Docker-compose, conditional statements? (e.g. add volume only if condition)

I want to add a volume to my service, but only if the final user gave a folder for it. Otherwise, no volume should be mounted, for the already-prepared image has valid data in a default folder.

That is, I want to do something like (pseudocode):

services:

  my_awesome_service:
  
    volumes:
      if ${VARIABLE} => ${VARIABLE}:/app/folder

Are such conditional statements definable in a docker-compose file?

The only way I see to make this possible is to first define a base docker-compose file, which does not have the volume mount, and the call on a second docker-compose file only if the $VARIABLE is defined. This is fine for a single or few conditions, but gets nasty if there are many.

Any solution?

Poor man's solution:

    volumes:
      ${VARIABLE:-/dev/null}:/app/folder

Or:

    volumes:
      ${VARIABLE:-/dev/null}:${VARIABLE:-/tmp}/app/folder

Nothing like this currently exists. Options to implement this that I can come up with include:

1. Make lots of compose file pieces, and merge together the parts you need to build up the final file.

2. Dynamically generate your compose file. Something like jsonnet may be a good starting point.

3. Skip compose, and just dynamically generate your docker run command. This starts to lack portability but some use cases are just easier to script yourself.

4. Submit a PR to the compose and docker/cli github repos to extend the compose functionality. Doing this with a golang template syntax would make the most sense to me.

Yeah, I do not think docker-compose's format supports conditional statements.

However, two solutions could be:

1. Pass a "complex" (list-like) variable to the docker-compose such as in this example:

docker-compose.yaml:

command: ${COMMAND_PARAMS}

bash:

#!/bin/bash
if test -z $CONDITION; then
  params="-param1 ${MIPARAM1}"
else
  params="-param1 ${MIPARAM1} -param2 ${MIPARAM2}"
fi
COMMAND_PARAMS=$params docker-compose up

(credits goes to original poster on github, @shin-)

2. Prepare the default folder in the docker image in a folder named something like folder_defaults, then have the volume always defined in docker-compose.yml, but then finally, have an internal script in the docker image that checks whether the volume folder is empty, and if so ln -s to the folder_defaults; otherwise leave it as it is.

Example of the conditional script:

if [ -z "$(ls -A /a/folder)" ]; then
  do something... using /a/folder_defaults
fi

If you are using Rancher for orchestration, there are escapes {{...}} available you can use for conditional statements in Rancher's version of docker-compose.

Read more about the integrated GO templating system here.