How to prevent overwriting the whole environment field while using YAML anchors

I am writing my docker-compose.yml file and to avoid writing redundant lines especially for using different profiles for dev, uat, prod etc, I was using yaml anchors. Here is a sample for one of the adapters I used for uat and for dev.

Adapter:&Adapter
  image:"my-image"
  profiles:
    - uat
  environment:
    - JAVA_OPTS="${JAVA_OPTS} -Dloader.main=com.my.class"
    - SPRING_PROFILES_ACTIVE=uat
  volumes:
    - /local/path/to/logs:/logs
  depends_on:
    - some_image

Adapter-dev:
  <<: *Adapter
  profiles:
    - dev
  environment:
    - SPRING_PROFILES_ACTIVE=dev

.env file contains:

JAVA_OPTS=-Xmx3g -Xms3g

The key part is that the JAVA_OPTS is a must have in order to run properly. The JAVA_OPTS is stored in a .env file, and I append to the JAVA_OPTS variable to specify my main class. I just found out that doing it in the way above, it overwrites the environment entirely and only passes the SPRING_PROFILES_ACTIVE to my docker container's environment variables. What I expected would be that only SPRING_PROFILES_ACTIVE would be overwritten, but the JAVA_OPTS would be preserved. How can I only overwrite the SPRING_PROFILES_ACTIVE in my docker-compose.yml without overwriting the JAVA_OPTS?

This might be a case where I'd use Compose's support for multiple Compose files, which are able to combine per-service settings. Compose profiles are better for selecting which service to run, but not necessarily for controlling how to run it.

You'd have a base docker-compose.yml file that listed all of the settings that were required in every environment

# docker-compose.yml
version: '3.8'
services:
  adapter:
    image: "my-image"
    environment:
      - JAVA_OPTS="${JAVA_OPTS} -Dloader.main=com.my.class"
    volumes:
      - /local/path/to/logs:/logs
    depends_on:
      - some_image

Then you'd have a file per environment that added settings for only that environment

# docker-compose.uat.yaml
version: '3.8'
services:
  adapter:
    environment:
    - SPRING_PROFILES_ACTIVE=uat

When you go to run docker-compose commands, you can then specify both files using docker-compose -f options. Note that you need to specify both files on every docker-compose invocation; setting a $COMPOSE_FILE environment variable may be a little easier.

docker-compose -f docker-compose.yml -f docker-compose.uat.yaml up -d

There are two versions of the Compose tooling, an older one that runs as a standalone tool and a newer one packaged as a docker CLI extension. This setup will work with both of them.

In Compose you can set up one service to extend another. This does essentially what you're showing with YAML anchors, but it also lets you provide additional settings.

version: '2.4'
services:
  adapter:
    image: "my-image"
    profiles: []
    environment:
      - JAVA_OPTS="${JAVA_OPTS} -Dloader.main=com.my.class"
    volumes:
      - /local/path/to/logs:/logs

  adapter-uat:
    extends:
      service: adapter
    profiles:
      - uat
    environment:
      - SPRING_PROFILES_ACTIVE=uat
    depends_on:
      - some_image

  adapter-dev:
    extends:
      service: adapter
    profiles:
      - dev
    environment:
      - SPRING_PROFILES_ACTIVE=dev
    depends_on:
      - some_image

depends_on: is documented to not be reused in extends: services, so I've copied it into each per-environment service here.

There are two versions of the Compose tool. If you are using the older version of the tool (docker-compose --version says it's version 1) then be aware that the Compose file format version 3 doesn't include extends:; you need to declare a version: 2.x at the start of the file and you will be unable to use some Swarm-related features in version 3 of the file format.

The specific solution used was to utilise a common-services.yml file. This allowed me to only run the services with the specified profile without running the default services. What I did was to extract the default service to an additional yml file like so:

#common-services.yml:
services:
  adapter:
    image: "my-image"
    profiles: []
    environment:
      - JAVA_OPTS="${JAVA_OPTS} -Dloader.main=com.my.class"
    volumes:
      - /local/path/to/logs:/logs

and the docker compose yaml will look like:

#docker-compose.yml
version: '3.9'
services:
  adapter-uat:
    extends:
      file: common-services.yml
      service: adapter
    profiles:
      - uat
    environment:
      - SPRING_PROFILES_ACTIVE=uat
    depends_on:
      - some_image

  adapter-dev:
    extends:
      file: common-services.yml
      service: adapter
    profiles:
      - dev
    environment:
      - SPRING_PROFILES_ACTIVE=dev
    depends_on:
      - some_image