Docker Compose

In a real project, your application usually needs more than just one container. You might need an API server, a database, and a cache.

Docker Compose is a tool for defining and running multi-container applications. You describe all your containers in a single file called docker-compose.yml.

Why Use Docker Compose?

Instead of typing 10 different docker run commands every time you want to start your project, you can just type:

docker-compose up

This starts all your containers at once and handles the networking between them automatically.

Example `docker-compose.yml` File

version: '3.8'

services:
  # The API Server
  api:
    build: .
    ports:
      - "3000:3000"
    depends_on:
      - db

  # The Database
  db:
    image: postgres:15
    environment:
      POSTGRES_PASSWORD: mysecretpassword

Key Docker Compose Commands

1. `docker-compose up`

Starts all the containers defined in your docker-compose.yml file.

Tip: Use docker-compose up -d to run them in the background.

2. `docker-compose down`

Stops and removes all the containers (and the networks they were using).

3. `docker-compose ps`

Shows you the status of all the containers in your project.

4. `docker-compose logs`

Shows you the combined output from all your containers.

Architecture Diagram (The Multi-Container System)

       [ DOCKER COMPOSE ]
               ↓ (Manages)
       ---------------------------
       ↓                         ↓
   [ API Container ]         [ DB Container ]
       ↓ (Talks to)              ↓
       [----------------- Docker Network -----------------]

How Junior Developers Encounter This

“I can’t get the database to start”: (Checking the db service in the docker-compose.yml file).
“I need to add a new service”: (Updating the services section in the file).
“The API is starting before the database is ready”: (Learning about depends_on).

Common Beginner Mistakes

Indentation Errors in YAML: YAML files are very strict about indentation. If your spaces aren’t perfectly aligned, Docker Compose will fail.
Forgetting to Rebuild: If you change your code and run docker-compose up, Docker will reuse your old image! Use docker-compose up --build to force it to re-build your images.
Hardcoding Passwords: Don’t put your database passwords directly in your docker-compose.yml file. Use an .env file instead.

Next Lesson

Once your database is running, you need to make sure its data doesn’t disappear when the container stops:

➡ Volumes and Persistent Data