Docker-compose: Replacement for the extends keyword

Question

From the docker docs:

Docker Compose’s extends keyword enables sharing of 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. Using extends you can define a common set of service options in one place and refer to it from anywhere.

For some reason this feature was removed in version 3.

Found also this thread, but it is inactive for 2 years.

I'm trying to find a replacement for this feature in the newer versions.

Would like to hear if somebody found a replacement for extends.

Thanks.

Answer 1

There are 2 ways to achieve what you need, you can decide to use one of them or both at the same time as they work slightly differently:

Multiple compose files

You can specify multiple compose files when running a docker compose command, you could for instance set up your project with:

docker-compose -f config1.yml -f config2.yml up

You could also use an environment variable to specify your files:

COMPOSE_FILE=config1.yml:config2.yml docker-compose up

What happens is that docker compose creates a single config merging what you defined in each of them.
Here the documentation showing how to merge multiple compose files.

You can also generate your final config file running the config command.

YAML Anchors

Since docker compose files are basically YAML files, you can take advantage of YAML Anchors to define a block of properties and reuse them in multiple parts of your config.

For example:

version: '3'

common: &common
  image: "myrepo/myimage"
  restart: "unless-stopped"
  volumes:
    - "volume:/mnt/myvolume"

services:
  service1:
    <<: *common
    ports:
      - "5000:5000"

  service2:
    <<: *common
    environment:
      - MYENV: value