🐙 Intro to Docker Compose

So far we’ve been running containers one at a time with docker run. That works fine for a single app — but real-world apps are rarely just one container.

Think about a typical Node.js app:

• 🟢 Node.js API — your backend server
• 🐬 MySQL or PostgreSQL — your database
• 🔴 Redis — for caching or sessions

You’d need to start each container separately, set up networking between them, manage environment variables for each… it gets messy fast.

Docker Compose solves this.

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🐙 What is Docker Compose?

Docker Compose is a tool that lets you define and run multiple containers as a single application using one configuration file — docker-compose.yml.

Instead of running three separate docker run commands with long flags, you write everything once in a YAML file and then run:

docker compose up

That’s it. All your containers start together, talk to each other, and behave like one cohesive app.

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🗂️ The docker-compose.yml File

Everything is defined in a file called docker-compose.yml at the root of your project.

Here’s a simple example with a Node.js API and a PostgreSQL database:

version: '3.8'

services:
  api:
    build: .
    ports:
      - "4000:4000"
    environment:
      - DATABASE_URL=postgres://postgres:secret@db:5432/mydb
    depends_on:
      - db

  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: mydb

What’s happening here:

• services → defines each container (called a service)
• api → builds from your local Dockerfile and maps port 4000
• db → pulls the official PostgreSQL image from Docker Hub
• depends_on → ensures the db container starts before api
• Both services are on the same network by default, so api can reach db by using the hostname db

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🚀 Key Commands

Command	What it does
docker compose up	Start all services
docker compose up -d	Start in detached (background) mode
docker compose down	Stop and remove containers
docker compose logs	View logs from all services
docker compose ps	List running services
docker compose build	Rebuild images

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🔌 How Containers Talk to Each Other

When you run docker compose up, Docker automatically creates a shared network for all your services.

Each service can reach the other using its service name as the hostname.

So if your Node.js app needs to connect to Postgres, the connection string is:

postgres://user:password@db:5432/mydb

Notice db — that’s the service name from docker-compose.yml, not localhost. Docker handles the DNS resolution internally.

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✅ Why Use Docker Compose?

• One command to start your entire app stack
• No manual networking — services talk to each other by name
• Consistent environments — same setup on every machine
• Easy to share — just share the docker-compose.yml and anyone can run your app

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🧠 Mental Model

Think of docker-compose.yml as the blueprint for your entire app — it describes every piece (API, database, cache) and how they connect. Docker Compose reads that blueprint and brings the whole thing to life with a single command.