Imagine building a city blueprint. Each building (service) has its own role: a hospital, a school, a power station. In Docker Compose, services are those buildings - each one represents a container with a specific purpose. By defining services, you tell Docker how to construct and connect the pieces of your application.

Services Foundations

1. What is a Service in Compose?

• A service is a container definition inside docker-compose.yml.
• Each service specifies the image, build context, ports, environment variables, volumes, and dependencies.
• Services can be scaled to multiple replicas.

2. Common Service Configuration Options

• restart: Controls restart policy (always, on-failure, unless-stopped).

depends_on: Defines service dependencies.

depends_on:
  - db

volumes: Mounts host directories or named volumes.

volumes:
  - dbdata:/var/lib/mysql

environment: Sets environment variables.

environment:
  - MYSQL_ROOT_PASSWORD=secret

ports: Maps host ports to container ports.

ports:
  - "8080:80"

build: Builds an image from a Dockerfile.

build: ./app

image: Defines which image to use.

image: nginx:latest

3. Service Dependencies

• Services can depend on each other (e.g., a web app depends on a database).
• Compose ensures dependent services start in the right order.

4. Scaling Services

• Services can be scaled horizontally.
• Runs three replicas of the web service.

docker-compose up --scale web=3

Things to Remember

• Services are the core building blocks of Compose.
• Each service defines how a container should run.
• Dependencies and scaling make services powerful for microservices.

Hands‑On Lab

Step 1: Define a Web + DB Stack

version: '3'
services:
  web:
    image: nginx
    ports:
      - "8080:80"
    depends_on:
      - db
  db:
    image: mysql:5.7
    environment:
      - MYSQL_ROOT_PASSWORD=secret
    volumes:
      - dbdata:/var/lib/mysql

volumes:
  dbdata:

Step 2: Run the Stack

docker-compose up -d

Step 3: Verify Services

docker-compose ps

Step 4: Scale the Web Service

docker-compose up -d --scale web=3

Practice Exercise

1. Create a docker-compose.yml file with three services:
   • frontend (nginx) exposed on port 8081.
   • backend (node.js app built from a Dockerfile).
   • database (mysql with environment variables).
2. Define dependencies (frontend depends on backend, backend depends on database).
3. Run the stack and confirm all services are connected.
4. Scale the frontend service to 2 replicas.

Visual Learning Model

docker-compose.yml
   ├── Service: frontend → port 8081
   ├── Service: backend → depends on database
   └── Service: database → persistent volume

The Hackers Notebook

Defining services in Compose is about describing how each container should run. Services specify images, builds, ports, environment variables, volumes, and dependencies. With scaling and restart policies, services become the foundation for building resilient, multi‑container applications.