Docker Compose

What Problem Compose Solves

Running a multi-container app with plain docker run means long commands, manual networking, and easy-to-forget flags. Start the database first, create a network, attach a volume, then run the API with the right -e flags and port maps — one missed step and nothing talks to anything.

Docker Compose replaces all of that with a single compose.yaml file and one command — docker compose up — to start every service, wire up networks, and attach volumes. The file is your source of truth; teammates get the same stack with zero guesswork.

Install Compose v2 alongside Docker on Ubuntu:

sudo apt install -y docker.io docker-compose-v2

Services

Services define the containers that make up your app — which image to use, ports to publish, and how each service starts. Each service name becomes a DNS hostname other services can reach:

services:
  web:
    build: .
    ports:
      - "8080:80"
  database:
    image: postgres:16

Networks

Networks control which services can talk to each other. By default Compose creates one network and attaches every service to it, but you can split traffic across named networks for isolation:

services:
  api:
    networks: [backend]
  database:
    networks: [backend]

networks:
  backend:

On a shared network, api connects to database:5432 — no IP addresses in your config.

Volumes

Volumes declare storage that survives container removal. Without them, a database container loses every row the moment you run docker compose down:

services:
  database:
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Environment Variables

Environment variables inject per-service config without rebuilding images. Database credentials, API keys, and feature flags belong here — not hardcoded in your Dockerfile:

services:
  database:
    environment:
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: myapp

Common Commands

Command	What it does
`docker compose up`	Start all services (builds images if needed)
`docker compose up -d`	Start in detached (background) mode
`docker compose down`	Stop and remove containers, networks, and volumes declared in the file
`docker compose logs -f web`	Stream logs for the `web` service
`docker compose exec web sh`	Open a shell inside a running `web` container

Key Takeaway Compose turns a pile of docker run commands into a declarative YAML file. Define your services, networks, volumes, and environment once — then bring the whole stack up or down with a single command.