Compose: the whole app in one file

📘What it really is

• Docker Compose reads a file (compose.yaml) describing services, and creates their containers, a shared network and volumes:
• docker compose up -d starts everything; docker compose ps lists it; docker compose logs -f app follows a service's output;

• docker compose down stops and removes the containers and network, keeping volumes.

• Each service lists its image (or a build folder), environment, ports, volumes and healthcheck.

• Start order: depends_on with condition: service_healthy waits until the database's health check passes before starting the API. Plain depends_on only waits for the container to start, which is before the database accepts connections.

• Copies: deploy: replicas: 2 (or docker compose up -d --scale app=3) runs several copies of one service, all sharing its name.

• Settings: ${DB_PASSWORD} in the file is filled from your shell or from a .env file next to it. Keep .env out of Git.
• Compose or Terraform? Compose is great for running an app on one machine, especially for development. Terraform (CD3) manages infrastructure of any kind, across many providers, and keeps a record (state) of what it created. The RouteRani projects use Terraform so the same skills carry over to cloud accounts.

The kit includes this compose.yaml (shortened). It runs the image tagged routerani:1.0.0, so build that tag first:

services:
  db:
    image: postgres:16.15-alpine
    environment:
      POSTGRES_USER: routerani
      POSTGRES_DB: routerani
      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
    secrets: [db_password]
    volumes: [db-data:/var/lib/postgresql/data]
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U routerani -d routerani"]
      interval: 2s
      retries: 15
  app:
    image: routerani:1.0.0
    environment:
      APP_VERSION: 1.0.0
      DATABASE_URL: postgres://routerani:${DB_PASSWORD}@db:5432/routerani
    depends_on:
      db: { condition: service_healthy }
    deploy: { replicas: 2 }
  lb:
    image: nginx:1.30.6-alpine
    volumes: [./infra/nginx.conf:/etc/nginx/nginx.conf:ro]
    ports: ["127.0.0.1:8081:80"]
    depends_on: [app]
volumes:
  db-data:
secrets:
  db_password:
    environment: DB_PASSWORD