Docker Compose and multi-container applications

Why and what

Compose describes services, networks and volumes in YAML. It is useful for repeatable development and small deployments; it does not by itself provide multi-host scheduling or high availability. Service names become DNS names on the Compose network.

Lab configuration

Create a .env file with strong lab-only MYSQL_PASSWORD and MYSQL_ROOT_PASSWORD values, restrict it to mode 600 and add it to .gitignore. Use this Compose file with an existing static-site image from the previous chapter:

services:
  web:
    image: academy-web:1
    ports:
      - '127.0.0.1:8080:80'
    restart: unless-stopped
  db:
    image: mysql:8.4
    environment:
      MYSQL_DATABASE: academy
      MYSQL_USER: academy
      MYSQL_PASSWORD: ${MYSQL_PASSWORD:?Set MYSQL_PASSWORD}
      MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD:?Set MYSQL_ROOT_PASSWORD}
    volumes:
      - mysql_data:/var/lib/mysql
    restart: unless-stopped
volumes:
  mysql_data:

This lab intentionally demonstrates two services; the static web image does not query MySQL. In a dynamic application, configure its database hostname as db, and add startup retry/readiness handling. Do not pretend that merely defining both services wires application code to the database.

Operate

docker compose config --quiet
docker compose up -d
docker compose ps
docker compose logs --tail 30 db
docker compose exec db mysql -u academy -p academy
docker compose down

down removes containers/networks while leaving named volumes by default. down -v removes declared named volumes and can erase the database. Environment variables can be inspected by privileged users; production secret handling should be stronger than a plain env file.

Readiness and failure

Start order is not application readiness. A database can have a running process while initialization is incomplete. Use health checks and retry logic with clear failure messages. Inspect container logs and verify persisted rows after recreating a container.

Assignment

Create a table and row, stop/recreate the stack without deleting volumes and confirm the row remains. Then describe a backup plan that survives host loss; a volume on the same host is persistence, not off-host disaster recovery.

Official reference

Docker Compose quickstart

Interview and revision check

Why is down -v different from down?

The volume-removal option can delete persisted database storage. Review exactly which volumes belong to the stack first.