Docker Compose's extends attribute lets you share common configurations among different files, or even different projects entirely.

Extending services is useful if you have several services that reuse a common set of configuration options. With extends you can define a common set of service options in one place and refer to it from anywhere. You can refer to another Compose file and select a service you want to also use in your own application, with the ability to override some attributes for your own needs.

[!IMPORTANT]

When you use multiple Compose files, you must make sure all paths in the files are relative to the base Compose file (i.e. the Compose file in your main-project folder). This is required because extend files need not be valid Compose files. Extend files can contain small fragments of configuration. Tracking which fragment of a service is relative to which path is difficult and confusing, so to keep paths easier to understand, all paths must be defined relative to the base file.

How it works#

Extending services from another file#

Take the following example:

services:
  web:
    extends:
      file: common-services.yml
      service: webapp

This instructs Compose to re-use only the properties of the webapp service defined in the common-services.yml file. The webapp service itself is not part of the final project.

If common-services.yml looks like this:

services:
  webapp:
    build: .
    ports:
      - "8000:8000"
    volumes:
      - "/data"

You get exactly the same result as if you wrote docker-compose.yml with the same build, ports, and volumes configuration values defined directly under web.

To include the service webapp in the final project when extending services from another file, you need to explicitly include both services in your current Compose file. For example (note this is a non-normative example):

services:
  web:
    build: alpine
    command: echo
    extends:
      file: common-services.yml
      service: webapp
  webapp:
    extends:
      file: common-services.yml
      service: webapp

Alternatively, you can use include.

Extending services within the same file#

If you define services in the same Compose file and extend one service from another, both the original service and the extended service will be part of your final configuration. For example:

services:
  web:
    build: alpine
    extends: webapp
  webapp:
    environment:
      - DEBUG=1

Extending services within the same file and from another file#

You can go further and define, or re-define, configuration locally in compose.yaml:

services:
  web:
    extends:
      file: common-services.yml
      service: webapp
    environment:
      - DEBUG=1
    cpu_shares: 5

  important_web:
    extends: web
    cpu_shares: 10

Additional example#

Extending an individual service is useful when you have multiple services that have a common configuration. The example below is a Compose app with two services, a web application and a queue worker. Both services use the same codebase and share many configuration options.

The common.yaml file defines the common configuration:

services:
  app:
    build: .
    environment:
      CONFIG_FILE_PATH: /code/config
      API_KEY: xxxyyy
    cpu_shares: 5

The docker-compose.yaml defines the concrete services which use the common configuration:

services:
  webapp:
    extends:
      file: common.yaml
      service: app
    command: /code/run_web_app
    ports:
      - 8080:8080
    depends_on:
      - queue
      - db

  queue_worker:
    extends:
      file: common.yaml
      service: app
    command: /code/run_worker
    depends_on:
      - queue

Exceptions and limitations#

volumes_from and depends_on are never shared between services using extends. These exceptions exist to avoid implicit dependencies; you always define volumes_from locally. This ensures dependencies between services are clearly visible when reading the current file. Defining these locally also ensures that changes to the referenced file don't break anything.

extends is useful if you only need a single service to be shared and you are familiar with the file you're extending to, so you can tweak the configuration. But this isn’t an acceptable solution when you want to re-use someone else's unfamiliar configurations and you don’t know about its own dependencies.

Relative paths#

When using extends with a file attribute which points to another folder, relative paths declared by the service being extended are converted so they still point to the same file when used by the extending service. This is illustrated in the following example:

Base Compose file:

services:
  webapp:
    image: example
    extends:
      file: ../commons/compose.yaml
      service: base

The commons/compose.yaml file:

services:
  base:
    env_file: ./container.env

The resulting service refers to the original container.env file within the commons directory. This can be confirmed with docker compose config which inspects the actual model:

services:
  webapp:
    image: example
    env_file:
      - ../commons/container.env

Reference information#

• extends