Minimize duplicated content in docker-compose.yml

[tschaffter]

The services defined in docker-compose.yml have a lot of configuration that overlap. There should be a way to reduce the amount of duplicated code using inheritance or templates.

[tschaffter]

The docker-compose keyword extends is no longer available in version 3 of docker-compose.

• extends not supported in version 3 schema docker/compose#4315

An alternative is to use a templating approach. The use of a script that injects the content into the template would be a drawback but not here since I'm already using one (debian.sh). However, this may mean that I would not longer be able to use docker-compose build <services>.

Looking at existing workarounds:

• Extending docker-compose configuration: This would mean one file per service. Something like:

docker-compose run \
  -f ./docker/base.yml \
  -f ./docker/extended.yml

An alternative as suggested in the above link is to place the overriding information in a Bash script to have them at a single location. However, this would make it difficult to validate the data. Alternatively I could place the overriding information in a JSON file, but I would list the built-in docker tool that validate the configuration file. I could write my own based on a JSON Schema that I would write for only the properties that I intend to support. However maintaining this schema would require additional effort.

• Using a templating program: The method adopted by the author of this comment relies on Liquid. After reading about Liquid, this looks like an overkill solution.

[tschaffter]

Update

The current approach is user friendly and enables to quickly build and test environment. The cost for reducing the amount of duplicated code in docker-compose.yml using the current version of docker-compose is outweighed by the importance of keeping a user-friendly interface. I will revisit this feature when it is really needed or when Docker adds a feature similar to extends.