docs/_includes/content/compose-extfields-sub.md

It is possible to re-use configuration fragments using extension fields. Those special fields can be of any format as long as they are located at the root of your Compose file and their name start with the x- character sequence.

version: '2.1'
x-custom:
  items:
    - a
    - b
  options:
    max-size: '12m'
  name: "custom"

The contents of those fields are ignored by Compose, but they can be inserted in your resource definitions using YAML anchors. For example, if you want several of your services to use the same logging configuration:

logging:
  options:
    max-size: '12m'
    max-file: '5'
  driver: json-file

You may write your Compose file as follows:

version: '2.1'
x-logging:
  &default-logging
  options:
    max-size: '12m'
    max-file: '5'
  driver: json-file

services:
  web:
    image: myapp/web:latest
    logging: *default-logging
  db:
    image: mysql:latest
    logging: *default-logging

It is also possible to partially override values in extension fields using the YAML merge type. For example:

version: '2.1'
x-volumes:
  &default-volume
  driver: foobar-storage

services:
  web:
    image: myapp/web:latest
    volumes: ["vol1", "vol2", "vol3"]
volumes:
  vol1: *default-volume
  vol2:
    << : *default-volume
    name: volume02
  vol3:
    << : *default-volume
    driver: default
    name: volume-local