Extensions

Extensions can be used to make your Compose file more efficient and easier to maintain.

Use the prefix x- as a top-level element to modularize configurations that you want to reuse. Compose ignores any fields that start with x-, this is the sole exception where Compose silently ignores unrecognized fields.

Extensions can also be used with anchors and aliases.

They also can be used within any structure in a Compose file where user-defined keys are not expected. Compose uses those to enable experimental features, the same way browsers add support for custom CSS features

Example 1

x-custom:
  foo:
    - bar
    - zot

services:
  webapp:
    image: example/webapp
    x-foo: bar

service:
  backend:
    deploy:
      placement:
        x-aws-role: "arn:aws:iam::XXXXXXXXXXXX:role/foo"
        x-aws-region: "eu-west-3"
        x-azure-region: "france-central"

Example 2

x-env: &env
  environment:
    - CONFIG_KEY
    - EXAMPLE_KEY

services:
  first:
    <<: *env
    image: my-image:latest
  second:
    <<: *env
    image: another-image:latest

In this example, the environment variables do not belong to either of the services. They’ve been lifted out completely into the x-env extension field. This defines a new node which contains the environment field. The &env YAML anchor is used so both services can reference the extension field’s value as *env.

Example 3

x-function: &function
 labels:
   function: "true"
 depends_on:
   - gateway
 networks:
   - functions
 deploy:
   placement:
     constraints:
       - 'node.platform.os == linux'
services:
 # Node.js gives OS info about the node (Host)
 nodeinfo:
   <<: *function
   image: functions/nodeinfo:latest
   environment:
     no_proxy: "gateway"
     https_proxy: $https_proxy
 # Uses `cat` to echo back response, fastest function to execute.
 echoit:
   <<: *function
   image: functions/alpine:health
   environment:
     fprocess: "cat"
     no_proxy: "gateway"
     https_proxy: $https_proxy

The nodeinfo and echoit services both include the x-function extension via the &function anchor, then set their specific image and environment.

Example 4

Using YAML merge it is also possible to use multiple extensions and share and override additional attributes for specific needs:

x-environment: &default-environment
  FOO: BAR
  ZOT: QUIX
x-keys: &keys
  KEY: VALUE
services:
  frontend:
    image: example/webapp
    environment:
      << : [*default-environment, *keys]
      YET_ANOTHER: VARIABLE

メモ

YAML merge only applies to mappings, and can't be used with sequences.

In the example above, the environment variables are declared using the FOO: BAR mapping syntax, while the sequence syntax - FOO=BAR is only valid when no fragments are involved.

Informative Historical Notes

This section is informative. At the time of writing, the following prefixes are known to exist:

Specifying byte values

Values express a byte value as a string in {amount}{byte unit} format: The supported units are b (bytes), k or kb (kilo bytes), m or mb (mega bytes) and g or gb (giga bytes).

    2b
    1024kb
    2048k
    300m
    1gb

Specifying durations

Values express a duration as a string in the form of {value}{unit}. The supported units are us (microseconds), ms (milliseconds), s (seconds), m (minutes) and h (hours). Values can combine multiple values without separator.

  10ms
  40s
  1m30s
  1h5m30s20ms