Week 6, Day 3 — docker-compose

docker-compose YAML structure, lifecycle commands, restart policies, and service networking

Part 1 — docker-compose.yml structure (12 min)

• Walk through a minimal docker-compose.yml on the board:
  services:
webserver:
image: nginx
ports:
- "8080:80"
volumes:
- ./webroot:/usr/share/nginx/html:ro
restart: unless-stopped
  This is equivalent to the docker run command from yesterday — much more readable. The service name (webserver) becomes the container hostname on the compose network.
• Key YAML concepts: indentation is two spaces (never tabs). Lists use - prefix. Strings with special characters should be quoted. The file must be named docker-compose.yml (or docker-compose.yaml) in the directory where you run docker-compose commands.
• Adding a second service — a database alongside the web server:
  services:
webserver:
image: nginx
ports:
- "8080:80"
depends_on:
- db
db:
image: mysql:8
environment:
MYSQL_ROOT_PASSWORD: secret
volumes:
- db_data:/var/lib/mysql
volumes:
db_data:
  The volumes: section at the bottom declares named volumes (managed by Docker, not a host path). depends_on ensures db starts before webserver.

Part 2 — Lifecycle commands (8 min)

• All commands run from the directory containing docker-compose.yml:
  docker-compose up -d — start all services in detached mode
  docker-compose down — stop and remove containers (keeps images and named volumes)
  docker-compose down -v — also removes named volumes
  docker-compose ps — show status of all services
  docker-compose logs — show logs from all services
  docker-compose logs -f webserver — follow a specific service's logs
  docker-compose exec webserver bash — exec into a running service
  docker-compose restart webserver — restart a specific service
  docker-compose pull — pull latest image versions for all services

Part 3 — Restart policies (5 min)

• Four restart policies:
  no — never restart automatically (default). Container stays stopped after it exits.
  always — always restart, even if manually stopped, even after docker daemon restart.
  unless-stopped — restart automatically, but not if manually stopped with docker stop. Survives host reboots. The best choice for production services.
  on-failure — only restart if the container exits with a non-zero exit code.
  For persistent services like DNS servers, web servers, and databases: use unless-stopped. The container comes back after a host reboot without any manual intervention.

Part 4 — Networks in compose (5 min)

• By default, docker-compose creates a private bridge network for each stack. All services in the same compose file can reach each other by service name. No port mapping is needed for container-to-container communication — only for host-to-container access.
• Services on the compose network: webserver can connect to db at db:3306 without knowing the IP. Docker's internal DNS resolves the service name. This is how production stacks work — services find each other by name, IPs change constantly.