Docker Composeによるマルチコンテナアプリ

Real apps rarely run in one container. You often need a web app, a database, and a cache. Docker Compose lets you define all of them in one YAML file and start them with a single command.

Compose reads a file named compose.yaml (or docker-compose.yml). Each entry under services describes one container.

A service can be built from a local Dockerfile or pulled as an existing image. You also set ports, environment variables, and dependencies here.

Pass configuration with environment or load from an .env file. Never hardcode secrets directly in the YAML committed to git.

Compose puts all services on a shared network. A service reaches another by its service name as the hostname. The web app connects to the database at db:5432.

Containers are ephemeral. Use a named volume so database data survives restarts and rebuilds.

depends_on controls start order, but it does not wait for a service to be ready. Combine it with a healthcheck so the web app waits until the database accepts connections.

Start everything in the background with up -d and stop with down. Add --build to rebuild images after code changes.

Aggregate logs from all services or follow one. This is essential for debugging how containers interact.

Run multiple copies of a stateless service to handle more load. Put a load balancer or reverse proxy in front to distribute traffic.

Use a base compose.yaml plus an override file for each environment. Development might mount source code for hot reload; production uses prebuilt images and stricter restart policies.

You learned to orchestrate multi-container apps with Docker Compose:

• Define services in compose.yaml
• Connect them via service-name networking
• Persist data with volumes and control order with depends_on/healthcheck
• Use up, down, logs, and scaling

Compose makes local and production multi-service setups reproducible.