Part 11: Docker Compose

In Part 10: Postgres in a container we ran Postgres as one container and the app locally against it. That works, but we want to run both of them at the same time with one command.

We use Docker Compose to put the app image and Postgres in one file, then run one command to start both services together.

Describe the stack

Ask the assistant for a Compose file with both services:

Write a compose.yaml with a postgres service and the app service built from
the Dockerfile

Give Postgres a named volume so data survives

Make the app wait for Postgres to be ready

This writes a compose.yaml at the repo root, the modern name that Compose now prefers while still reading docker-compose.yml too.

The generated file looks like this:

services:
  postgres:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: snakearena
      POSTGRES_PASSWORD: snakearena
      POSTGRES_DB: snakearena
    volumes:
      - snake_pgdata:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U snakearena -d snakearena"]
      interval: 5s
      timeout: 5s
      retries: 5
      start_period: 5s

  app:
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      SNAKE_ROYALE_DATABASE_URL: postgresql://snakearena:snakearena@postgres:5432/snakearena
    ports:
      - "8000:8000"
    depends_on:
      postgres:
        condition: service_healthy

volumes:
  snake_pgdata:

Compose makes this convenient in two ways:

• The app reaches the database at the hostname postgres, the service name on the Compose network, so there's no host.docker.internal juggling.
• depends_on with the Postgres healthcheck holds the app back until the database is ready to accept connections.

Bring the stack up

Bring the whole stack up with one command:

docker compose up --build

The app is at localhost:8000, with the API docs alongside it. Sign up and submit a score, then bring the stack down and up again - the snake_pgdata volume keeps the data. Stop the stack with docker compose down.

Commit the Compose setup:

git add .
git commit -m "Add Docker Compose for app and Postgres"

We take this same app image to a real server in Part 12: Deploy to AWS with infrastructure as code.