Overview of Docker Compose

Docker Compose is an orchestration tool provided by Docker for managing multi-container applications. It uses a YAML file to define services, networks, and volumes, allowing developers to launch and manage complex environments with a single command. This is especially useful in development, testing, and staging environments.

Step 1: Create a Dockerfile

Begin by defining a Docker image using a Dockerfile in your application's root directory. Below is an example of a Dockerfile for a Python application:

# Use a lightweight Python image
FROM python:3.9-alpine

# Set the working directory inside the container
WORKDIR /opt/app

# Copy local files into the container
COPY . .

# Install dependencies from requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Expose port 5000 for the application
EXPOSE 5000

# Set an environment variable
ENV GREETING Hello

# Start the application
CMD ["python", "main.py"]

Step 2: Define Services in docker-compose.yml

Create a docker-compose.yml file in the root directory to define your services. Here's an example:

version: '3.8'

services:
  app:
    build: .
    ports:
      - "5000:5000"
    volumes:
      - .:/opt/app
    environment:
      - GREETING=Hi

  cache:
    image: redis:latest
    ports:
      - "6379"

In this configuration, two services are defined: app and cache. The app service is built from the Dockerfile in the current directory, and it maps port 5000. The cache service uses the official Redis image.

Step 3: Launch and Manage Services

Run the following command in the directory containing the docker-compose.yml file to start all services:

docker-compose up

To run in detached mode, add the -d flag:

docker-compose up -d

If you make changes to your configuration or application, rebuild the images using:

docker-compose up --build

Step 4: Control Service Lifecycle

Use the following commands to manage running services:

• Stop services: docker-compose stop
• Stop and remove containers: docker-compose down
• View logs: docker-compose logs
• List running services: docker-compose ps

Step 5: Scale Services

You can scale a service to run multiple instances using the --scale option. For example, to start three instances of the app service:

docker-compose up --scale app=3

Debugging and Log Inspection

To inspect logs for a specific service:

docker-compose logs app

Replace app with the name of any service defined in your YAML file, such as cache.