Docker + Django in 10 Days: From Zero to Production [Day: 06]

🎯 Goal of Day 6

By the end of today, you will:

• Understand what environment variables are

• Use a .env file with Docker Compose

• Clean up Django settings.py

• Understand volumes deeply

• Know how Docker keeps your database data safe

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❓ Why Environment Variables Matter

Right now, you have this in settings.py:

PASSWORD = "django_pass"

That’s bad because:

• Secrets leak to GitHub

• Hard to change per environment

• Unprofessional ❌

Industry rule:

❝ Code is public. Secrets are not. ❞

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🧠 What Are Environment Variables?

Environment variables are key-value pairs available to a running process.

Example:

DB_NAME=django_db
DB_PASSWORD=super_secret

Docker passes these safely into containers.

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📁 Step 1: Create .env File

Create a file named .env (same level as docker-compose.yml):

DEBUG=1

POSTGRES_DB=django_db
POSTGRES_USER=django_user
POSTGRES_PASSWORD=django_pass
POSTGRES_HOST=db
POSTGRES_PORT=5432

⚠️ Never commit .env to GitHub.

Add to .gitignore:

.env

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🐳 Step 2: Update docker-compose.yml

Replace hard-coded values:

services:
  web:
    build: .
    command: python manage.py runserver 0.0.0.0:8000
    volumes:
      - .:/app
    ports:
      - "8000:8000"
    env_file:
      - .env
    depends_on:
      - db

  db:
    image: postgres:15
    env_file:
      - .env
    volumes:
      - postgres_data:/var/lib/postgresql/data

volumes:
  postgres_data:

Now:

• Docker reads .env

• Injects variables automatically

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🧠 Important: How .env Works in Compose

• .env → loaded by Docker Compose

• Variables available inside containers

• Same file works for:

  • Django

  • PostgreSQL

Very powerful.

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🐍 Step 3: Update Django settings.py

Use os.environ:

import os

DEBUG = os.environ.get("DEBUG") == "1"

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        "NAME": os.environ.get("POSTGRES_DB"),
        "USER": os.environ.get("POSTGRES_USER"),
        "PASSWORD": os.environ.get("POSTGRES_PASSWORD"),
        "HOST": os.environ.get("POSTGRES_HOST"),
        "PORT": os.environ.get("POSTGRES_PORT"),
    }
}

Now your settings are:

• Clean

• Secure

• Environment-friendly

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🧠 What Are Volumes? (Deep Understanding)

You already used:

volumes:
  - postgres_data:/var/lib/postgresql/data

What this means:

• Database files stored outside container

• Container can be deleted

• Data survives

📌 Without volumes:

Stop container → data gone 😱

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🧪 Test Data Persistence (Do This)

1. Start containers:

   docker compose up
   

2. Create a superuser

3. Stop everything:

   docker compose down
   

4. Start again:

   docker compose up
   

5. Login again

✅ Data is still there → volumes working.

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🧠 Bind Mount vs Volume (Important Difference)

Bind Mount	Volume
.:/app	postgres_data
Local ↔ container	Managed by Docker
Code sync	Persistent data
Dev use	DB use

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📚 Official Docs (Read Today)

• Environment variables in Compose
  https://docs.docker.com/compose/environment-variables/

• Docker volumes
  https://docs.docker.com/storage/volumes/

• Django settings best practices
  https://docs.djangoproject.com/en/stable/topics/settings/

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⚠️ Common Beginner Mistakes

• Committing .env ❌

• Using print(os.environ) (leaks secrets)

• Forgetting env vars exist only at runtime

• Mixing dev & prod secrets

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📝 Homework (Important)

1. Move all secrets to .env

2. Confirm Django reads from env

3. Restart containers

4. Test data persistence

5. Explain (to yourself) why volumes exist

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✅ End of Day 6 Summary

• You learned environment variables

• You secured your Django project

• You understand volumes clearly

• Your setup looks professional now

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