Docker Compose YAML Anchors (&) - Eliminate Repeated Config

Problem

When running multiple similar services in Docker Compose, common settings like image, volumes, and restart get copy-pasted across every service definition. With 10 services, that’s 10 copies of the same config.

Solution

# Define common config with an anchor (&)
x-app-common: &app-common
  image: my-app:latest
  volumes:
    - ./cert:/app/cert:ro
  restart: unless-stopped
  dns:
    - 172.16.0.1

services:
  # Merge (<<) common config, add service-specific settings
  app01:
    <<: *app-common
    container_name: app01
    environment:
      - APP_ID=01
      - PORT=3001

  app02:
    <<: *app-common
    container_name: app02
    environment:
      - APP_ID=02
      - PORT=3002

  app03:
    <<: *app-common
    container_name: app03
    environment:
      - APP_ID=03
      - PORT=3003

Key Points

• Keys starting with x- are extension fields that Docker Compose ignores. They’re the ideal place for shared configuration.
• &name defines an anchor, <<: *name merges it. Any key specified after the merge overrides the anchored value.
• The more services you have, the bigger the payoff. When managing 10 services, changing a common setting in one place eliminates configuration drift.