Q: What is Docker Compose and when would you use it?

Answer:

Docker Compose is a tool for defining and running multi-container applications using a single YAML configuration file (docker-compose.yml or compose.yaml). Instead of running multiple docker run commands with complex flags, you declare everything in one file and spin up the entire stack with a single command.

Without Compose (Painful)

docker network create myapp
docker volume create db_data
docker run -d --name db --network myapp -v db_data:/var/lib/postgresql/data \
    -e POSTGRES_PASSWORD=secret postgres:16
docker run -d --name redis --network myapp redis:7
docker run -d --name api --network myapp -p 3000:3000 \
    -e DATABASE_URL=postgres://db:5432 \
    -e REDIS_URL=redis://redis:6379 myapi

With Compose (Clean)

# compose.yaml
services:
  api:
    build: ./api
    ports:
      - "3000:3000"
    environment:
      DATABASE_URL: postgres://db:5432/mydb
      REDIS_URL: redis://redis:6379
    depends_on:
      - db
      - redis

  db:
    image: postgres:16
    volumes:
      - db_data:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: secret

  redis:
    image: redis:7-alpine

volumes:
  db_data:

# Start everything
docker compose up -d

# View logs
docker compose logs -f api

# Stop and remove everything
docker compose down

# Stop, remove, AND delete volumes (nuclear option)
docker compose down -v

Key Features

  • Automatic networking: All services in a compose.yaml automatically join a shared network and can reach each other by service name.
  • Volume management: Volumes are declared and managed alongside services.
  • Build integration: You can specify build: context directly instead of pre-building images.
  • Profiles: Group services into profiles for conditional startup.
  • Override files: Use compose.override.yaml for environment-specific config.

When to Use Compose

  • Local development: Spin up your full stack (API + DB + cache + queue) in one command.
  • CI/CD: Run integration tests against real services.
  • Single-host production: Small deployments that don't need Kubernetes.

[!NOTE] Docker Compose is not an orchestration tool. It runs containers on a single host. For multi-host orchestration, use Docker Swarm or Kubernetes.