🧩 Docker Compose – Run Multiple Containers Easily

Running one container is fine.
But real apps usually need many containers:

• Web app
• Database
• Cache
• Backend API

Starting each one with long docker run commands is painful 😖.

👉 Docker Compose solves this.

⚡ What is Docker Compose?

Docker Compose lets you define and run multiple containers using one file called docker-compose.yml.

In simple words:

• One file
• One command
• Entire application up and running 🚀

📌 Mostly used in development and testing, but also useful in small production setups.

🍱 Easy Analogy

Imagine ordering food 🍔🍟🥤:

• Burger = Web app
• Fries = Database
• Drink = Cache

Instead of ordering one by one, you order a combo.

👉 Docker Compose = Combo order for containers.

📄 docker-compose.yml (The Heart of Compose)

This file describes:

• Which containers to run
• Images to use
• Ports
• Volumes
• Networks
• Environment variables

Everything in YAML format.

🛠️ Basic Docker Compose Example

Web + Database App

version: "3.9"

services:
  web:
    image: nginx
    ports:
      - "80:80"

  db:
    image: mysql:8
    environment:
      MYSQL_ROOT_PASSWORD: root

version: "3.9"

services:
  web:
    image: nginx
    ports:
      - "80:80"

  db:
    image: mysql:8
    environment:
      MYSQL_ROOT_PASSWORD: root

What happens:

• Nginx container starts
• MySQL container starts
• Both run together as one app

▶️ Running Docker Compose

Start everything:

docker compose up

docker compose up

Run in background:

docker compose up -d

docker compose up -d

Stop containers:

docker compose down

docker compose down

👉 One command replaces many docker run commands.

📦 Services in Docker Compose

Each container is called a service.

Example:

services:
  frontend:
  backend:
  database:

services:
  frontend:
  backend:
  database:

Compose automatically:

• Creates a network
• Allows services to talk using service names
  (backend can talk to database directly)

🌐 Networking in Docker Compose

By default:

• Compose creates a bridge network
• All services connect to it
• Containers talk using service names

👉 Example:

backend:
  depends_on:
    - db

backend:
  depends_on:
    - db

Backend can reach DB using:

mysql -h db

mysql -h db

No IP needed 👍.

💾 Volumes in Docker Compose

Used to persist data.

services:
  db:
    image: mysql
    volumes:
      - db_data:/var/lib/mysql

volumes:
  db_data:

services:
  db:
    image: mysql
    volumes:
      - db_data:/var/lib/mysql

volumes:
  db_data:

Now:

• Delete container ❌
• Data still safe ✅

🔐 Environment Variables

You can define env variables directly:

environment:
  APP_ENV: production

environment:
  APP_ENV: production

Or use .env file (cleaner way):

MYSQL_ROOT_PASSWORD=root

MYSQL_ROOT_PASSWORD=root

Compose automatically loads it.

🧱 Build Image Using Compose

Instead of using prebuilt images:

web:
  build: .
  ports:
    - "3000:3000"

web:
  build: .
  ports:
    - "3000:3000"

Then run:

docker compose up --build

docker compose up --build

👉 Great for local development.

⏱️ depends_on (Startup Order)

web:
  depends_on:
    - db

web:
  depends_on:
    - db

This ensures:

• DB starts first
• Web starts after

📌 Note: It doesn’t wait for DB to be ready, only started.

🧹 Cleaning Up

Stop and remove containers:

docker compose down

docker compose down

Remove volumes also:

docker compose down -v

docker compose down -v

🆚 Docker Compose vs Docker Swarm

👉 Compose = local orchestration
👉 Swarm = cluster orchestration

✅ When to Use Docker Compose?

Use Docker Compose when:

• Developing locally
• Running multi-container apps
• Testing microservices
• Learning Docker concepts