Intermediate

Hardcoding IP addresses in your containers? Docker Compose provides automatic DNS-based service discovery. Each service can reach another using just the service name. How it works Docker Compose creates a default network and registers each container with an internal DNS server. The DNS name matches the service name in your compose.yml. services: web: image: nginx environment: # Just use the service name! API_URL: http://api:3000 DB_HOST: postgres api: image: myapi environment: DATABASE_URL: postgres://user:pass@postgres:5432/mydb postgres: image: postgres:15 No configuration needed. The web service connects to api using http://api:3000, and api connects to postgres using the hostname postgres. ...

“Connection refused” errors? The app starts before the database is ready. Here’s the fix. What doesn’t work This only waits for the container to start, not for it to be ready: services: app: depends_on: - db # Container starts, but database isn't ready yet What actually works Add health checks: services: db: image: postgres:16 environment: POSTGRES_PASSWORD: secret healthcheck: test: ["CMD-SHELL", "pg_isready -U postgres"] interval: 10s timeout: 5s retries: 5 start_period: 10s app: image: myapp depends_on: db: condition: service_healthy # Now it actually waits for the database Common health checks PostgreSQL: ...

Same compose.yml, different environments. Here’s the cleanest approach. Basic setup Create different env files for each environment: .env.dev DATABASE_URL=postgresql://localhost:5432/dev_db API_KEY=dev_key_12345 LOG_LEVEL=debug REPLICAS=1 .env.prod DATABASE_URL=postgresql://prod-db.example.com:5432/prod_db API_KEY=${SECURE_API_KEY} # From CI/CD secrets LOG_LEVEL=error REPLICAS=3 How to use them # Development docker compose --env-file .env.dev up # Production docker compose --env-file .env.prod up # Override specific vars API_KEY=test_key docker compose --env-file .env.dev up Layering configs You can use multiple env files: # Base + environment-specific docker compose \ --env-file .env.base \ --env-file .env.prod \ up Note: Later files override earlier ones. ...