose defines multi-container applications in YAML (compose.yaml) and manages services, networks, and volumes together.

1. Master compose.yaml

services:
  app:
    build: .
    ports:
      - "8000:8000"
    environment:
      DB_HOST: db
      DB_PORT: "5432"
    depends_on:
      - db
  
  db:
    image: postgres:16
    environment:
      POSTGRES_DB: appdb
      POSTGRES_USER: appuser
      POSTGRES_PASSWORD: change-me
    volumes:
      - db_data:/var/lib/postgresql/data

volumes:
  db_data:

Compose services on the same network can discover each other by service name (e.g., DB_HOST: db).

2. Compose Commands

docker compose up -d (Start in background)
docker compose down (Stop and remove containers/networks)
docker compose logs -f (Tail logs)
docker compose exec app sh (Open shell in app service)

Compose Data Warning: docker compose down -v will remove named volumes and therefore delete your database data. Never use it casually!

Part I: Environment Variables & Secrets

1. Environment Variables

Use -e KEY=value for one variable or --env-file .env for a file.

docker run -e APP_ENV=dev my-app:1.0

2. Secrets Security

Hinglish: Secrets (passwords, API keys) Dockerfile mein hard-code mat karo! Use environment variables injected at runtime, or proper secret-management mechanisms.

Unit 3: Storage & Compose