Day-07.md

Day 7: Docker Compose - Introduction & Basic Services

Concepts:

• Docker Compose: A tool for defining and running multi-container Docker applications. You define your application's services, networks, and volumes in a single docker-compose.yml file.
• docker-compose.yml: The core configuration file for Compose, written in YAML.
• Services: Individual containers that make up your application. Each service corresponds to a container that will be run.
• up and down commands: How to start and stop your entire application stack.
• Automatic Network Creation: Compose automatically creates a default user-defined bridge network for all services defined in the docker-compose.yml file, allowing them to communicate by service name.
• Port Mapping in Compose: Defining host-to-container port mappings in the yml file.

Examples:

1. Create a new directory for your Compose project: my-first-compose-app.
2. app.py (Simple Flask app for web service):

from flask import Flask
from redis import Redis
import os

app = Flask(__name__)
# 'redis' will be the hostname for the Redis service as defined in docker-compose.yml
redis_host = os.getenv('REDIS_HOST', 'redis')
redis = Redis(host=redis_host, port=6379) 

@app.route('/')
def hello():
    try:
        count = redis.incr('hits')
        return f'Hello from Docker! I have been seen {count} times.\n'
    except Exception as e:
        return f"Error connecting to Redis: {e}\n", 500

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000, debug=True)

3. requirements.txt:

Flask
redis

4. Dockerfile (for the Flask app - placed in my-first-compose-app):

FROM python:3.9-slim-buster

WORKDIR /app

COPY requirements.txt .

RUN pip install -r requirements.txt

COPY . .

EXPOSE 5000

CMD ["python", "app.py"]

5. docker-compose.yml (in my-first-compose-app directory):

version: '3.8' # Specify Compose file format version

services:
  web: # Define a service named 'web'
    build: . # Build the image from the Dockerfile in the current directory
    ports:
      - "8000:5000" # Map host port 8000 to container port 5000
    environment:
      REDIS_HOST: redis # Pass environment variable to the container
    depends_on: # Ensure redis service starts before web (does not wait for readiness)
      - redis 
  redis: # Define a service named 'redis'
    image: "redis:alpine" # Use the official Redis Alpine image from Docker Hub

6. Start the application: Navigate to the my-first-compose-app directory in your terminal.

sudo docker-compose up -d

(The -d flag runs containers in detached mode.)

7. Access the application: Open http://localhost:8000 in your browser and refresh to see the counter increase.

8. View logs for all services or specific services:

docker-compose logs
docker-compose logs web
docker-compose logs redis

9. Check running services:

docker-compose ps

10. Stop and remove containers and network created by Compose:

docker-compose down

Challenge 7: Compose a Simple Nginx and Static Site

• Create a new directory for this challenge.
• Create a simple index.html file (e.g., "Hello from Nginx!").
• Write a docker-compose.yml file to:
  • Define a service for Nginx using the nginx:latest image.
  • Map a host port (e.g., 8080) to Nginx's default web port (80).
  • Use a bind mount to serve your index.html from your host machine into the Nginx container's web root (/usr/share/nginx/html).
• Bring up the service with docker-compose up -d and verify you can access your static page.