Debugging

When using dynamic port mapping or multiple services, it’s not always obvious which host port maps to which container port. docker compose port tells you exactly. Basic usage # Which host port maps to container port 80 on the web service? docker compose port web 80 # Output: 0.0.0.0:8080 Why dynamic ports matter When you let Docker assign ports automatically, the host port changes on every docker compose up: services: web: image: nginx ports: - "80" # Dynamic host port, container port 80 docker compose port web 80 # Output: 0.0.0.0:55432 (assigned dynamically) This is common in CI or when running multiple instances of the same project. ...

Know the difference between exec and run! Each has its place in your debugging toolkit. The key difference exec: Runs commands in an existing container run: Creates a new container # Exec: enters running container docker compose exec web bash # Run: starts new container docker compose run web bash When to use exec Use exec for debugging running services: # Debug a running web server docker compose exec web bash # Check logs inside container docker compose exec web tail -f /var/log/app.log # Run database queries docker compose exec db psql -U postgres # Check process list docker compose exec web ps aux # Test connectivity from inside docker compose exec web curl http://api:3000/health Important: Container must be running! ...

Track everything happening in your Compose stack! Events provide real-time insights into container lifecycle changes. Basic event monitoring Watch events as they happen: # Stream all events docker compose events # JSON format for parsing docker compose events --json # Specific services only docker compose events web worker # Since a specific time docker compose events --since "2026-02-06T10:00:00" Event types Common events you’ll see: container create # Container created container start # Container started container stop # Stop initiated container die # Container exited container destroy # Container removed health_status # Health check changed network connect # Network attached network disconnect # Network detached Processing events Parse with jq: ...

Stop scrolling through endless output. Master docker compose logs options to find issues fast and monitor services effectively. Basic commands # All service logs docker compose logs # Single service docker compose logs web # Multiple services docker compose logs web worker Follow logs in real-time Watch logs as they happen: # Follow all services docker compose logs -f # Follow specific service docker compose logs -f api # Start fresh and follow docker compose logs -f --since 1m Tail recent logs Get last N lines: ...