Why does Docker Compose fail with an indentation error?

YAML uses indentation to define hierarchy. Mixing tabs and spaces, or inconsistent indentation under a key, causes the parser to misread the structure and report confusing errors.

What does depends_on do in Docker Compose?

depends_on controls startup order, ensuring the db service starts before the web service. It does not wait for the database to be ready — use a health check or startup script for that.

How do I use environment variables in docker-compose.yml?

Reference variables from a .env file in the same directory using ${VARIABLE_NAME} syntax, or use the env_file key to load a file of variables into a container.

Validate a docker-compose.yml File

Docker Compose files are notoriously sensitive to indentation because YAML uses whitespace to define structure. A single extra or missing space breaks the file silently or with a confusing error. This example shows a multi-service compose file with volumes, environment variables, and port mappings. Validate before running docker compose up to catch errors early.

Example

version: '3.9'
services:
  web:
    image: node:20-alpine
    ports:
      - "3000:3000"
    environment:
      NODE_ENV: production
      DATABASE_URL: postgres://db:5432/myapp
    depends_on:
      - db
    volumes:
      - ./app:/usr/src/app

  db:
    image: postgres:16
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: user
      POSTGRES_PASSWORD: secret
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

[ open in Docker Compose Validator → ]

FAQ

Why does Docker Compose fail with an indentation error?: YAML uses indentation to define hierarchy. Mixing tabs and spaces, or inconsistent indentation under a key, causes the parser to misread the structure and report confusing errors.
What does depends_on do in Docker Compose?: depends_on controls startup order, ensuring the db service starts before the web service. It does not wait for the database to be ready — use a health check or startup script for that.
How do I use environment variables in docker-compose.yml?: Reference variables from a .env file in the same directory using ${VARIABLE_NAME} syntax, or use the env_file key to load a file of variables into a container.

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